# Could you have designed a better P-39?



## gjs238 (Jan 7, 2010)

With hindsight (ain't hindsight a great thing?) could you have made the P-39 a truly great plane?
Perhaps scaling-up the basic design for greater fuel capacity, better turbocharger inter/aftercooler arrangement, armament, etc?
The mid-engine layout appears to be an efficient way to incorporate a turbocharger into a single-engine fighter/interceptor (all the ducting of the P-47 is not necessary.)
The mid-engine layout also lends itself to impressive nose armament.

I'm not suggesting improving an existing P-39, rather how the basic mid-engine turbocharged design concept could have been.


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## davebender (Jan 7, 2010)

Replace the Allison engine with a Packard built RR Merlin engine.


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## Colin1 (Jan 7, 2010)

If we're back in the day
then we're not going to find a better turbocharger solution than the one that was rejected, not without turning the fuselage into something the size of a P-47. Turbocharger design of the time was in its infancy, was not fully understood and the principal manufacturers mistrusted them, seeing them as troublesome, requiring development time and for that read an uncompetitive tender. 
The P-39 could have soldiered on with the V-1710 in an Army support role superbly, it could best the Bf109E convincingly at those altitudes and could have held the line until the V-1650 came along. In an ideal world, I would have gone with an experimental Packard V-1650 installation to take the P-39 to altitude and a wet wing plus drop tanks to start working on the range issue.

So a supercharger solution, rather than a turbocharger.

In the real world, getting hold of a V-1650 would have been considerably harder, maybe buying a dozen Merlin 61s direct from Rolls-Royce and shipping them over. It is highly likely that that wouldn't have proved any easier.


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## davebender (Jan 7, 2010)

We want a Army fighter aircraft that works right now. (i.e. around 1940)

1. Turborchargers are still experimental for fighter aircraft during 1940.

2. The Allison engine is a dog at all but low altitude.

3. The RR Merlin engine is state of the art and arrangements are already being made for mass production in the USA. Britain is investing heavily into further performance improvments for this engine.

Seems like a no brainer to me. Pay Packard to expand production capacity so they can produce RR Merlins for the P-39 as well as for the Lancaster bomber.


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## gjs238 (Jan 7, 2010)

Colin1 said:


> If we're back in the day then we're not going to find a better turbocharger solution than the one that was rejected, not without turning the fuselage into something the size of a P-47. Turbocharger design of the time was in its infancy, was not fully understood and the principal manufacturers mistrusted them, seeing them as troublesome, requiring development time and for that read an uncompetitive tender.


How is that? (not a challenge, just a question)
Turbos were accepted and worked out well in several US aircraft. With more room why wouldn't a turbo work in an enlarged P-39?
I know this is apples oranges by a long shot, but a modern front-engine car doesn't have the turbo located back in the trunk connected by ductwork ala P-47, the turbo is up front w/the engine (or down back if a rear or mid-engine setup.)


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## Colin1 (Jan 7, 2010)

davebender said:


> 3. The RR Merlin engine is state of the art and arrangements are already being made for mass production in the USA. Britain is investing heavily into further performance improvments for this engine.
> 
> Seems like a no brainer to me. Pay Packard to expand production capacity so they can produce RR Merlins for the P-39 as well as for the Lancaster bomber.


With Merlin production going to the Lancaster and later the Mustang, how easily do you think the Bell lines are going to procure them?

Once the Merlin Mustang shows its colours as a high-altitude, long-legged escort, who do you think is going to look at the low-altitude, short-legged Bell fighter? The USAAF had their Mustang, they didn't need another one, just like they didn't need the Curtiss XP-40Q.

I'd definitely look at a V-1650 installation, the problem still might be getting hold of one.


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## Colin1 (Jan 7, 2010)

gjs238 said:


> ...a modern front-engine car doesn't have the turbo located back in the trunk connected by ductwork ala P-47, the turbo is up front w/the engine (or down back if a rear or mid-engine setup.)


I'll answer more fully in a minute, I'm busy with something but a car's turbo isn't being driven by a 27 - 36 litre engine


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## gjs238 (Jan 7, 2010)

Colin1 said:


> With Merlin production going to the Lancaster and later the Mustang, how easily do you think the Bell lines are going to procure them?
> 
> Once the Merlin Mustang shows its colours as a high-altitude, long-legged escort, who do you think is going to look at the low-altitude, short-legged Bell fighter? The USAAF had their Mustang, they didn't need another one, just like they didn't need the Curtiss XP-40Q.
> 
> I'd definitely look at a V-1650 installation, the problem still might be getting hold of one.



- Merlin Mustang?
- XP-40Q?
- V-1650? 
All too late.
I'm talking about a response to USAAC Circular Proposal X-609 in February 1937.
The P-39 as we know it first flew (according to Wiki) 6 April 1938.

Turn the clock back to February 1937.
Could what became to be known as the P-39 have turned out "better, faster, stronger?"


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## Shortround6 (Jan 7, 2010)

gjs238 said:


> - Merlin Mustang?
> - XP-40Q?
> - V-1650?
> All too late.
> ...


.

Better? possibly.
Faster? at what altitude? under 15,000ft no. over 20,000ft yes.
Stronger? never knew the P-39 was weak

Enlarging the basic design to house an adequate inter-cooler (the basic failing of the original design) and more the fuel ( the 2nd failing) are both going to require a bigger, heavier airframe. The turbo is going to provide NO additional HP at the lower altitudes and only give you superior power from around 15,000ft and up. With the same power and a bigger airplane speed and climb will both be inferior until the planes reach a height at which the turbo installation offers a better power to weight/power to drag ratio than the non-turbo plane. Probably somewhere over 20,000ft. The time to altitude of the larger airplane might be suspect also. With a lower climb rate at the lower altitudes, what altitude do the planes have to climb to for the turbo plane catch up?


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## MikeGazdik (Jan 8, 2010)

I agree with Davebender, put the Packard Merlin in the plane and your are done, other than range.

If we take the time-line back to 37-38, well then I again look at both Allison and the USAAC needing to install an altitude capable supercharger on the V-12 they had in production. That not only changes the game for the P39, but the P-40 as well. ( and even possibly the P-38 )

And like mentioned, wet wings for the Bell would have been critical for range as the fuselage is used up with the cannon, nose gear, engine and , radiator.

I would rather do that instead of increase the size of the plane to accept fuselage fuel or to facilitate the turbo-charger. I think the P-39 really was an aerodynamically clean airframe for its time. I wonder what the speed at say 25k feet would have been with a Merlin? I would guess in the 415mph range !


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## Demetrious (Jan 8, 2010)

> And like mentioned, wet wings for the Bell would have been critical for range as the fuselage is used up with the cannon, nose gear, engine and , radiator.



Be careful with that. Range is very important, but wet wings (that don't use sealing tanks) make for reduced survivability, and the weight penalty can be significant.


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## riacrato (Jan 8, 2010)

Put an armor piercing cannon in it, call it an attacker and you're done.


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## davebender (Jan 8, 2010)

> how easily do you think the Bell lines are going to procure them?


No problem at all if the U.S.Army Air Corps makes money available. Hundreds of American military- industrial complexes were built from scratch during the late 1930s to early 1940s. Pour a hundred million dollars on the ground in 1940 and you have a state of the art RR Merlin aircraft engine plant working at design capacity by 1942.

Personally I'd use the new Merlin engines for P-38s, P-40s and P-51s rather then the P-39. But that is an an entirely different issue. Someone in the U.S. Army Air Corps procurement branch was convinced the P-39 was a great fighter aircraft. Switching to the Merlin engine will help some. At least nobody will be able to claim the P-39 was given a poor engine.


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## Milosh (Jan 8, 2010)

Why would switching to the Merlin be an improvement? There was no Merlin 60 series engines at the time.


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## delcyros (Jan 8, 2010)

[+] reduce wing area. =increase top speed and roll rate. Accept the higher wingloading.
[+] install either LE-slats or slots for the outer wing panels = improve low speed handling, less prone to spin violently from near stall conditions.
[+]replace the 37mm nose cannon by a .50cal BMG. Replace wing LMG´s by .50 cal BMG´s. = lowering the loaded weight and easening gun firing by a uniform BMG calibre weapon, typical for US practice.
[+]install thrust producing ejector nozzles = increase top speed further.
[+]install a reflector type gunsight instead of the telecope type. Replace it later with computing gunsights, if aviable.

All suggestions are possible in the timeframe and do not require major changes or technology still under development. The resulting plane will not only be significantly faster in top speed, it will also enjoi a lower weight and thus better acceleration, a uniform and more precise gun laying distribution and improved maneuverability on the longitudinal axis traded for a wider turn radius. More importantly, it will be easier to fly close to the stall speed regime.


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## krieghund (Jan 8, 2010)

The YP-39 during testing at Wright-Pat during 1939 reached 390 at 20kft (of course armament was not fitted) It was estimated that with armament and war kit it would have reached 375 at 20kft.

The V1710-17 with B-5 Turbo was rated at 1000 HP at 20kft. Later with turbo adjustments it was rated at 1000 at 25kft which by my estimations would put at around 380-385 at 25kft climbing to 20kft in 6 minutes with a range of about 600 miles. However the Army changed all that.

(A note about the Merlin: The V1650-1 in the P-40F was not real improvement over the V1710-85 models)

The armament would have been 1x37, 2x.50 and 2x.30 all in the nose as in the P-39C. I would have opted for all five guns to be .50 cal. to solve the aiming problems in a deflection shot. 

The real fix to the P-39 is the P-63 Kingcobra. the P-39 is too small and has no growth potential.


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## riacrato (Jan 8, 2010)

delcyros said:


> [+]replace the 37mm nose cannon by a .50cal BMG. Replace wing LMG´s by .50 cal BMG´s. = lowering the loaded weight and easening gun firing by a uniform BMG calibre weapon, typical for US practice.



I got the feeling that would completely off-balance the plane. Also the cannon was the whole purpose of this thing.


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## davebender (Jan 8, 2010)

> The real fix to the P-39 is the P-63 Kingcobra. the P-39 is too small and has no growth potential.


In other words, scrap the design and start over with something better. 

Perhaps Bell could purchase rights to license build the He-112B and DB601 engine. You get an overall better aircraft and can fire the 37mm cannon through the prop hub. 8)


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## krieghund (Jan 8, 2010)

You meant the He100D?


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## timshatz (Jan 8, 2010)

delcyros said:


> [+] reduce wing area. =increase top speed and roll rate. Accept the higher wingloading.
> [+] install either LE-slats or slots for the outer wing panels = improve low speed handling, less prone to spin violently from near stall conditions.
> [+]replace the 37mm nose cannon by a .50cal BMG. Replace wing LMG´s by .50 cal BMG´s. = lowering the loaded weight and easening gun firing by a uniform BMG calibre weapon, typical for US practice.
> [+]install thrust producing ejector nozzles = increase top speed further.
> ...



Like all of them Decl except the 50 cal in the wing and nose. Take the guns out of the wings and add a 20MM to the nose. It will be almost identical to the 109F/G in armament and they did pretty well with that (and very similar to the Yak3). Actually, now that I think about it, maybe the Soviet 23MM cannon would be a better call. I've read on this board that it was superior to the 20MM (at least I think it was this board). 

Otherwise, go with it. Especially like the leading edge slats and wing area ideas. Always thought the wings were a tad small on the P39 but if you can shave more off, all the better.


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## davebender (Jan 8, 2010)

Luftwaffe Resource Center - A Warbirds Resource Group Site - Heinkel He 112
No. I mean the He-112B. Prototype first flight was May 1937. So you've got plenty of time for Bell to license the design and get it into production.


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## Shortround6 (Jan 8, 2010)

krieghund said:


> The YP-39 during testing at Wright-Pat during 1939 reached 390 at 20kft (of course armament was not fitted) It was estimated that with armament and war kit it would have reached 375 at 20kft.



There is some debate about this claimed performance and a some evidence that no such flights with those performance figures were ever flown.


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## Shortround6 (Jan 8, 2010)

davebender said:


> In other words, scrap the design and start over with something better.
> 
> Perhaps Bell could purchase rights to license build the He-112B and DB601 engine. You get an overall better aircraft and can fire the 37mm cannon through the prop hub. 8)



The better aircraft part is debatable. 
the DB 601 wasn't really a production engine in 1937 was it? 
The German's didn't seem to get much of any gun to work firing through the prop hub until after 1940.
Max inside diameter of the tube through the 601 engine was in the order of 70mm wasn't it?
Might be tad small for the American 37mm gun:

File:M4 cannon drawing.jpg - Wikipedia, the free encyclopedia

even if the barrel fits there is quite a bit of gun to fit behind the engine without intruding into the cockpit or moving the cockpit rearwards.


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## MikeGazdik (Jan 9, 2010)

I think we are getting too radical. Improving the P-39 is the name of the game, not replacing it all together.

In my opinion, 3 thing items detracted from the P-39 : Altitude capable engine, Range, Armament.

If the 1st is addressed, the other two can be dealt with through evolution. The armament would be changed as many fighters were during the war. The range would be handled in a smaller scale by drop tanks and maybe some internal changes.

The plane itself will not be made into something that will negate the need of the P-51, but it may be a large step between the historical P-39 and the P-51.

The P-39 obviously had limitations. But I think because the P-40 was just enough better, and produced in greater numbers early in the war, the P-39 was easy and quick to be put aside by the USAAC. It wasn't a pile of junk, as the Russians disproved, its main detractor was high altitude performance.


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## Colin1 (Jan 9, 2010)

MikeGazdik said:


> The P-39 obviously had limitations. But I think because the P-40 was just enough better, and produced in greater numbers early in the war, the P-39 was easy and quick to be put aside by the USAAC. It wasn't a pile of junk, as the Russians disproved, its main detractor was high altitude performance.


In what way was the P-40 'just enough' better?
It couldn't fly any higher, never carried cannon and no version of the P-40 was as fast as the P-39.


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## MikeGazdik (Jan 9, 2010)

Colin1 said:


> In what way was the P-40 'just enough' better?
> It couldn't fly any higher, never carried cannon and no version of the P-40 was as fast as the P-39.



But the P-40 was a traditional layout with the engine in front, so I think it was more readily accepted by pilots and possibly maintenance. And it also had slightly more range, and the armament after the B/C models was standardized. 

Thats what I meant by "better", I should have clarified. Speed wise the Airacobra was faster, and in handling they are very close.


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## davebender (Jan 9, 2010)

The P-39 had some bad handling characteristics. Like entering a stall while conducting the low level strafing runs it was designed to do. The P-40 was somewhat underpowered (power to weight ratio) but it had no bad habits. At least that's my impression (which may not be correct).


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## krieghund (Jan 10, 2010)

The company was trying to better the design

Case in point, the XP-39E (Model 23) had the following improvements

A 2 stage 2 speed supercharger on the V1710-47 1325HP at T.O. and 1150 HP at 21,300ft

A larger wing with laminar airfoil, b=35' 10" S=236

Speed 386 Mph at 21,680ft
Ceiling 35200ft

Weight empty = 6936 lbs
Loaded= 8200
Gross= 8918

Fuel normal 100 US gal Max 150 US gal

Contracted=11 Apr 1941 First flown 26 Feb 1942

Also the XP-39 (model 11) was ordered on 7 Oct 1937
First flight 6 Apr 1939 Flight to 390 mph at 20000ft Wednesday 11 Apr 1939

Also you guys don't want the He-112 you want the He100D in this same time period.


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## krieghund (Jan 10, 2010)

The NACA did its best to help industry realize these dramatic increases of speed in production aircraft. This effort can be seen clearly in Langley's cleanup of the Bell XP-39 Airacobra, eleventh in the series of military planes subjected to the NACA operation. Bell's chief engineer Robert J. Woods (a former LMAL employee in Eastman Jacobs's VDT section) had designed the unconventional plane-its power plant amidships, at the center of gravity, and its cannon in the nose-as a 400-MPH fighter. At Wright Field in the spring of 1939, the unarmed XP-39 prototype (with a turbosupercharged Allison engine, rating 1150 horsepower) flew to a [199] maximum speed of 390 MPH at 20,000 feet. The aircraft reached this speed, however, with a gross weight of only 5550 pounds, thought to be about a ton less than a heavily armored production P-39. That meant that the existing aircraft, when normally loaded, would have a hard time exceeding 340 MPH. Still, the test performance impressed the Air Corps enough for it to issue a contract, three weeks later, for 13 production model YP-39s. Gen. Henry H. "Hap" Arnold, desperate for a new fighter, hoped that the speed of the airplane could be increased to over 400 MPH by cleaning up the drag. On 9 June 1939 he formally requested NACA approval for immediate testing of the XP-39 in the Full-Scale Tunnel.(21)
Actually Langley had received the XP-39 from Wright Field three days before Arnold's request, which had been put in writing on 6 June to satisfy NACA headquarters. On 8 June, Robert Woods and other representatives from Bell arrived at Langley to see the NACA's experimental setup and witness the initial round of tests. For the next two months the FST team systematically investigated the airplane's various sources of drag. On 10 August, Lawrence D. Bell, president of the Bell Aircraft Company, visited Langley to discuss the test results obtained to date. Bell was shown preliminary data from the FST indicating that the prototype in a completely faired condition had a drag value of only 0.0150 compared to 0.0316 in the original form. This meant a maximum increase in speed, if all the NACA's suggestions for drag improvement were met, of 26 percent. The NACA realized, of course, that not all of the changes to the configuration studied in the FST were feasible for the production aircraft. Fifteen days later, the head of the FST team reported that by cuffing the propeller at the point where it met the hub, streamlining the internal cooling ducts of the wings, lowering the cabin six inches, decreasing the size of the wheels so that they could be completely housed within the wing, and removing the turbosupercharger and certain air intakes, the speed of the XP-39 airplane for a given altitude and engine power could be increased significantly. Extrapolating from the same weight airframe to a more powerful (1350-horsepower) engine with a geared supercharger, he estimated that the top speed attainable with the aircraft might be as high as 429 MPH at 20,000 feet. The FST head did not know precisely how much additional air would be required to cool the bigger engine, but he did believe that even if this increase was very large, it would not prohibit the plane from obtaining at least 410 MPH.(22)
Bell incorporated enough changes recommended by the NACA to improve the speed of the airplane by about 16 percent. These changes included installation of an engine that could be equipped with a gear-driven supercharger but had only 1090 horsepower - 60 horsepower less than the engine which had driven the unarmed XP-39 to 390 MPH at Wright Field in the spring of 1939 (and 260 horsepower less than that used hypothetically by the FST head in his paper study).
Source: NASA History website ch7

20. See Paul L. Coe, Jr., "Review of Drag Clean-Up Tests in Langley Full-Scale Tunnel (From 1935 to 1945) Applicable to Current General Aviation Airplanes," NASA TN D-8206 (Washington, 1976) and Laurence K. Loftin, Jr., Subsonic Aircraft: Evolution and the Matching of Size to Performance, NASA RP-1060 (Washington, 1980), pp. 265-268.
21. Arnold to NACA, "Full-Scale Wind-Tunnel Tests of XP-39," 9 June 1939, RA file 674.
22. Smith J. DeFrance to Chief, Aerodyn. Div., "Estimated High Speed of the XP-39 Airplane," 25 Aug. 1939; Abe Silverstein and F. R. Nickle, "Tests of the XP-39 in the Full-Scale Tunnel," 27 Sept. 1939. Both in RA file 674.
23. "Comments of Representatives of Bureau of Aeronautics on Report of Drag Reduction on XP-39 Airplane," 2 Nov. 1939, ibid


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## Shortround6 (Jan 10, 2010)

The claimed 390mph of the XP-39 before the wind tunnel tests must be viewed with some suspicion.

Unless something has changed in the last few years, no real record of such a flight has been found. No date or name of pilot who achieved this speed. 

The original extension shaft had some torsional vibration issues and was redesigned with a larger diameter and thicker walls. The new shaft was not fitted to the plane until it came back from Langley. Some sources claim that because of the shaft problem NO full power flights were made during this time period. RPM being limited to 2600rpm I believe. There were also issues with both radiator and oil cooling.

The NACA also reported that the inter-cooler design allowed way too little airflow at climbing speeds and way too much airflow (adding to drag) at high speed. Perhaps their calculations were in error?

There were also reported problems with directional stability. 

How many hours of flight time did the XP-39 achieve before it was sent to Langley?


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## davebender (Jan 10, 2010)

> Also you guys don't want the He-112 you want the He100D in this same time period.


That depends on the cut off date for obtaining a license agreement. I think something could be worked out during 1937 when the He-112B was available. But I doubt the American Government would allow a license agreement after September 1939.

Even better idea.... 
The Fw-190 prototype was flying 1 June 1939, powered by the BMW139 radial engine. Rush the license agreement and get the blueprints out of Germany prior to the invasion of Poland. Now make an American version of the Fw-190 powered by the Wright R-2600 engine. You might beat the German version into mass production as the BMW801 engine wasn't ready for prime time until 1942.


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## krieghund (Jan 11, 2010)

Not trying to deviate too much....

The He100V1 flew 22 Jan 1938 and it pretty much has the shape of the 'D' production series
The He100D-0 production armed batch flew Sept 1939 and pretty much retained the demensions of the 1st prototype.

Just imagine it with a DB601E installed and 3 x MG151/15

The Fw190v1 flew 1 Jun 1939 and had shape differences of the 'A' series
The first of what could be reconized as the FW was the the Fw190V5 in Apr 1940 and the first Fw190A-0 production series on Mar 1941.

The He100 was more developed at this date and would have caused problems during the BoB as it had alot more endurance than the Bf190E and 50 mph faster


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## davebender (Jan 11, 2010)

We have no choice. Making the P-39 into a decent combat aircraft is easier said then done.


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## gjs238 (Jan 11, 2010)

Returning to USAAC Circular Proposal X-609 in February 1937 and the planes that were designed as a response...

Turbochargers were perceived as the solution to high altitude performance (P-38, P-39, and later Seversky/Republic designs.)
The P-38 Seversky/Republic designs made the turbo work, why not the P-39?

It seems the basic design was just too small - more room was needed for proper inter/after cooling and fuel capacity (and to accommodate the pilot.)
It also seems that an enlarged P-39 (ala P-63) could accommodate the turbo and necessary cooling without requiring extensive ductwork as with front engine/rear turbo designs.

?


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## Shortround6 (Jan 11, 2010)

gjs238 said:


> It also seems that an enlarged P-39 (ala P-63) could accommodate the turbo and necessary cooling without requiring extensive ductwork as with front engine/rear turbo designs.



Considering that the P-63 had no inter-cooler (or room for one) and tried to rely on water/alcohol injection instead I would say that any enlarged P-39 is going to have to be even larger than a P-63.

Please note that two different mock inter-cooler set ups were tried on a later P-39. Both reduced the speed of the P-39 by 30mph or better at lower altitudes because of increased drag. A third or fourth design may have done a bit better but you are going to wind up trading better high altitude performance for less low altitude performance.

Also noting that other modifications were made to the engine to allow it to make the 1325hp or more of the later models besides just putting on a different supercharger means that a 1939-40 version of the plane will have the extra weight and drag of the turbo installation and larger fuselage but no additional power untill the plane exceeds 15,000ft or so of altitude.


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## Shortround6 (Jan 11, 2010)

Still haven't seen anything to convince me that either Heinkel would have been any better than the American planes of the time (1940-41).

Would either one carry the fuel the Americans required for range?

Considering that the engine mounted cannon never worked in SERVICE use until until late 1940 or early 1941 and that the MG/FF cannon was feed from from 60 round drum means a rather limited armament. 

Are you proposing that the Americans adopt Germans guns JUST for this fighter?

The Americans could probably have built faster, better climbing fighters in 1938-1940 if they adopted both limited fuel and limited armament (and nobodies planes had armor or self sealing tanks at the time) and lighter structure.


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## davparlr (Jan 11, 2010)

The P-39 should have been much better. XP-39 appears to have been better until AAF redesigned it.

Install Merlin (preferred) or Allison w/turbo, hire North American engineers to redo aero, replace wings with larger, cleaner wings, more fuel in new wings and anywhere else, one 20 mm and four .50s

I don’t see a problem with putting in the P-38 Allison with turbo and intercooler. Looks like it would fit and high altitude performance would improve significantly.


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## Shortround6 (Jan 12, 2010)

davparlr said:


> The P-39 should have been much better. XP-39 appears to have been better until AAF redesigned it.



It appears that the XP-39 never flew at the often touted speeds and climb figures that are given for it. 

Given the NON-OPERATIONAL status of the turbo charger at the time the AAF may have had little choice in the redesign if they wanted planes in squadron service in 1941. 

Merlin's track record in 1939 was being installed in Hurricanes and Spitfires with fixed pitch props and going into Fairey Battles with constant speed props. It also offered 100-150 less horsepower for take-off.


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## Colin1 (Jan 12, 2010)

The P-39 would have benefitted from a better supercharger, this could have potentially come from the V-1650 so Bell and the USAAC surrendering the notion of a turbocharged fighter was not the death-knell in any sense for the Bell fighter.

The P-39 was a tight ship, it was the smallest possible wrapper around the biggest (for the period) armament and powerplant. It's safe to speculate that the V-1650, with better supercharging, would not have posed much of an intrusion into space that the V-1710 wasn't currently occupying.

The altitude issue was not beyond redemption, the biggest problem for the P-39 in such a tight design was that someone forgot the fuel.

A wet wing needs to be considered out of necessity. An arrangement with the guns bay running ventrally (for servicing access) to the nose and a fuel cell on top of this directly in front of the engine, with the pilot accomm directly forward of this might also be considered. This arrangement gives the pilot an unprecedented view of his battlespace and the fuel cell could well have little adverse effect on c of g in this location.

The downside is the proximity of pilot to the airscrew, if he has to belly in and the blades fold back in the characteristic fashion, are they going to chop in to the cockpit and cause him harm? An engine emergency cut-out might allay fears. The second point would be in the instance of a rough belly-in, if the powerplant is dislodged, it will invariably move forwards and rupture a fuel cell containing an unspecified amount of fuel with the inevitable consequences for the pilot.

Somewhere down the P-39 line I would take the canopy arrangement the same way as the P-47 and P-51; the omission of the doors would facilitate more conventional side-panel instrumentation in the same way that it did with the Typhoon and the now-solid side walls add to the structural rigidity. A sliding hood would give better all-round vision previously cramped by framing and the mast.


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## gjs238 (Jan 12, 2010)

While I think that a better supercharger was technically possible at the time of the P-39 debut, one was not developed and not available. The most feasible way to obtain good altitude performance was via turbcharging, as was done with the P-38 and the Seversky/Republic designs.

The turbo improves high altitude performance, but weight and drag penalties degrade low level performance.
The P-38 and Seversky/Republic designs are subject to the same benefits detriments, yet they managed to implement the turbo successfully.

Why the disparity?
Why can't the turbo work for (an enlarged) P-39, yet be successful for the others?

If the turbo is so bad, then why not remove the turbos to lighten up simplify the P-38 or P-47 on the argument that low to medium altitude performance will improve?


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## Shortround6 (Jan 12, 2010)

The P-39 was (to begin with) an 1100hp fighter. the P-38 was a 2200HP fighter and the Seversky/Republic designs, once you get passed the the P-43, were going to be 1400-2000HP fighters. the 1400HP version was superseded by the 2000HP version.
The P-38 and the P-47 had enough power to bull their way through at low altitude in spite of the turbo equipment. 

If you enlarge the P-39 to include an effective inter-cooler and two stage supercharger AND more fuel, you have increase weight, bulk and drag and yet have NOT increased power below 15,000ft. 

And, as in many things, timing is everything.

With 939 P-39s built by the end of 1941 compared to ONE P-47 and with the totals at the the end of 1942 running 2871 to 533 delaying production of the P-39 by even a few months would have put the US in an even worse situation than it found it self in. The Extra time take to get the the turbos "right" meant that the P-38 and the P-47 were not available until a bit later time frame.


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## davparlr (Jan 13, 2010)

Shortround6 said:


> It appears that the XP-39 never flew at the often touted speeds and climb figures that are given for it.



I have no reason to argue this point.



> Given the NON-OPERATIONAL status of the turbo charger at the time the AAF may have had little choice in the redesign if they wanted planes in squadron service in 1941.



I think the turbo supercharger was well developed by 1939, certainly it appears the B-17 was operating successfully. Also, development on the P-47 seems uneventful. P-38 problems with the turbo looks to be mainly associated with installation, not from engine-turbo integration.




> The P-39 was (to begin with) an 1100hp fighter. the P-38 was a 2200HP fighter and the Seversky/Republic designs, once you get passed the the P-43, were going to be 1400-2000HP fighters. the 1400HP version was superseded by the 2000HP version.
> The P-38 and the P-47 had enough power to bull their way through at low altitude in spite of the turbo equipment.



I don’t agree with this. The P-38 supercharger system was about 300 lbs. Adding this to the basic weight of the P-39 and it is close to half the weight of the P-38 (P-39D w/turbo, 6600 lbs, P-38D 12700lbs). Comparing power to weight of contemporary US fighters, P-38 .18, P-39 .17, P-40 .19, P-47 .19, F4F-3 .20, it is apparent that the P-39 w/turbo is not significantly different. Additionally, airspeed performance is only slightly affected by gross weight, e.g., the max airspeed at SL of a P-51D at 8000 lbs is 369 mph, at 12000 lbs, the airspeed is 364, only 5 mph difference with a 50% increase in weight.



> If you enlarge the P-39 to include an effective inter-cooler and two stage supercharger AND more fuel, you have increase weight, bulk and drag and yet have NOT increased power below 15,000ft.



I don’t believe the P-39 would need to be larger to incorporate an efficient intercooler. In comparing the P-39 engine installation with the P-38L installation, it appears that the P-39 has similar room for an efficient intercooler (I am assuming the P-38L intercooler is efficient). Additional fuel would need to be stored in the new, more efficient wing. With better wings, performance should improve, overall, in spite of the increased weight.



> The Extra time take to get the the turbos "right" meant that the P-38 and the P-47 were not available until a bit later time frame.



I am not sure that the turbos were the long pole for the delay in getting the P-38 ready for war. The gestation period for the turbocharged P-47 and the supercharged F4U, both with the R2800 engine, were very similar, approximately four years (assuming the YP-43 kind of a prototype of the P-47). I didn’t find any issues with the P-47 turbo integration.

It appears to me the P-39 could have, should have, had the same performance as the P-38, maybe slightly slower, but its aero was not as good and the turbo was removed. However it would never have had the range of the P-38 so it was more of a point defense fighter. It would have been very formidable in the Pacific and more formidable in Russia.


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## Hawk 75 (Jan 13, 2010)

Forget the P39, they should had purchased the licence to produce the Spitfire


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## Colin1 (Jan 13, 2010)

davparlr said:


> I think the turbo supercharger was well developed by 1939, certainly it appears the B-17 was operating successfully. Also, development on the P-47 seems uneventful. P-38 problems with the turbo looks to be mainly associated with installation, not from engine-turbo integration


I'm not familiar with the dates, but at what point (read date) were R-1820s turbocharged? As I understand it, the first turbocharged models were the R-1820-51s and I'm pretty sure that takes a turbocharged B-17 out of the mid-30s.



davparlr said:


> ...airspeed performance is only slightly affected by gross weight, e.g., the max airspeed at SL of a P-51D at 8000 lbs is 369 mph, at 12000 lbs, the airspeed is 364, only 5 mph difference with a 50% increase in weight


Your counterpoint has limits, the real effect of a 50% weight increase would be felt in combat speed, rather than max airspeed.



davparlr said:


> I don’t believe the P-39 would need to be larger to incorporate an efficient intercooler. In comparing the P-39 engine installation with the P-38L installation, it appears that the P-39 has similar room for an efficient intercooler (I am assuming the P-38L intercooler is efficient). Additional fuel would need to be stored in the new, more efficient wing. With better wings, performance should improve, overall, in spite of the increased weight.
> 
> I am not sure that the turbos were the long pole for the delay in getting the P-38 ready for war. The gestation period for the turbocharged P-47 and the supercharged F4U, both with the R2800 engine, were very similar, approximately four years (assuming the YP-43 kind of a prototype of the P-47). I didn’t find any issues with the P-47 turbo integration.
> 
> It appears to me the P-39 could have, should have, had the same performance as the P-38, maybe slightly slower, but its aero was not as good and the turbo was removed. However it would never have had the range of the P-38 so it was more of a point defense fighter. It would have been very formidable in the Pacific and more formidable in Russia.


It returns to the same point, American turbocharging simply wasn't considered reliable in mid- to late-1930s aero design. Also consider that USAAC officials weren't at this stage paying much attention to the Bf109 or A6M series, it would be mid 1942 before they got their hands on a Zero although prior to that, they could have exploited information on the type made available much earlier, from Claire Chennault. Remaining with the USAAC, it should be remembered that their leaders saw bomber aviation as the primary application of airpower in the 1930s, that there wouldn't be any fighter escort or even any expectation for high-altitude fighter combat. It was the USAAC that revised Type Specification 616 and removed the turbocharger from the spec in doing so. The aero industry's part in it (and it wasn't just Bell) was the utter lack of hesitation in booting the turbocharger out, they saw the clear financial sense in omitting what was potentially expensive and troublesome development.


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## Shortround6 (Jan 17, 2010)

davparlr said:


> I think the turbo supercharger was well developed by 1939, certainly it appears the B-17 was operating successfully. Also, development on the P-47 seems uneventful. P-38 problems with the turbo looks to be mainly associated with installation, not from engine-turbo integration.



The second B-17 to receive turbos (and the first production airframe to do so) wasn't flown until June 27 1039 and the last of 39 wasn't completed until March 30,1940. These were the B models. British had quite a bit of trouble with the C models they were given. The second P-38 wasn't completed until Sept 1940. With the first one crashed in Feb 1939 I don't think there was really a whole lot of experience to base things on. 





davparlr said:


> I don’t agree with this. The P-38 supercharger system was about 300 lbs. Adding this to the basic weight of the P-39 and it is close to half the weight of the P-38 (P-39D w/turbo, 6600 lbs, P-38D 12700lbs). Comparing power to weight of contemporary US fighters, P-38 .18, P-39 .17, P-40 .19, P-47 .19, F4F-3 .20, it is apparent that the P-39 w/turbo is not significantly different. Additionally, airspeed performance is only slightly affected by gross weight, e.g., the max airspeed at SL of a P-51D at 8000 lbs is 369 mph, at 12000 lbs, the airspeed is 364, only 5 mph difference with a 50% increase in weight.



Power to weight doesn't have as much influence on speed as power to drag. While the P-39 had the second best drag figures of common American fighters it was not half that of the P-38 and P-47. Power to weight has a much bigger influence on climb speed and effective ceiling. Adding weight and drag to the existing P-39 design would have made the already poor climbing performance even worse at altitudes under 15,000ft or so. 
Power to weight may also affect acceleration more than top speed. 




davparlr said:


> I don’t believe the P-39 would need to be larger to incorporate an efficient intercooler. In comparing the P-39 engine installation with the P-38L installation, it appears that the P-39 has similar room for an efficient intercooler (I am assuming the P-38L intercooler is efficient). Additional fuel would need to be stored in the new, more efficient wing. With better wings, performance should improve, overall, in spite of the increased weight.



OK, just where is this "room". Bell engineers tried two different external turbo/intercooler setups, now this may have been to try to put a turbo on the P-39 with a minimum of changes but in the changes from the XP-39 to the P-39 the area where turbo had been placed was used for larger radiator and oil cooler ducting because of inadequate cooling of both systems in the original design. Please note that the P-38L housed it's intercoolers in the "chin" ducts while the radiators were located behind the wing. 

Please also note the "improvements" in the XP-39E which turned into something of a lead sled.




davparlr said:


> I am not sure that the turbos were the long pole for the delay in getting the P-38 ready for war. The gestation period for the turbocharged P-47 and the supercharged F4U, both with the R2800 engine, were very similar, approximately four years (assuming the YP-43 kind of a prototype of the P-47). I didn’t find any issues with the P-47 turbo integration.



Please note that the P-47 didn't achieve combat status until early 1943, and that there were reported problems with the turbos on the P-43s in service. Also note that the F4U used a two stage inter-cooled supercharger. No turbo problem but flow matching to avoid surging and inter-cooler air flows did have to be worked out. 


davparlr said:


> It appears to me the P-39 could have, should have, had the same performance as the P-38, maybe slightly slower, but its aero was not as good and the turbo was removed. However it would never have had the range of the P-38 so it was more of a point defense fighter. It would have been very formidable in the Pacific and more formidable in Russia.


A big part of the P-39s problems came from an over ambitious armament. Compared to it's contemporaries the P-39 was attempting to carry way too much armament for the available power. Just the four wing .30s and their ammo weighed more than the entire armament of a Spitfire I or Hurricane I. The 37mm and it's ammo weighed almost as much the cowl guns and a single 20mm on the 109E. Add in even a single .50 cal and the armament weight goes way over the 109s.
Or compare the armament weight of the P-39 to the early P-40s. The P-40C carried the same .50 cowl guns (with too much ammo) and the same wing guns (with 120pounds less ammo) and NO 200lb PLUS 37mm cannon with the same engine.


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## Markus (Jan 17, 2010)

davebender said:


> We have no choice. Making the P-39 into a decent combat aircraft is easier said then done.



Depends on the definition of a decent combat A/C. The USAAF wanted a bomber interceptor. That requires

-a high rate of climb
-a engine with a high critical altitude
-a powerful armament

Long range is not required. IMO all that´s needed to fullfill the demands of the USAAF is an Allison with a 2-stage supercharger. Such a P-39 had its 1st flight in April 1942:



> Empty and loaded weights were 6936 lbs and 8918 lbs respectively, making the XP-39E the heaviest of all Airacobra variants. During tests, a maximum speed of 386 mph at 21,680 feet was attained, which was much better high-altitude performance than other Airacobra variants. An altitude of 20,000 feet could be reached in 9.3 minutes.



That beats a P-40 and F4F and would be fine for places like Guadalcanal or Port Moresby.


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## krieghund (Jan 18, 2010)

Ok let's take one step at a time anyone know this aircraft?


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## drgondog (Jan 18, 2010)

IMHO - only the mating of a Merlin would bring about the increase in performance for the P-39 series prior to P-63.

It would add 300+ pounds. Placed in front of cockpit, it requires a re-design of the lowere cowl, probably require dropping the wing and also extending the fuselage to account for a major cg shift. On the plus side the increase in length would provide for aft fuel cell but even that has disadvantage as the disposable fuel changes the cg again... pondering more, it would seem that the only way to NOT fool with basic lines is to stuff the Merlin aft of the cockpit and I wonder about the issues there also as the aft cg becomes even more of a problem..

As everyone pointed out even the Merlin was not a high altitude solution until ~ 1942. My personal opinion is that only the late model Allisons (i.e. -119) could have made a really strong contribuon to the P-39.

The primary influence of adding weight is twofold. 1.) for the same airfoil, there is an increase in AoA (for lift and DRAG) as the aircraft strives for level balanced flight which affects range more than speed but affects both, and 2.) similar but more pronounced effects to climb and turn performance.


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## krieghund (Jan 18, 2010)

I think the V1710-47 would have been satisfactory not the Merlin


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## Colin1 (Jan 19, 2010)

krieghund said:


> I think the V1710-47 would have been satisfactory not the Merlin


Satisfactory for what?
If it couldn't get the P-51A to altitude, why would it have changed the P-39's fortunes?


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## Clay_Allison (Jan 19, 2010)

Pick either the P-39 or the P-40 and give one of them a high-alt (larger impeller) supercharger, make both capable of using drop tanks. Let one fly top cover and escort strategic bombers, let the other do ground attack and escort medium bombers for low alt tactical bombing missions. 

We had the ability to design a single stage supercharger with any desired critical alt rating, somehow we entered the war with no mechanically supercharged high-altitude fighter. IMO, I'd rather it be the P-40 that gets the high altitude role, but the P-39 was lighter and thus perhaps better suited.

If a more improved M4 cannon was added, maybe necked down to 30mm with a higher MV, that would have made it a scary fighter to be bounced by.


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## Colin1 (Jan 19, 2010)

Clay_Allison said:


> Pick either the P-39 or the P-40 and give one of them a high-alt (larger impeller) supercharger, make both capable of using drop tanks. Let one fly top cover and escort strategic bombers, let the other do ground attack and escort medium bombers for low alt tactical bombing missions.
> 
> We had the ability to design a single stage supercharger with any desired critical alt rating, somehow we entered the war with no mechanically supercharged high-altitude fighter. IMO, I'd rather it be the P-40 that gets the high altitude role, but the P-39 was lighter and thus perhaps better suited


I can't see the P-39 escorting strategic bombers
even the term suggests distances that simply weren't within the scope of the P-39's abilities. The late-mark P-51 carried, for its time, an ungodly amount of fuel to get it to the far end of Germany - where would you put it on the P-39? And don't say drop tanks, it wouldn't be enough; the P-39 would still be running in third place behind the P-47 for escort duty range.


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## gjs238 (Jan 19, 2010)

Clay_Allison said:


> Pick either the P-39 or the P-40 and give one of them a high-alt (larger impeller) supercharger, make both capable of using drop tanks. Let one fly top cover and escort strategic bombers, let the other do ground attack and escort medium bombers for low alt tactical bombing missions.
> 
> We had the ability to design a single stage supercharger with any desired critical alt rating, somehow we entered the war with no mechanically supercharged high-altitude fighter. IMO, I'd rather it be the P-40 that gets the high altitude role, but the P-39 was lighter and thus perhaps better suited.
> 
> If a more improved M4 cannon was added, maybe necked down to 30mm with a higher MV, that would have made it a scary fighter to be bounced by.



The P-39 was supposed to be a high altitude bomber bouncer.
Hence turbocharger and cannon.

The P-40 was never designed with a turbocharger.


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## Clay_Allison (Jan 19, 2010)

gjs238 said:


> The P-39 was supposed to be a high altitude bomber bouncer.
> Hence turbocharger and cannon.
> 
> The P-40 was never designed with a turbocharger.


Ignore the bit about strategic bomber escort, I forgot how short legged the P-39 was.

Re-read my post. I never once said turbocharger. relying on turbos to the exclusion of supercharger development was our biggest mistake pre-war. A mechanically driven single stage supercharger designed for high altitude could give either plane a critical alt of 20k+ feet.


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## drgondog (Jan 19, 2010)

Colin1 said:


> I can't see the P-39 escorting strategic bombers
> even the term suggests distances that simply weren't within the scope of the P-39's abilities. The late-mark P-51 carried, for its time, an ungodly amount of fuel to get it to the far end of Germany - where would you put it on the P-39? And don't say drop tanks, it wouldn't be enough; the P-39 would still be running in third place behind the P-47 for escort duty range.



Dead on.


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## Shortround6 (Jan 19, 2010)

Clay_Allison said:


> We had the ability to design a single stage supercharger with any desired critical alt rating, somehow we entered the war with no mechanically supercharged high-altitude fighter. IMO, I'd rather it be the P-40 that gets the high altitude role, but the P-39 was lighter and thus perhaps better suited.



No, we did not have that ability. Nobody had that ability.

A single stage supercharger at the beginning of the war had a pressure ratio of about 2.3 to 1 with an efficiency of below 70%. Rolls-Royce, Allison, Daimler-Benz, they were all about the same. Pressure ratios did go up during the war for single stage superchargers but it took Hooker to figure that some of the design formulas had errors and also a lot of experimental work to reach the higher numbers, which by the end of the war were around 4 to one. The 2 stage supercharger on the first Merlin 60 series engines was good for around a 5.5 to 1 pressure ratio. Some of the early American turbo set ups (early P-38s and B-17s) were good for around a 4.5 pressure ratio, combined between the turbo and the engine stages. Later versions were good for around 6 to 1 pressure ratios. 

The V1710-47 was a two stage engine but because of the Allison drive system it was around a foot and half longer than a regular Allison and a few hundred pounds heavier. There was also no intercooler which limited WEP.



Clay_Allison said:


> A mechanically driven single stage supercharger designed for high altitude could give either plane a critical alt of 20k+ feet.



No, it couldn't. Early Allisons needed 45.5in of intake pressure at sea level to make their rated take off power(1150HP). That is using a pressure ratio of 1.5 to 1 at sea level. At 20,000ft the air pressure has fallen to from 29.92in to 13.75in. to get a pressure of 45.5 in the intake manifold the supercharger would need to provide a pressure ratio of 3.3 to 1. Just not happening with early war superchargers no matter what gear ratio you use to drive them.


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## Clay_Allison (Jan 20, 2010)

Shortround6 said:


> No, we did not have that ability. Nobody had that ability.
> 
> A single stage supercharger at the beginning of the war had a pressure ratio of about 2.3 to 1 with an efficiency of below 70%. Rolls-Royce, Allison, Daimler-Benz, they were all about the same. Pressure ratios did go up during the war for single stage superchargers but it took Hooker to figure that some of the design formulas had errors and also a lot of experimental work to reach the higher numbers, which by the end of the war were around 4 to one. The 2 stage supercharger on the first Merlin 60 series engines was good for around a 5.5 to 1 pressure ratio. Some of the early American turbo set ups (early P-38s and B-17s) were good for around a 4.5 pressure ratio, combined between the turbo and the engine stages. Later versions were good for around 6 to 1 pressure ratios.
> 
> ...


I was under the impression that if you wanted high alt performance you could get it by trading low alt performance IE the MiG-3?


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## Shortround6 (Jan 20, 2010)

Clay_Allison said:


> I was under the impression that if you wanted high alt performance you could get it by trading low alt performance IE the MiG-3?



The trade off comes with the power needed to run the supercharger at higher speeds.
Some engines, like the early DB engines and the AM-35 in the MiG-3 operated at a lower manifold pressure than the Allison and Merlin and so the same pressure ratio supercharger could maintain the needed pressure to a higher altitude. 
The Merlin III was a prime example. It was rated at 1030hp at 16,250ft I believe. But at low level it was only good for 880hp. By changing gear ratios more power could be had at low altitude but at the cost of lowering the critical altitude. The similar Merlin that used a two speed drive to the same compressor got about another 1,000ft of altitude but gained a lot of low level power because the low altitude gear set needed a lot less power to drive it.
They could have used the the higher gear ratio from the two speed engine in the single speed engine and picked up the extra altitude but only at the cost of even less power for take off.
The early Allisons had less altitude performance than the early Merlins but did have more power available for take-off and low altitude. IF the 9.60 supercharger gear set had been available earlier (and it took enough more power to drive that the gears had to be widened/strengthened to handle the load) the early Allisons might have been able to make their rated power at 15,500ft instead of 12,000ft. BUT only at the cost of less power for take off. Given the extra weight of the P-39 and P-40 over the Spitfire this may not have been a good idea.


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## Markus (Jan 20, 2010)

@drgondog: An mid-Merlin with a 2-speed supercharger had IIRC a critical altitude of 18 to 20,000 feet. Even with the reduced engine power above a plane with such an engine could have been useful in the PTO as contemporary Japanese planes had no better superchargers.

@Clay_Allison: Both the P-39 and the P-40 could carry drop tanks but since the tanks had to be dropped when entering combat a fighter on a combat mission could only the same amount of fuel externally as internally. In case of the P-39 with her internal fuel capacity of 120 gal. this means 240 gal max. Which is by coincidence the same as the amount of fuel a P-51 carries internally. And a 50/50 split seems optimistic. A P-40(app. 150gal internally) usually carried an external 75gal. tank which gave her a combat radius of 300 miles.


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## krieghund (Jan 20, 2010)

GUYS THIS ENGINE IS RATED AT 1150HP @ 21400 FT MILITARY AND ONLY USED IN THE XP-39E AND IN THE P-63 PROTOTYPES

IT PROPELLED THE XP39E TO 386MPH AT 21500 FT JUST THINK WHAT IT COULD HAVE DONE IN THE P-51A



MY PREVIOUS POSTED PICTURE IS A P-39C NOSE ATTACHED TO THE XP-39. ALSO IN PLACE OF THE WING .30 CALS AND AMMO ARE INTERNAL FUEL TANKS TO ROUND OUT THE INTERNAL FUEL LOAD AT 180 GALS INSTEAD OF 120 GALS.

AND WITH THE B-5 TURBO ADJUSTED PROPERLY IT SHOULD DO 380 MPH ABOVE 18000 FT. THE ONLY FIGHTER IN 1940 TO CATCH IT IS THE HE100D.


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## Shortround6 (Jan 20, 2010)

krieghund said:


> GUYS THIS ENGINE IS RATED AT 1150HP @ 21400 FT MILITARY AND ONLY USED IN THE XP-39E AND IN THE P-63 PROTOTYPES
> 
> IT PROPELLED THE XP39E TO 386MPH AT 21500 FT JUST THINK WHAT IT COULD HAVE DONE IN THE P-51A
> 
> ...



And with the extra length and weight it just wouldn't fit in a standard P-39 fuselage which is why the XP-39 was over a foot longer. 

Why do you think the P-39 could have hit anywhere near 380mph at 18000ft? the orginal XP-39 from what evidence there is never actually flew at the numbers claimed for it and the Guys at Langley with the full size wind tunnel estimated it was 40mph below the Bell calculated figures. Now if had really flown 390mph at 20,000ft as claimed and the guys at Langley said it would only do 350-360 don't you think there might be more dispute over the Langley results?

By the way, just what weight do you estimate for your super P-39?


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## Colin1 (Jan 20, 2010)

Whoops
my bad, I stated that the V-1710-47 powered the P-51A (post #50), which it didn't, the -81 did

Sorry!


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## Clay_Allison (Jan 21, 2010)

Shortround6 said:


> And with the extra length and weight it just wouldn't fit in a standard P-39 fuselage which is why the XP-39 was over a foot longer.
> 
> Why do you think the P-39 could have hit anywhere near 380mph at 18000ft? the orginal XP-39 from what evidence there is never actually flew at the numbers claimed for it and the Guys at Langley with the full size wind tunnel estimated it was 40mph below the Bell calculated figures. Now if had really flown 390mph at 20,000ft as claimed and the guys at Langley said it would only do 350-360 don't you think there might be more dispute over the Langley results?
> 
> By the way, just what weight do you estimate for your super P-39?


I guess it would be about halfway between a P-39 and a P-63.


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## krieghund (Jan 23, 2010)

The story of the test flight maximum speed of the XP-39 being at 390 mph has been reprinted so many times it has become somewhat of an urban legend.
The flight history (all in Dayton Ohio) is thus;
6 Apr 1939, retired Navy Lt. James Taylor takes it up for 20 minutes and says handling is pretty good. (legend says this test flight he did 390)
7 Apr 1939, he flew it again for Gen Hap Arnold but had to return early due to oil pressure and oil temperature problems.
22 Apr 1939, he flew it two times this day for a total of 47 minutes. He recorded all his findings and observations in his dairy. (This would be a crucial find for this subject)
Daniel D Whitney's "Vees for Victory - The Story of the Alison V1710 Aircraft Engine 1929-1948" pp79-988 has a bit to say on the development of the P-39 and the removal of the turbo.

> During 1937 when Alison had sold only 15 V1710 they pushed to develop altitude rated rather than turbo versions as they felt that their sales problems were partly due to Air Corps focusing on inadequately developed turbo charging. 
>XP-39 Flight testing was delayed several months while Alison fixed vibration problems, resulting from torsional vibration of the extension shafts, found during ground running tests. This had been only partially alleviated by first flight so certain RPM had to be avoided.
> He says "It is clear from recently located NACA test reports on the XP-39 that it was not meeting the contracted performance guarantees. While it has been reported that the aircraft was able to climb from takeoff to 20,000 ft in five minutes, and that the maximum speed at that altitude was 390mph, with the airplane weighing 5550 lbs the data does not show it.
...
Furthermore, given that General Arnold was hurriedly arranging to have NACA put the airplane in its wind tunnel for drag reduction tests only a month after the first flight, suggests that all was not well."
> The XP-39 was 10% overweight at 6104 lb on official weigh in versus a contracted 5550 lb.
> Flight testing had shown inadequate cooling of oil, Prestone and inter-cooler. NACA found for example the radiator required 10,250 scfm and that during climb at 160mph only 7,880 scfm was provided.
> NACA tests showed that the various air inlets were badly arranged with internal constrictions from the way the aircraft had been packaged (wing spars, landing gear obstructions etc) reducing air flow.
> A table of drag breakdown is given showing turbo charger related drag (Turbo + waste gate + intercooler) was about 16.4 % of the aircraft total. 
> Intercooler air flow was only 1,600 scfm at high speed while 5,000 to 7,000 was required if the engine was to achieve full power without detonation.
> External arrangement of air inlets was causing asymmetrical stalling with the left wing root stalling at several degrees angle of attack before the right.
> As received by NACA they estimate speed to be 340mph at 20,00ft with 1150bhp
> Following testing of modifications recommended by NACA drag was reduced to a level where it was estimated that 392mph at 20,000 if 1150 bhp was provided by the engine.
> NACA considered an altitude rated V1710 and with the other changes this was expected to give about 402mph at 13,200 feet with the elimination of the drag from the turbocharger and intercooler even before allowing for the reduction in weight from the removal of the turbo. NACA estimated that the performance at 20,000 feet would be "about the same" as the turbo version 
> NACA recommendations were
- Improve streamlining of wheel well doors
- Lower the canopy
- Remove turbosupercharging due to high installation drag.
- Relocate carburetor scope from left side of fuselage to just behind the canopy.
- Install the altitude rated engine.

To me this indicates the 390 mph at 20,000 ft is a probably a myth, being the NACA predicted value if the sum of all recommendations had been applied and that the V1710 provided 1150 bhp at that altitude.

I am presently working up the computer model of the XP-39A to show the performance of the aircraft if it had proceeded to the development stage as the P-39C.

*I will show that the combat weight will be similar to that of the production P-39D but with over 230 BHP available at 20000ft and 443 BHP at 25000ft at military power. *


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## Piper106 (Feb 20, 2010)

Shortround6 hit the nail on the head. Too much space and weight taken by armament. 

Four .50 machine guns would have be enough. (note 1 below) Three would be mounted to fire synchronized through the propeller disc (200 rpg), and one would be fire through the propeller hub (270 rounds). (Note 2 below) Compared to the 37 mm + two .50 + four .30 guns carried by the D through N models, this new four .50 gun arrangement would have weighed 175 pounds less. Compared to 37 mm + .50 + .30 ammo, there would have been at least 50 pounds less ammo on board at takeoff. 

With the .30 wing guns removed, the empty gun bay in each wing could be used to hold an additional 30 gallon fuel tank, allowing up to 186 gallons internal fuel for longer missions. (note 3 below). 

I would combine those changes with some serious arm twisting at Allison to get more power. NOW!!!. 
They needed to get their 9.6:1 high allitude blower gears (or the bigger late war G series 10.25" diameter supercharger OR a license built Merlin 45 blower grafted onto the V-1710 engine) plus clearance to operate at 3200 rpm for war emergencies at lot sooner then history tells us. 

Piper106 

Note 1: By early 1941, British reports from the Battle of Britian would have shown reasonable people that .30 caliber machine guns were ineffective, and that the hitting power of the 37 mm cannon was overkill for anyting the Axis powers were putting into the air. 

Note 2: The P-39C had two .50s and two .30 in the nose in addition to the 37 mm cannon. On the P-39D the two nose .30 guns were removed to make room for more 37 mm ammo. With the 37 mm cannon out of the way, three synchonized machine guns would have been practical. 

Footnote 3; Post-war, the two P-39s converted for air racing, Cobra I and Cobra II had a 50 gallon fuel tank in each gun bay. Agreed, these were tanks made from thin sheet metal, but even allowing for the thicker rubber used for self-sealing tanks, it would seem reasonable that 30 gallons could have fit into the space available.


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## Wavelength (Feb 21, 2010)

The first thing I would have done was get rid of the car doors.

How did Diamler Benz get okay altitude performance out their 600 series motors?

This is just a what if, but what would a P-39 with a DB motor have been?


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## Milosh (Feb 21, 2010)

Wavelength said:


> The first thing I would have done was get rid of the car doors.
> 
> How did Diamler Benz get okay altitude performance out their 600 series motors?
> 
> This is just a what if, but what would a P-39 with a DB motor have been?



By using a variable speed supercharger. If the DB's supercharger had been fitted to the Allison engine......


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## Shortround6 (Feb 21, 2010)

Milosh said:


> By using a variable speed supercharger. If the DB's supercharger had been fitted to the Allison engine......


it wouldn't have changed much. The DB supercharger wasn't providing as much boost as the Allison supercharger. The DB engine didn't need (and wouldn't stand) the same level of boost as the Allison.


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## Wavelength (Feb 21, 2010)

What about employing the DB design concept for driving their supercharger to drive the Allison's supercharger impellers?


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## Shortround6 (Feb 21, 2010)

Wavelength said:


> What about employing the DB design concept for driving their supercharger to drive the Allison's supercharger impellers?



Allison did use the " DB concept" to dive the 1st stage of their later 2 stage engines. There is nothing "magic" about using a variable speed drive. It will allow for more efficient use of the supercharger at LOWER altitudes than a single speed or two speed supercharger and it has no NOTCH effect as do 2 and 3 speed superchargers. It is however, a bit heavier and more complicated (expensive)than a 2 speed drive and it does heat up the engine oil when it is 'slipping' at lower altitudes requiring a slightly larger oil cooler. The 'DB concept" is actually fairly close to the torque converter in most automatic car transmissions.


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## Markus (Feb 21, 2010)

Piper106 said:


> Shortround6 hit the nail on the head. Too much space and weight taken by armament.
> 
> Four .50 machine guns would have be enough. (note 1 below) Three would be mounted to fire synchronized through the propeller disc (200 rpg), and one would be fire through the propeller hub (270 rounds). (Note 2 below) Compared to the 37 mm + two .50 + four .30 guns carried by the D through N models, this new four .50 gun arrangement would have weighed 175 pounds less. Compared to 37 mm + .50 + .30 ammo, there would have been at least 50 pounds less ammo on board at takeoff.



I did some math on that. 

The weight of the guns is 1*238lb, 2*70lb and 4*25lb = 478 lb
The weight of the ammo is 60*1.72lb, 400*0.3 and 4,000*0.065lb = 482lb
That makes a total of 960lb!

Your proposed armament would weight 280lb(4 guns)+261lb(870 rds of ammo), makes 541lb, saves 419lb.

Though 200 to 270 rounds per gun is not much. The four-gun Wildcats had 430 rounds per gun and the USN pilots were not happy when the six-gun version reduced that to just 240. I prefer three cal.50s in the nose and 400 to 450 rounds per gun. That gives the pilots at least twice the firing time and still saves at least 355lb.


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## Clay_Allison (Feb 22, 2010)

Alternately, the U.S. Military was aware of the Gast Gun and a compact two-barrel setup capable of 1600 RPM might have been been made to fire through the prop hub of the P-39. A tight center-line stream like that could make for very easy shooting.


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## V-1710 (Feb 24, 2010)

I think the real 'problem' with the P-39 was the U.S.A.A.C., not the aircraft itself. The original mission of the P-39 was that of a high altitude bomber interceptor, powered with a turbocharged Allison V-1710 (similar to the YP-37) and armed with the Oldsmobile built 37mm. cannon. The aircraft didn't need to have a long range, and of course less fuel kept the weight down for a fast climb. The 37mm. cannon would have been very effective against bombers. High wing loading would not have been an issue as the P-39 was orginally designd to combat large bombers, not other fighters. The turbocharged Allison, though complex and tempermental, had fine high altitude performance characteristics. During development, the U.S.A.A.C. basically informed Bell that the interceptor P-39 was not needed, and directed Bell to develop the P-39 as a ground attack aircraft. In that role, the turbocharger was not needed. Armor and additional machine guns were added, and coupled with the less powerful single-stage supercharged Allison, performance suffered. However, the Russians proved time and time again that the P-39 was quite effective in the ground attack role, so I think it is safe to say the transition was successful. The P-39 was simply not ever intended to be a pure fighter.


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## Colin1 (Feb 24, 2010)

V-1710 said:


> The P-39 was simply not ever intended to be a pure fighter


Your source?
The P-39 was faster than the A6M in some regimes, it was more heavily armed than the P-51. It was undeniably manoeuvrable, the Bf109 could not compete with the P-39 in a turn. Even with the Bf109 on the P-39's tail to begin with, the P-39 would reverse the advantage within two turns.

NACA had a big say in the deletion of the turbocharger, not that Bell were complaining about it; US turbochargers of the time were unreliable. And bulky, even with the negation of the ducting facilitated by the position of the powerplant, it was still a job and a half to shoehorn it into the dainty proportions of the P-39. The point is that notwithstanding the absence of the turbocharger, adequate supercharging would have made the P-39 a superb fighter, possibly constrained to the interceptor role.


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## V-1710 (Feb 25, 2010)

My sources were 'Vees for Victory' by Daniel D. Whitney (THE definitive work on the Allison 1710) and 'the American Fighter' by Angelucci/Bowers. 'Vees' has interesting commentary of NACA's role testing and improving the P-39's aerodynamics. It appears to be the opinion of the author that the decision to remove the turbocharger had been made by "the powers that be" before NACA had begun testing. In fact, it would seem that NACA was given the task of redesigning the intake system for the single-stage V-1710. That would be consistant if the U.S.A.A.C. had decided to change the P-39's role to ground attack from interceptor. Further, remember the U.S.A.A.C. held the belief in the mid-30's (and probably up until the time of the Battle of Britain) the air-to-air combat between fighters would not be much of a factor in any envisioned military action to defend the U.S. (read isolationism). This mindset lead to the emphasis of heavy long range bombers over every other type of combat aircraft, as such aircraft would be the most effective method of repelling enemy naval attacks on the continental U.S. (the only thing the U.S. military was concerned with at the time). The FM-1, YP-37, P-38, P-39, P-40, and P-43 were all born out of this 'original sin', as each aircraft was either designed as an interceptor or a ground attack/support fighter. Only the P-38 had the ability to function as a world class air superiority fighter at the time.


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## Shortround6 (Feb 25, 2010)

What I find amazing about this "ground support" theory is that none of those planes were fitted with bomb racks in their early forms.

It is not because this was a new idea, even in US service, because many US biplanes could carry small bombs and the P-26 could carry a respectable bomb lad for a plane of it's size and power. The P-35 was rated for something like a 300lb bomb load. 
Export P-36s could be fitted to carry a respectable bomb load so it is not like Curtiss didn't know about it or promote it.

I have read that the army didn't want what few fighters they had being drawn off to ground support missions or perhaps it was an attempt to get more money from congress for more planes? each type having a role so more total aircraft were needed rather than getting fewer multi-mission aircraft. 

The original P-40 was a particularly strange plane to be considered as ground support. A liquid cooled engine, which the army already didn't want for attack planes (see attack planes of the period or early thirties) and just a pair of .50 cal Mgs for armament with NO bomb racks? A very expensive and inefficient way to get a pair of .50s into strafing position. 

Considering that the Army had planes like this: Factsheets : Northrop A-17A

Both in service and on order for ground support the ground support theory for US fighters doesn't seem to hold water very well. Mid to low altitude fighter to control that airspace FOR the attack planes may make more sense.


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## V-1710 (Feb 25, 2010)

Good points. In addition to the A-17A, there was also the A-24. In any event, the U.S.A.A.C. seemed convinced most fighter operations would take place at low to medium altitudes.l


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## Markus (Feb 25, 2010)

V-1710 said:


> In any event, the U.S.A.A.C. seemed convinced most fighter operations would take place at low to medium altitudes.l



If so, why did they spend two decades to get a turbosupercharger working and designed planes like the FM-1 and P-38? I think the USAAC´s infatuation with heavy, high flying bombers convinced them they needed high flying fighters to shoot down the enemy´s heavy bombers.


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## V-1710 (Feb 25, 2010)

Markus said:


> If so, why did they spend two decades to get a turbosupercharger working and designed planes like the FM-1 and P-38? I think the USAAC´s infatuation with heavy, high flying bombers convinced them they needed high flying fighters to shoot down the enemy´s heavy bombers.



I agree. Back to the original point that the P-39 was designed as an interceptor, as was the P-38 and FM-1. I think the U.S.A.A.C. made a destinction between the roles of a 'fighter' (as in P-40 or P-43) and an 'interceptor' (P-38 and original P-39).


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## Piper106 (Feb 28, 2010)

Markus said:


> I did some math on that.
> 
> The weight of the guns is 1*238lb, 2*70lb and 4*25lb = 478 lb
> The weight of the ammo is 60*1.72lb, 400*0.3 and 4,000*0.065lb = 482lb
> ...



First none of the P-39s carried 60 rounds of 37 mm ammo, the standard references indicate that the capacity of the ring magazine in the D and later models being 30 rounds. Second, while the ammo boxes may have been able to hold 4000 rounds of .30 ammo, hopefully they were never filled.  A more realistic accounting would be that the ammo weight at take-off would have been reduced about 120 pounds. 

References indicate that the .50 Browning fired at about 600 rpm when syncronized, so 200 rpg would have given about 20 seconds worth of fire. 
Same time of fire for the centerline mounted 50 with 270 rounds. 
But I could buy into your concept also, three .50s with 400 to 450 rpg.

Piper106


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## Markus (Mar 1, 2010)

Piper106 said:


> First none of the P-39s carried 60 rounds of 37 mm ammo, the standard references indicate that the capacity of the ring magazine in the D and later models being 30 rounds. Second, while the ammo boxes may have been able to hold 4000 rounds of .30 ammo, hopefully they were never filled.
> References indicate that the .50 Browning fired at about 600 rpm when syncronized, so 200 rpg would have given about 20 seconds worth of fire.
> 
> Piper106



I stand corrected on the number of 37mm shells. 60 is the number of a P-400´s 20mm shells. And I meant the cal.30-06 machine gun had 1,000 rounds each. IMO the ideal number of guns and amount of ammo depends on what the plane is supposed to do. Your four-gun proposal would probably be the better choice for an interceptor.


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## krieghund (Mar 7, 2010)

Hey Guys, sorry for the delay getting back to you on the P-39A (aka XP-39) but I'm up to my ears in finishing a Statement of Work to add an additional 10 C-130 aircraft to our on going avionics upgrade to the C-130H and L100 aircraft.

As soon as possible I will put my info out...I have computer modeled the performance and have exact weights.



Der Dog


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## Ottawageezer (May 24, 2011)

As a newcomer, I find the P 39 re-design discussion civil, well-informed, friendly and fun. Here are some thoughts on the subject: possible changes in design that might have improved the P 39 performance as a fighter or attack aircraft. Though perhaps bending the original challenge, it seems interesting to consider changes that could have been made in later years (items 1 and 2 below) as well as in 1938.

1. Revisit the airframe structural design to have it conform to the adequate British (or German) stress limits rather than the more conservative American values. Relaxed stress limits contributed from the start to the fine performance of the F8F Bearcat and substantially improved the already excellent P 51 Mustang. The empty weight of the P51F, for example, was 5635 lbs. compared to 7125 lbs. for the P 51 D, a huge 20.9% improvement. Similar weight saving in the P 39 would improve, in probable order of value, rate-of-climb, range, maneuverability, top speed, landing speed and take-off run and would allow some latitude for weight-increasing modifications. Admittedly there would seem almost no chance of the Air Corps agreeing to this until well into the war.
The extremely rugged (sometimes over-rugged) construction of U.S. fighters seems to have been the common cause of rates-of-climb lower than they should have been. 

1.1. As part of the airframe re-think, add armor, but make it part of the load-bearing structure of the airplane as was done with the IL 2 Shturmovik and the Fairchild A 10. The compact grouping of the pilot, cooling system and engine make the P 39 well suited to protection from below by one undersurface sheet of armor. A particularly thick sheet would be a nice feature of a ground attack version of the plane 

2. I agree with several of you that, especially for the fighter role, the engine should be changed to a Merlin. There seems little chance, though, that this could be considered before September 1940 when it was agreed that Packard would produce the Merlin in the United States.

3. Increase the size of the leading-edge air intakes, ducts, and, if need be, radiators, to provide satisfactory cooling. The intakes have always LOOKED too small, noticeably smaller in fact than the leading-edge intakes on the radial air-cooled F4U Corsair. By all means keep the intakes on the leading-edge, however, because they are an asset there. As with intakes right at the nose, such as on the P 47 and F6F, they avoid airflow shear or turbulence from upstream surfaces, but do so without increasing the drag-producing cross-sectional area. In may opinion, leading-edge intakes, sometimes used only for the oil-cooler or carburetor, likely contributed to the good performance of aircraft such as the B 17, Mosquito, F4U Corsair, F8F Bearcat, Hawker Tempest and Fury, Yak 3 and 9, and the promising Westland Whirlwind. 

3.1. Optimize ducting to fully exploit the 'Meridith effect' (1934), the jet thrust of heated air from the radiator, as was done with the Spitfire and Mustang.. 

3.2. Remove the carburetor air-intake from behind the canopy and incorporate it into a leading-edge intake. This would slightly reduce drag and accommodate the change to the Merlin which, unlike the Allison, has an updraft carburetor.

4. Remove the nose wheel. The Airacobra has always seemed too small to afford the luxury of a tricycle landing gear and performance in the air should take precedence over landing and ground handling. (The Bf 109 clearly followed this precedence a bit too far.) Removing the nose wheel frees valuable space in the nose (expensively bought by the unorthodox engine location), saves weight, and means less to go wrong or to maintain. As demonstrated by the Airacobra's tail-dragging naval cousin, the Airabonita, the main wheels can be attached to the front spar and stowed on retraction between the front and main spar. With the Airabonita, however, the radiators and ducts were moved backwards, out of the way, to the undersurface of the wing. In my opinion this should not be done. Instead, the wheels should be attached further out on the wing even if this requires reinforcement of the front spar. Alternatively, they might be retracted directly backwards as on the F4U. With the latter option, a notch would be needed in the main spar to accommodate the landing gear strut and the spar would need reinforcement at the notch.

5. Change the armament. The elimination of the nose wheel allows the general sentiment of Piper106, Shortround6, Markus, kreighund and delcyros to be taken further. With no nose wheel, a pure fighter version of the Airacobra would benefit from removing the 37mm cannon and going to 5 or 6 closely grouped guns in the nose, all 50 caliber with a common trajectory. A bomber interceptor might similarly be armed with, say, five identical 20mm cannon. Also, with no 37mm gun and no nose wheel to contend with, what the British claimed was extremely poor armament accessibility could be improved. A ground attack Airacobra, however, should retain the 37mm cannon and combine it with, say, four 20 mm cannon in the nose, even if this means fairing over unavoidable bulges.

6. On a fighter version of the plane, change the seating to give the pilot a much more reclining posture as in the Bf 109. This is effective and important in reducing drainage of blood from the brain in a tight turn or zoom. In the Airacobra, it also makes more space available for fuel,under the pilot. With no the 37 mm cannon, there would be room forward for the pilots feet. On a ground attack P 39, the original seating should be kept.

7. I agree with delcyros that automatic wing slats should be added to improve maneuverability and lower the stalling and landing speeds (as on the Bf 109) and perhaps mitigate, or even eliminate, the tendency of the Airacobra to tumble and go into a flat spin. The disadvantage, as experienced on the Bf 109, is possible disruption in aim when the slats suddenly deploy.

8. I agree that getting rid of the automobile doors is a very sensible idea. This would save weight and would simplify and improve both fuselage and canopy structure. A further minor point, if the new canopy slides, an antenna mast could still be avoided, as on the FW 190, with a pulley/spring arrangement inside the canopy.


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## Lighthunmust (May 24, 2011)

I wonder if the Red Air Force would agree that it needed to be re-designed. They appeared to believe it a great plane only needing a few tweaks.


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## gjs238 (May 25, 2011)

Lighthunmust said:


> I wonder if the Red Air Force would agree that it needed to be re-designed. They appeared to believe it a great plane only needing a few tweaks.


That doesn't mean they wouldn't have appreciated an even better P-39.


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## michaelmaltby (May 25, 2011)

.... called the P-63 KING COBRA .

MM


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## gjs238 (May 25, 2011)

A day late and a dollar short


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## tomo pauk (May 25, 2011)

Armament changed to either 5 x HMGs, or cannon + 2 HMGs, cockpit moved in front, new fuel tank between cockpit engine.
A pair of under-wing drop tanks would've been nice.


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## Lighthunmust (May 25, 2011)

gjs238 said:


> That doesn't mean they wouldn't have appreciated an even better P-39.



It also doesn't mean that they would. The tweaks I mentioned were about the equipment the Soviets removed not adding or changing equipment. From what I have read the Soviets did not have too many complaints about the P-39 for air to air. They understood the limitations and strengths of the design and used it accordingly. As I recall Chuck Yeager thought the Airacobra was a pretty good plane within its limitations. 



michaelmaltby said:


> .... called the P-63 KING COBRA .
> 
> MM


 


gjs238 said:


> A day late and a dollar short



The P-63 was not so much the product of a redesign as an entirely new aircraft that just looked similar to a P-39. Sorta like the difference between a P-51D and P-51H. I don't think you really could redesign the P-39 to be much better because of the inherent limitations of its size and layout.

Even with the benefit of experience they still didn't get a P-63 with the capability of a P-51 or P-47 in altitude or range. However if the fight was below 10K both of those would be in trouble against a P-63.

What is remarkable about the P-39 is how beautiful it is, what a great racer it was after the war, and how effective it was when flown by American and Soviet pilots who knew how to use it.

My primary source is "Cobra!" by Birch Matthews


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## gjs238 (May 25, 2011)

tomo pauk said:


> Armament changed to either 5 x HMGs, or cannon + 2 HMGs, cockpit moved in front, new fuel tank between cockpit engine.
> A pair of under-wing drop tanks would've been nice.


 Oooohh, I like this. Can we start a pool and build our own?


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## tomo pauk (May 25, 2011)

Maybe this one, as posted in thread about pusher designs:


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## gjs238 (May 25, 2011)

Perhaps having the cockpit moved forward will help with the CG issues?


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## tomo pauk (May 25, 2011)

Maybe having 3-4 hull HMGs 'stead of 2?

A low-to-mid altitude companion to P-38, the DoubleCobra. Two x 37mm, 4 x .50cals (or 6 x .50cals only), 2 x 75 plus a 150-gal drop tanks, additional fuel cells in outer wings (2 x 90 gals?), plus fuel cells in inner wing (60 gals?). Perhaps 390-400 mph by 1943?


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## Shortround6 (May 25, 2011)

Lighthunmust said:


> The P-63 was not so much the product of a redesign as an entirely new aircraft that just looked similar to a P-39. Sorta like the difference between a P-51D and P-51H. I don't think you really could redesign the P-39 to be much better because of the inherent limitations of its size and layout.
> 
> Even with the benefit of experience they still didn't get a P-63 with the capability of a P-51 or P-47 in altitude or range. However if the fight was below 10K both of those would be in trouble against a P-63.
> 
> What is remarkable about the P-39 is how beautiful it is, what a great racer it was after the war, and how effective it was when flown by American and Soviet pilots who knew how to use it.



I think that kind of sums it up.

some other thoughts on this subject.

I really can't understand this mania for trying to turn a pig's ear into a silk purse.

The P-39 had a lot of limitations due to it's initial design and no amount of minor "tweaks" were going to do more than help a bit. 

many of the "tweaks" proposed in this thread (and others) are not really "tweaks" but designing a whole new airplane that just sort-of looks like a P-39. 

P-39 was built the way it was because at the time it was ordered the only other game in town was the P-40. It was first ordered in the Summer of 1939 and first deliveries were in the Dec of 1940 (not combat capable) in 1941 it not only was the second most produced US fighter but it beat the production of the next 4 or 5 types put together. In 1942 it was still the second most produced fighter although other were catching up. Spend a number of months "ing" with the P-39 design and you wind up with hundreds fewer fighters in 1942. 

The P-39 was limited in it's small size (for an american fighter), there is only so much room to put stuff. P-40s had a bigger wing and a narrower track. While the landing gear folded back the landing gear legs were housed in a protruding fairing under the wing. the wing was longer front to back than it would be further out with a wider track and the wing is thicker top to bottom which gives more room to house the wheels without resorting to lumps and bumps on the wing.

It is all very well to talk about "just use the Meredith effect" but getting it to actually do what you want may be another story. While the 'idea' was fairly well known, having been put in a paper and read to a society in 1935 turning the formula or equation into practice took a lot more work. Look at a P-51 and see how much volume (cubic feet) they used to get the radiator/oil cooler duct to expand and contract in the right way and to fit the proper sized radiator to get the results that they got. Were do you find that kind of volume in the P-39? 

The P-39 was handicapped by an American obsession with heavy gun armament. Not only was the plane designed around the gun rather than around the engine but the inclusion of the wing .30s and a totally foolish amount of ammo fished things off. (one of the "tweaks" the Russians used, dump the wing guns and ammo). 

A P-39 carried about 886-890lb worth of guns and ammo, not including ammo boxes , gun mounts, gun heaters, chargers and other bits and pieces. 
A Spitfire MK V with belt fed Hispanos carried about 620lbs worth of guns and ammo. Try bolting 270lb worth of lead ballast into the Spitfire and then limiting it's throttle so it gives about 200-300hp less than it really did and then compare it's performance to the P-39.
I have no idea why the USAAC thought you could jam that weight of armament into an 1150hp fighter and get something that was going to perform.


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## gjs238 (May 25, 2011)

Shortround6 said:


> I think that kind of sums it up.
> 
> some other thoughts on this subject.
> 
> ...



That's a misundestanding that happens alot with these what-if threads.
Usually when I start one of these what-if threads, I'm imagining a redesign of the aircraft from day one with the magic of hindsight, not tweaks of an existing aircraft.


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## Shortround6 (May 25, 2011)

gjs238 said:


> That's a misundestanding that happens alot with these what-if threads.
> Usually when I start one of these what-if threads, I'm imagining a redesign of the aircraft from day one with the magic of hindsight, not tweaks of an existing aircraft.


 
I can understand some "what-ifs" and also "this would have been a neat plane if" but when the what-if wants to rewrite history "if they had just done "so and so" they could have/would have *WON* the battle of Upper Wenslydale or dominated the sky over Pago-Pago without ever considering what negative effects the modifications would have on either performance or production it gets a little strange. 

As I have said before one of my personal "what if's" is a MK II Whirlwind (without Merlins) if not a MK III so I am not immune to the idea 

I have no illusion that it would change the course of the war, at least not by much.


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## tomo pauk (May 26, 2011)

For the sake of the discussion, what should be the features of the Mk II / III Whirly?


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## Lighthunmust (May 27, 2011)

Ditch the wing guns and 37mm cannon. Use a 20mm or .50 in the nose. Tell your pilots to do what Hartmann said "Stick your nose in their cockpit".


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## Shortround6 (May 27, 2011)

tomo pauk said:


> For the sake of the discussion, what should be the features of the Mk II / III Whirly?



Belt fed 20mm guns for starters, at least two if not 3 installations were mocked up and 1 or two flown? That is no stretch. 
Set up fuel system to cross fed so if one engine goes out you can fed the remaining engine with both fuel tanks.
Fit full feathering propellers.
Fit fuselage fuel tanks. again already schemed/drawn up. 
Fit an under fuselage hard-point to go along with a hard point under each wing. Plumb hard-points for drop tanks. 
Engines is were it gets a little bit tricker 

Fit a two speed gear box to the supercharger so you have sort of a Peregrine X engine like the Merlin X. Won't do much for altitude but should increase take-off and low level power by around 100hp per engine. Increase boost to 12lbs. 
MK III gets a Merlin XX style supercharger or Merlin 46 style supercharger with 2 speed drive. Sized for the Peregrine engine. Might be able to get just under 1000hp per engine for take-off instead of historic 775hp which would help with under wing loads. low level climb should get a good boost. Might get 860hp at 18,500-19,000ft instead of 885 at 15,000ft. A useful increase if not amazing. Cuts into Typhoon territory quit nicely.


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## Messy1 (May 28, 2011)

Sure, 60 years of hindsight is a marvelous tool.


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## Readie (May 28, 2011)

Bell P39Q-15BE 44-2911 Airacobra

Have you seen this?
Cheers
John


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## P-40K-5 (May 28, 2011)

gjs238 said:


> With hindsight (ain't hindsight a great thing?) could you have made the P-39 a truly great plane?
> Perhaps scaling-up the basic design for greater fuel capacity, better turbocharger inter/aftercooler arrangement, armament, etc?
> The mid-engine layout appears to be an efficient way to incorporate a turbocharger into a single-engine fighter/interceptor (all the ducting of the P-47 is not necessary.)
> The mid-engine layout also lends itself to impressive nose armament.
> ...


 
a two-stage supercharger would have been a blessing for the Allison engine ( high alltitude escorts). 2 50cals in the nose a 20mm cannon firing through the prop hub
would have been sufficient to bring down almost anything. plus it would keep the plane light(er), or to improve the armor for the engine cooling system.


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## drgondog (May 28, 2011)

Shortround summed it up - "what coulda happened with the P-39 - did happen! It became the P-63" end of story.

Small airframe limited the P-39/P-63 from ever being effective long range fighter, engine limited in power at altitude interesting to western allies but fine for USSR... crappy 37mm gun. The Sovs made it into a good low altitude performer by putting in a good gun and stripping the useless .30 cals out.


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## tomo pauk (May 29, 2011)

I'll just point into some things that either a costumer (USAAC) or a producer (Bell) did not think that were worth pursuing.
1st, P-39 was never issued with wing drop tank installation. That's hardly a fault of basic design, so either USAAC didn't ordered it, and/or Bell never proposed it. 
2nd, Indeed, 4 x .30cals were never looked as an asset in air combat, up to the point of discarding them by Soviets. The 350lbs worth of those guns ammo, and the volume occupied by those, might have been better used as fuel load (2 fuel tanks + 2 x 25 gals maybe; 1 gal = 6lbs) for 170 gals total. I've attached the wing cross section to make myself more clear.

As for Soviet use of the P-39: no new gun was installed (other than US .50cal under wing of late model P-39s; wonder if they deleted that one too?). The 37mm was never regarded as crappy by Soviets. If the basic design was a bad one, it would've never been a good performer, at any altitude.


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## Shortround6 (May 29, 2011)

tomo pauk said:


> I'll just point into some things that either a costumer (USAAC) or a producer (Bell) did not think that were worth pursuing.
> 1st, P-39 was never issued with wing drop tank installation. That's hardly a fault of basic design, so either USAAC didn't ordered it, and/or Bell never proposed it.



You have to wonder why that was, did they just not think of it? or did they think of it, do some quick calculations of the back of an envelop or napkin and decide it wasn't worth taking any further?



tomo pauk said:


> 2nd, Indeed, 4 x .30cals were never looked as an asset in air combat, up to the point of discarding them by Soviets. The 350lbs worth of those guns ammo, and the volume occupied by those, might have been better used as fuel load (2 fuel tanks + 2 x 25 gals maybe; 1 gal = 6lbs) for 170 gals total. I've attached the wing cross section to make myself more clear.



Long, flat protected tanks are heavy. Fuel system for a P-38J weighed 506lbs for a capacity of 410 gallons. P-39 was around 270lb (or more ) for 120 gallons. Both P-47 and P-51 use less weight per gallon. The P-39 was a ground loving airplane. The only single seat US fighter that needed more runway was the P-47. The P-39Q (the best P-39 at taking off) needed 1350ft of pavement at 7600lbs at sea level on a ) degree day. that is with 87 gallons of fuel on board and the four .50s. Adding either a 500lb bomb or a 75 gallon drop rank requires another 300ft of pavement. Making it over the 50 ft barrier adds 450 of distance over the plane when clean. 
1150-1325hp just isn't enough to lift large external loads without very long runways. Long runways came along later in the war but by then the P-39 was on the way out. 


tomo pauk said:


> As for Soviet use of the P-39: no new gun was installed (other than US .50cal under wing of late model P-39s; wonder if they deleted that one too?). The 37mm was never regarded as crappy by Soviets. If the basic design was a bad one, it would've never been a good performer, at any altitude.



From what I have read they did delete the wing guns, in fact some of the last delivers may have been made with guns never installed. The 37mm took some sorting out. There was a problem with the ejection chute that caused many jams, you can't get new rounds into the gun if the old ones can't get out. This was sorted out and the gun didn't seem to have too many other problems. 

The design wasn't necessarily "bad" but it did have limits. I had a couple of old Triumph sports cars, they were pretty good at somethings and terrible at others, like taking 5 friends to a party or trying to move a refrigerator.

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## tomo pauk (May 29, 2011)

Shortround6 said:


> [about wing drop tanks]You have to wonder why that was, did they just not think of it? or did they think of it, do some quick calculations of the back of an envelop or napkin and decide it wasn't worth taking any further?



Still wondering; care to share some info?



> Long, flat protected tanks are heavy. Fuel system for a P-38J weighed 506lbs for a capacity of 410 gallons. P-39 was around 270lb (or more ) for 120 gallons. Both P-47 and P-51 use less weight per gallon.



Fuel system + fuel of real P-39 weighted 990 lbs. We have weight bonus of 350lbs (LMGs + ammo deleted), giving us around 35% more fuel - 42gals? Not the +50 gals I've claimed, but 160 gals internal never the less. 



> The P-39 was a ground loving airplane. The only single seat US fighter that needed more runway was the P-47. The P-39Q (the best P-39 at taking off) needed 1350ft of pavement at 7600lbs at sea level on a ) degree day. that is with 87 gallons of fuel on board and the four .50s. Adding either a 500lb bomb or a 75 gallon drop rank requires another 300ft of pavement. Making it over the 50 ft barrier adds 450 of distance over the plane when clean.
> 1150-1325hp just isn't enough to lift large external loads without very long runways. Long runways came along later in the war but by then the P-39 was on the way out.


I'm not suggesting that we fill the P-39 with 400 gals of fuel, along with 1 or 2 thousands lbs of bombs. Carrying 160 gals of internal fuel doesn't add up a single pound when compared with P-39s with 4 x LMGs.
Further about runaways: the P-39 with more fuel can put a decent runaway into a good use (and a plane itself). The one with scarce fuel cannot.



> From what I have read they did delete the wing guns, in fact some of the last delivers may have been made with guns never installed. The 37mm took some sorting out. There was a problem with the ejection chute that caused many jams, you can't get new rounds into the gun if the old ones can't get out. This was sorted out and the gun didn't seem to have too many other problems.



Agreed. 



> The design wasn't necessarily "bad" but it did have limits.



Ditto.



> I had a couple of old Triumph sports cars, they were pretty good at somethings and terrible at others, like taking 5 friends to a party or trying to move a refrigerator.



Those are among the reason why I just love my VW Transporter


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## Lighthunmust (Jun 5, 2011)

tomo pauk said:


> I'll just point into some things that either a costumer (USAAC) or a producer (Bell) did not think that were worth pursuing.
> 1st, P-39 was never issued with wing drop tank installation. That's hardly a fault of basic design, so either USAAC didn't ordered it, and/or Bell never proposed it.
> 
> 
> ...


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## Shortround6 (Jun 5, 2011)

[E=Lighthunmust;791008]QUOT1st, P-39 was never issued with wing drop tank installation. That's hardly a fault of basic design, so either USAAC didn't ordered it, and/or Bell never proposed it.[/QUOTE]

They did use a 75 gallon belly tank. And an even bigger ferry tank 175 gallons. total fuel with the ferry tank 295 gallons. They probably had a very good idea of the take-off performance (or lack thereof) and range.

While ditching the wing guns saves weight it is not enough to mount large under wing stores. Even 52 gallon drop tanks are going to go over 360lbs installed. What does it get? 104 gallons under wing and a belly tank of 75 for 179 gallons or one belly ferry tank of 175 gallons? 

Even with two 500lb under wing stations you get two 75 gallon tanks. 270 gallons total compared to 195 gallons with the belly tank. Is an extra 75 gallons going to turn it into an escort fighter? An extra 75 gallons is worth about 321miles at max cruising speed (750hp) but that is in clean condition. Wings will have to be stressed for the under wing loads. 

The plane is not going to carry all three stations loaded. Not without one looooong runway. 

[E=Lighthunmust;791008]
In the USAAF the doctrine of a strong defensive armament on bombers would insure "The Bomber will always get through" was sacrosanct. To even hint that long range fighter escorts for bombers were needed was sacrilege. How do you justify to Congress spending depression era dollars on expensive bombers that are less than invulnerable? The suggestion of the need for drop tanks would not be tolerated. Even "Hap" Arnold would not tolerate the idea, although if I recall it was also due to a belief it would degrade fighter performance. This short explanation is far from comprehensive, but I hope gives some idea of why no one was considering drop tank installation capability. 
[/QUOTE]

What is strange about this is that a number of US biplane fighters used either drop tanks or auxiliary external tanks or external tanks that could be dropped in an emergency? Both Curtiss Hawk biplanes and Boeing Biplanes in both Army and Navy service. The idea of drop tanks was neither new or untested.


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## Lighthunmust (Jun 5, 2011)

I don't think Bell is responsible for much in the way of design faults for the P-39 regarding range or weapons package. They responded to conditions established by USAAF. I am aware that there was some use of auxiliary tanks prior to the war. This certainly eliminates any possibility that USAAF leaders did not know they could be a viable option. They simply did not want them. They also made little effort to create anything more than glorified ferry tanks unsuitable for high altitude if memory serves me well. The most recent source of my information are Warren Bodie's books.


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## tomo pauk (Oct 3, 2011)

Turbo 39. Radiators relocated at leading edge, with intercooler at another. Compressor is under engine. Exhaust manifolds meet under hull, a feet or two prior entering the turbo. Wing guns ammo deleted, fuel tanks instead (optionally; 2 x 20 gals). Fuel tank in front part of wing central section (30-40 gal). Under-wing drop tanks (2 x 75 gals). 
At 20-25K perhaps 380mph (1941-42; 1150 HP), 400 (1942-43; 1325 HP), 410 (1943, 1425 HP), 420 (1600 HP @ WER); P-63 was managing 400-410 mph with 1150-1200HP @ 25-20K (exhaust thrust comes to aid).

(red box - turbocharger; blue box - intercooler; orange box - fuel tank in central wing section)


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## MikeGazdik (Oct 3, 2011)

I'll take mine in sand and spinach, over light grey please!!! Matter of fact, think its time to change my avatar again to my favorite underdog!!


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## Shortround6 (Oct 3, 2011)

tomo pauk said:


> Turbo 39.
> 
> (red box - turbocharger; blue box - intercooler; orange box - fuel tank in central wing section)



For an idea of what you are trying to deal with, here are some pictures, diagrams of a B-17 turbo installation, 1200hp so the airflow requirements shouldn't be too different. 

Warbird Information Exchange • View topic - b-17 wing intercooler removal


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## wuzak (Oct 3, 2011)

tomo pauk said:


> Turbo 39. Radiators relocated at leading edge, with intercooler at another. Compressor is under engine. Exhaust manifolds meet under hull, a feet or two prior entering the turbo. Wing guns ammo deleted, fuel tanks instead (optionally; 2 x 20 gals). Fuel tank in front part of wing central section (30-40 gal). Under-wing drop tanks (2 x 75 gals).
> At 20-25K perhaps 380mph (1941-42; 1150 HP), 400 (1942-43; 1325 HP), 410 (1943, 1425 HP), 420 (1600 HP @ WER); P-63 was managing 400-410 mph with 1150-1200HP @ 25-20K (exhaust thrust comes to aid).
> 
> (red box - turbocharger; blue box - intercooler; orange box - fuel tank in central wing section)



Apart from the intercooler (and maybe the fuel tank, you have just described the XP-39.

http://aerofiles.com/bell-p39b.jpg

Or at least you have put the turbo in the same place. The radiators are fed from leading edge inlets, but the radiators themselves are in the fuselage. 

I'd suggest placing the turbo behind the engine with the turbine axis pointing down and to the rear with an exit for the turbo and wastegate below the rear fuselage. There wouldn't be any space for an air to air intercooler, so I would look at using a liquid to air intercooler, like on the two stage Merlins. The radiator required for that may be small enough to fit with the engine coolant and oil radiator group. If not, we could place that radiator under the chin, as in 2 stage powered Mosquitos.


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## tomo pauk (Oct 4, 2011)

Shortround6 said:


> For an idea of what you are trying to deal with, here are some pictures, diagrams of a B-17 turbo installation, 1200hp so the airflow requirements shouldn't be too different.


 
Thanks for the link 
Maybe I was not crystal clear: the exhaust-to-turbo ducting in mostly external, as is the intercooler-to-carb. The turbo-to-intercooler duct is now in place of wing part of oil cooler duct.



wuzak said:


> Apart from the intercooler (and maybe the fuel tank, you have just described the XP-39.
> 
> http://aerofiles.com/bell-p39b.jpg
> 
> Or at least you have put the turbo in the same place. The radiators are fed from leading edge inlets, but the radiators themselves are in the fuselage.



Yep, I know about the XP-39.
IIRC, the exhaust ducting there was too short, so any increase in power would've damaged the turbo? The intercooler was either under-sized, or at bad place, or both? The radiators in fuselage (actually, in central wing section) and their ducts are eating too much of precious space, hence they're in front of the wings. Of course, we can put them under front part of wing, for looks a-la later Spitfires.



> I'd suggest placing the turbo behind the engine with the turbine axis pointing down and to the rear with an exit for the turbo and wastegate below the rear fuselage. There wouldn't be any space for an air to air intercooler, so I would look at using a liquid to air intercooler, like on the two stage Merlins. The radiator required for that may be small enough to fit with the engine coolant and oil radiator group. If not, we could place that radiator under the chin, as in 2 stage powered Mosquitos.



With turbo behind the engine, there is 300+ lbs some 5 feet behind CoG - not the best thing for P-39. Too bad Bell wasn't producing P-63s as turbo, 'stead of mech two stage. 
The chin radiator would've interfere with front wheel system?

OTOH, we could contemplate a razorback P-39 - glycol oil tanks moved above engine, making weight allowance space for turbo intercooler.


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## Shortround6 (Oct 4, 2011)

tomo pauk said:


> Thanks for the link
> Maybe I was not crystal clear: the exhaust-to-turbo ducting in mostly external, as is the intercooler-to-carb. The turbo-to-intercooler duct is now in place of wing part of oil cooler duct.



Sticking bits and pieces on the outside increases the drag. Same problem as as Bell's attempt's at an external turbo add on. You may get your 380-400mph at 20-25,000ft in 1941-43 but if you cut the sea level speed from about 310mph to 270-280mph and the speed at 10,000ft from 350mph to 310-320mph you wind up with a very specialized airplane. It will also climb worse than a standard P-39 up to about 15,000ft or so. Pulling the wing guns may not be enough (especially if you stick in extra fuel). It is only when the 1425 hp engine becomes available that this starts to look attractive and by then the need for it is fading. You do need bigger radiators and oil coolers for the higher powered engines and even the intercooler needs to change from the early version to the late version. The diagrams and pictures were to give you an idea of the size of the pipes/ducts you are dealing with. You can shorten them and change them form square to round or vis versa but you can't make them smaller in cross section (especially forhe higher HP engines) 




tomo pauk said:


> Yep, I know about the XP-39.
> IIRC, the exhaust ducting there was too short, so any increase in power would've damaged the turbo? The intercooler was either under-sized, or at bad place, or both?



The intercooler was both. They also had sever problems with both the radiator and oil cooler.


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## wuzak (Oct 4, 2011)

tomo pauk said:


> With turbo behind the engine, there is 300+ lbs some 5 feet behind CoG - not the best thing for P-39. Too bad Bell wasn't producing P-63s as turbo, 'stead of mech two stage.
> The chin radiator would've interfere with front wheel system?
> 
> OTOH, we could contemplate a razorback P-39 - glycol oil tanks moved above engine, making weight allowance space for turbo intercooler.



I'd suggest that the wings could move, or other equipment, to get the weight balance back.


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## Shortround6 (Oct 4, 2011)

Sort of what they did on the P-63. look at were the cockpit is in relation to the wing leading edge on both planes. 

Since the P-63 was what the Bell engineers thought they needed for a two stage supercharged Allison engine fighter ( mechanical drive instead of exhaust drive for the auxiliary stage) I am not sure why some people think they can stuff the same amount of equipment (or more) into the P-39 airframe. While they didn't cut metal on on anything except the two mockups of the add-on turbos we have no idea how many (or how few) sketches they did of other arrangements. If these are the arrangements that made it to the shop I can only conclude that they were considered the best options at the time. Granted they may have been constrained by the need for minimum changes to the P-39 in order to keep up production after the P-39C/D was adopted. Moving wings, and wholesale re-arrangements of internal components and ducts requires new tooling and assembly procedures (new in the sense of what parts get added first for best access and things like that).

If the rework is supposed to take place at the time of the XP-39 before the P-39 goes into even limited production you are faced with the same problem the airforce was faced with historically. The turbo simply wasn't ready for squadron service in any airplane at the time in question. Delaying the start of production by about a year (what the USAAF estimated as the time needed to bring the turbo up to usable standard) meant an unacceptable loss of production.

To illustrate this they had completed 208 P-38s by the end of 1941, 128 of them in Nov and Dec. at that time they had completed 939 P-39s. Delaying the P-39 program by, about year could have resulted in 600-620 fewer P-39s available in Jan of 1942, this keeps going, it means 600 fewer P-39s available in June of 1942. 

While about 3 times the number of P-40s had been built the P-39 in Jan of 1942 outnumbered all the P-38s, P-51s, F4Fs built with enough to spare to cover most of either the P-43 production or Buffalo Production, take your pick. 

Bell started work on the Model 33 in Feb 1941, US Army orders two prototype XP-63s on June 27 1941. I would think they had a pretty good idea how much room they needed for what they wanted in the aircraft.


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## davparlr (Oct 4, 2011)

What would I do?

1.	Get someone else to do the aero. As those who have read my post on the P-59 know, I am not a fan of Bell aerodynamics. A look at the P-39 and one cannot be anything but impressed by the clean design with imbedded radiator with leading edge air inlets and its overall smoothness. With its small wing it should have been faster. Its CD0 was higher than the P-47! This should not have been true. I think the problem was in the details. Cleaning up the details would have improved airspeed, especially at low level.
2.	Better supercharger. Perhaps installing the F4F-3 supercharger which seems to have had a higher operating range (1000 hp at 19k). This would have improved high altitude performance.
3.	I like the two 50cals and one (good) 20mm and no wing guns. 
4.	More fuel. Added wing tanks may have helped here. Overall gross weight for combat must not increase.
5.	Improve roll rate to P-40 levels.

This would have made the P-39 much more formidable.


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## Shortround6 (Oct 4, 2011)

davparlr said:


> What would I do?
> 
> 2.	Better supercharger. Perhaps installing the F4F-3 supercharger which seems to have had a higher operating range (1000 hp at 19k). This would have improved high altitude performance.
> 3.	I like the two 50cals and one (good) 20mm and no wing guns.
> ...



2. F4F-3 supercharger was a two stage with inter-cooler. back to the weight and space problems of the turbo/P-63
3. This one is a good idea. 
4. Might work but increase will be small and is only going to work if wing guns are left out. 
5. getting weight out of wings may help but his may mean no extra tanks way out in the wings.


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## krieghund (Oct 4, 2011)

According to Dean's "America's Hundred Thousand" the Cdo's fall out this way but one must also take into account the equivalent flat plate area;
CDo......................EFPA sq ft.........DRAG AT 250 MPH
P-51D = .0176......................4.1............................. 845
P-39N = .0217..................... 4.63........................... 864 
P-47B = .0213..................... 6.39..........................1465
F6f-3 = .0272......................9.08.......................... 1673

The following chart shows the real speed of the prototype at 20000ft (340 mph) and the speed it could have attained if the NACA recommendations were applied (390 mph) The Cdo originally was .03 and after the NACA mod would have been .021


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## tomo pauk (Oct 4, 2011)

Shortround6 said:


> Sticking bits and pieces on the outside increases the drag. Same problem as as Bell's attempt's at an external turbo add on.



Not quite as Bell's add on turbo; my idea puts far less parts at the plane's exterior. 



> You may get your 380-400mph at 20-25,000ft in 1941-43 but if you cut the sea level speed from about 310mph to 270-280mph and the speed at 10,000ft from 350mph to 310-320mph you wind up with a very specialized airplane.



Now why the P-39 with 1325 HP will do only 320 mph @ 10K? P-63 was capable of 350 @ 10K with 1300 HP, despite having 10 % more of equivalent flat plate area (P-39 was making 360 there, with 1100 HP). The 'notorius' Bf-109G-2 was loosing 12-13 mph with wing cannons installed; no increase in power there. 
So even if we take external plumbing into account, saying that it would've cut 50 mph, while providing 200 HP more, is highly questionable. 



> It will also climb worse than a standard P-39 up to about 15,000ft or so. Pulling the wing guns may not be enough (especially if you stick in extra fuel).



Seeing that P-39 climbed better than most (all?) SE fighters made in USA (apart from P-63), I'd say that turbo P-39 wouldn't be at any disadvantage. Regular 39 out-climbed the Spit Vs, IIRC.
We could also mention that, at higher altitude, the advantage would've been even greater (not vs. paddle blade P-47s?). Sure enough, if a commander wants his planes to intercept something, those planes wouldn't be carrying the drop tanks, but have only internal 160 gals.



> It is only when the 1425 hp engine becomes available that this starts to look attractive and by then the need for it is fading.



If the low-production models are made (1150 HP), followed with mass produced 1325 HP model (early 1942-mid '43), then such a plane plays far more important role than regular P-39 or P-40. It's also more easily available than P-38. RAF/RAAF are also satisfied costumers. A next step (1425 HP in mid 1943; meaning we have 1600 HP @ WER) is as much expected as P-38H was, but here we talk about all theaters, not just PTO. 
Of course, by 1944 a slight redesign of front hull is needed (HMGs staggered, so their ammo boxes can be enlarged to hold 300 rds, 37mm replaced by belt fed Hispano - sorry, P-38). 4-bladed prop. Instead of 2 x 75 gals, it's 2 x 108 in drop tanks.



> You do need bigger radiators and oil coolers for the higher powered engines and even the intercooler needs to change from the early version to the late version. The diagrams and pictures were to give you an idea of the size of the pipes/ducts you are dealing with. You can shorten them and change them form square to round or vis versa but you can't make them smaller in cross section (especially forhe higher HP engines)



Same things were true for all planes experiencing increase in engine power, so I see no issues here.


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## post76 (Oct 5, 2011)

thrust vectoring.....


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## krieghund (Oct 5, 2011)

This P-39 looks to be doing 353mph at 10k ft.


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## davparlr (Oct 5, 2011)

Shortround6 said:


> 2. F4F-3 supercharger was a two stage with inter-cooler. back to the weight and space problems of the turbo/P-63


The P-39 definitely needed the performance. There seems to be enough space to work with for the F4F-3 type supercharger. Weight is unknown but should be lighter than the more powerful P-63 engine, so I would guess 150 lbs. This is not insignificant, but I think workable, especially with the removal of the guns. Cg change would need to be evaluated and, if necessary, worked. This could be an issue on such a light aircraft.



> 5. getting weight out of wings may help but his may mean no extra tanks way out in the wings.


I think improving the efffectivity of the ailerons should solve this problem.

Overall, the P-39 was a small aircraft which would limit its ability to grow and adapt to new missions. It didn't seem to fit in the, in my opinion, correct AAF philosophy of building aircraft to fight over enemy territory not American.


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## Shortround6 (Oct 5, 2011)

Here is a picture of one of the two intercoolers on the F4F-3. 

http://data3.primeportal.net/hangar/sal_provenzano/f4f-3_wildcat/images/f4f-3_wildcat_16_of_17.jpg

here is a picture of the air intakes for the intercoolers. 

http://data3.primeportal.net/hangar/sal_provenzano/f4f-3_wildcat/images/f4f-3_wildcat_01_of_17.jpg

Here is a picture of the duct leading from the intake to the inter cooler. 

http://data3.primeportal.net/hangar/sal_provenzano/f4f-3_wildcat/images/f4f-3_wildcat_10_of_17.jpg

The more power over 1000hp you want at altitude the bigger the inter-coolers have to be. 

Grumman apparently exhausted the inter-coolers into the wheel wells, P-39 is going to need intakes, ducts and outlets. The second stage apparently increased the weight of the engine by 55-65lbs not including the inter-coolers but this supercharger may not have been big enough to supply more than 1000hp or so worth of air at 19,000ft. It all has to go behind the engine.


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## pinsog (Oct 5, 2011)

Everything I had always read about the P39 said it was a terrible airplane that essentially couldn't get out of it's own way. 

Now, thanks to this site, I have learned:
1. It was really fast down low
2. Had very good manueverability
3. Had an initial climb rate, I just read, of 4000 fpm

Why was it's combat performance so bad? Did it's enemies simply orbit above it where it's performance trailed off and zoom and boom it?


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## gjs238 (Oct 5, 2011)

Impressions made early in the U.S. part of the war, before good U.S. tactics were developed, may have been lasting.
Same goes for the P-40.

Later war planes benefitted in the reverse.
Not only were they more capable, but better tactics had been developed and adopted by then.
RE: F6F, F4U, P-51, etc.


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## tomo pauk (Oct 5, 2011)

pinsog said:


> Everything I had always read about the P39 said it was a terrible airplane that essentially couldn't get out of it's own way.
> 
> Now, thanks to this site, I have learned:
> 1. It was really fast down low
> ...



It's combat performance was not so bad; the main customer (Soviets) asked for more P-39s (and Spitfires), instead of P-40s (and Hurricanes). The main shortcomings (low performance above 15K, short range) were not issues at Eastern front air battles. They were for USAAC RAF, however: USAAC was contested (mainly over Guadalcanal, but also elsewhere) by high-flying IJA/IJN bombers their escorts, while air war at ETO demanded both good hi-alt performance good combat range. The combat at Pacific also required good/great combat range, and P-39 was unable to provide it.
Soviets were also stripping down its Cobras (LMGs ammo deleted, some armor radio gear too), in order to extract better performance out of their planes.


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## gjs238 (Oct 5, 2011)

Gotta wonder how the AVG would have managed with P-39s.
Or for that matter with anything else besides P-40s.


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## pinsog (Oct 5, 2011)

Does anyone have horsepower figures of P39 vs Spitfire vs Me109 at various altitudes?


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## Shortround6 (Oct 5, 2011)

gjs238 said:


> Gotta wonder how the AVG would have managed with P-39s.
> Or for that matter with anything else besides P-40s.



Bitter defeat for the AVG using P-39s. Good as they were for dragging the USAAC into the 1940s and giving pilots mechanics something to fly and work on on the second half of 1941 the P-39 was not combat ready at that time. Ships with the P-40s for the AVG sailed in July or Aug of 1941 (?). They needed more maintenance and trouble shooting than the P-40 and would have lead to fewer numbers in the air at any given time. 37mm jammed with monotonous regularity at that time so after the first 2-3 shots from the cannon they were no better armed than the P-40.


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## post76 (Oct 5, 2011)

pinsog said:


> Everything I had always read about the P39 said it was a terrible airplane that essentially couldn't get out of it's own way.
> 
> Now, thanks to this site, I have learned:
> 1. It was really fast down low
> ...


 
A lot of it also depends on the historical context and when the aircraft was written about.
I find that perceptions and impressions of aircraft change from generation to generation and country to country.
I think seeing later designs like the F4U and F-51 still in use after 1950 may have hogged the stage for less popular designs like the P-39, but if you research Soviet history they give much more detail on their use. 
Impressions have nothing left to be weighted against until more information becomes available with time.


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## post76 (Oct 5, 2011)

Shortround6 said:


> Bitter defeat for the AVG using P-39s. Good as they were for dragging the USAAC into the 1940s and giving pilots mechanics something to fly and work on on the second half of 1941 the P-39 was not combat ready at that time. Ships with the P-40s for the AVG sailed in July or Aug of 1941 (?). They needed more maintenance and trouble shooting than the P-40 and would have lead to fewer numbers in the air at any given time. 37mm jammed with monotonous regularity at that time so after the first 2-3 shots from the cannon they were no better armed than the P-40.



The main issue there would be the tricycle gear. It just wasn't strong enough when used under harsher conditions.
A lot of the P-40s all up weight was said to have stemmed from the undercarriage allowing it to take off or land in less than adequate conditions, something that made it more beneficial to use than a P-39. 
I've heard the AVG stuck with the P-40 for other reasons, noting that maintenance was always an issue for them anyway seeing as they had limited replacement parts for the planes they did have.


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## Shortround6 (Oct 5, 2011)

post76 said:


> A lot of it also depends on the historical context and when the aircraft was written about.
> I find that perceptions and impressions of aircraft change from generation to generation and country to country.
> I think seeing later designs like the F4U and F-51 still in use after 1950 may have hogged the stage for less popular designs like the P-39, but if you research Soviet history they give much more detail on their use.
> Impressions have nothing left to be weighted against until more information becomes available with time.



Another thing to remember is that most Merlin P-51s performed about the same. They were also only on the scene for a short period of time compared to some other aircraft. The Merlin P-51s fought for just under 1 1/2 years in Europe. 

The P-39s had more variations and fought over a much longer span of time. A early P-39 flown by the book and limited to 1150hp is going to perform at low altitude much different than a one in late 1942/early 43 that is allowed to use 1490hp of WER. 

Flown against the experienced Japanese pilots by green pilots in the south pacific and using the pre-WER power levels is a different "historical context" than flown at low altitude only against less maneuverable planes by experienced pilots on the Russian front.


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## pinsog (Oct 5, 2011)

Didn't the Russians, in addition to removing wing guns, armor and other extra weight, also run the Allison engine well beyond it's standard horsepower rating? 

Does anyone know how much horsepower the Russians were able to get out of their P39's?


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## Shortround6 (Oct 5, 2011)

I don't know but which power rating are you talking about?

The early P-39s were rated at 1150hp for take-off and held that to 12,000ft, they used 42in of manifold pressure, given good gasoline they were later rated for 1490hp at up to 4,300ft at 56in manifold pressure. They really didn't like over revving much and tended to fail in just a few hours after even a minute or two of over revving (under power not over speeding in a dive). using too much boost is a real gamble depending on the fuel and perhaps the weather. you can get away with a bit more at -30^ than you can at +110^ a little detonation is like knocking in a car. bad detonation can hole pistons, bend or break connecting rods, stretch or break cylinder hold down bolts or studs in just seconds. You tend to hear about the pilots who used it and got home, You don't hear much about the pilots who tried it and crash landed/baled out.

The P-39 K and L used Allisons that were good for 1325hp for take off and 11150 at 11,800ft but were rated for 1580hp WER (60 in pressure) at 2,500ft. This would taper off to the 1150hp at 11,800ft. 

The bulk of the P-39s (over 7,000 of them) used an engine good for 1200hp at take off and good for 1125hp at 15,500ft. (44.5 in boost) it was rated at 1410hp at 9,500ft (57in pressure). it used 9.60 supercharger gears which gave the extra performance. They also heated the intake air more and the engine was running closer to the detonation limit which is why it ran the lower pressure at WER. 

If you know the pressure the Russians were using you can make a guess at the power.


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## pinsog (Oct 6, 2011)

Shortround6 said:


> I don't know but which power rating are you talking about?
> 
> The early P-39s were rated at 1150hp for take-off and held that to 12,000ft, they used 42in of manifold pressure, given good gasoline they were later rated for 1490hp at up to 4,300ft at 56in manifold pressure. They really didn't like over revving much and tended to fail in just a few hours after even a minute or two of over revving (under power not over speeding in a dive). using too much boost is a real gamble depending on the fuel and perhaps the weather. you can get away with a bit more at -30^ than you can at +110^ a little detonation is like knocking in a car. bad detonation can hole pistons, bend or break connecting rods, stretch or break cylinder hold down bolts or studs in just seconds. You tend to hear about the pilots who used it and got home, You don't hear much about the pilots who tried it and crash landed/baled out.
> 
> ...



I wish I knew the power rating. I'm not even sure where I read that information. I think I got it off a link off of this sight but I couldn't find it to save my life. As I recall, where ever I read this info, they said that the engines had a VERY short life and had to be changed often. I am 99% sure that I got the info from a link off of this sight and it was an interview with a Russian ace. Maybe someone here knows what I'm talking about.


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## Ratsel (Oct 6, 2011)

The RAF was doing the same engine mods in their P-40E's in N.Africa, up to 70" IIRC. Somewhere around 1600-1700 HP by bypassing the boost controller. Wasn't a common practice and the engines did not last long.


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## Jabberwocky (Oct 6, 2011)

Ratsel said:


> The RAF was doing the same engine mods in their P-40E's in N.Africa, up to 70" IIRC. Somewhere around 1600-1700 HP by bypassing the boost controller. Wasn't a common practice and the engines did not last long.


 
Perils P-40 page has a December 1942 General Motors Allison document that refers to RAAF pilots overboosting V-1710-39 and -73 engines with the 8.8:1 supercharger ratios (basically a low altitude blower) to 66” and 70” Hg at low altitudes for “prolonged periods”. 

At 66”, the overboosting produced 1745 hp at SL and 1770 hp at 2000 ft. The engines were also subject to over-revving.

At 70”, the overboosting at 3200 rpm produced 1780 hp at 3000 ft. 

Allison was worried that the overboosting with the 8.8 blower ratio would produce a ‘bad precedent’ once engines with a 9.6:1 supercharger ratio entered service. However, the engines with the low alt ratio were “apparently” standing up well to the overboosting. Allison recommended an emergency rating of 60”, producing 1570 hp for the engine, for the 8.8:1 engines and 57” for the 9.6:1 engines.


EDIT: Did some more checking and it looks like just a week after the Allison/GM memo, the -39 was re-rate to a maximum of 56" (1470 hp at SL) and the -73 was re-rated to a maximum of 60" (1550 hp at SL). Looks like the engines with the 9.6:1 supercharger ration were limited to a maximum of 57" (1360 hp at SL, 1480 hp at 10,000 ft) at least until April 1944.


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## post76 (Oct 6, 2011)

pinsog said:


> I wish I knew the power rating. I'm not even sure where I read that information. I think I got it off a link off of this sight but I couldn't find it to save my life. As I recall, where ever I read this info, they said that the engines had a VERY short life and had to be changed often. I am 99% sure that I got the info from a link off of this sight and it was an interview with a Russian ace. Maybe someone here knows what I'm talking about.


 
Interview with L.Kulakov
If you back up to "articles" you will find other interviews.

The short engine life (50 hours vs 120 hours) before overhaul was largely because of a different oil quality.
The Russians were told to use a cleaner grade to prevent excessive wear, other than that it was not anymore prone to wear than other engines.
There's also something about a throttle/prop linkage that had to be removed to get more out of the aircraft. 
You can read more on that site. 
They speak highly of the P-40 and more so the P-39, referring to them as "modern" aircraft compared to their I-16s and LaGGs.


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## pinsog (Oct 6, 2011)

post76 said:


> Interview with L.Kulakov
> If you back up to "articles" you will find other interviews.
> 
> The short engine life (50 hours vs 120 hours) before overhaul was largely because of a different oil quality.
> ...



Thank you. The article I read said the decreased engine life was due to overboosting and/or overrevving (I can't specifically) but, it was an interview with a pilot and that was probably just his opinion/assumption. Sounds like the actual culprit was poor oil quality. 

"At 66”, the overboosting produced 1745 hp at SL and 1770 hp at 2000 ft. The engines were also subject to over-revving.

At 70”, the overboosting at 3200 rpm produced 1780 hp at 3000 ft. 

Allison was worried that the overboosting with the 8.8 blower ratio would produce a ‘bad precedent’ once engines with a 9.6:1 supercharger ratio entered service. However, the engines with the low alt ratio were “apparently” standing up well to the overboosting. Allison recommended an emergency rating of 60”, producing 1570 hp for the engine, for the 8.8:1 engines and 57” for the 9.6:1 engines.


EDIT: Did some more checking and it looks like just a week after the Allison/GM memo, the -39 was re-rate to a maximum of 56" (1470 hp at SL) and the -73 was re-rated to a maximum of 60" (1550 hp at SL). Looks like the engines with the 9.6:1 supercharger ration were limited to a maximum of 57" (1360 hp at SL, 1480 hp at 10,000 ft) at least until April 1944." 

If the Allisons were standing up to that level of power, why didn't Allison crank them up like that from the factory?


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## post76 (Oct 6, 2011)

pinsog said:


> If the Allisons were standing up to that level of power, why didn't Allison crank them up like that from the factory?



Sounds like they did if they had to establish a WEP setting as well as take off power. 

Recognizing their conservative ratings, it may have been done so production could be kept up with demand, especially at a time when
the war was not decided.
We look at it in hind-site, but its probably a fair enough reason not knowing what future engine demands would be and for which aircraft. This was pre-packard merlin.


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## tomo pauk (Oct 6, 2011)

pinsog said:


> Does anyone have horsepower figures of P39 vs Spitfire vs Me109 at various altitudes?



Disclaimer: I did not rectified discrepancy PS vs. HP here. The lines for V-1710 are not set in stone: for any pixel of those lines that might be wrong, I am to blame.


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## davparlr (Oct 6, 2011)

Shortround6 said:


> Here is a picture of one of the two intercoolers on the F4F-3.
> 
> Grumman apparently exhausted the inter-coolers into the wheel wells, P-39 is going to need intakes, ducts and outlets. The second stage apparently increased the weight of the engine by 55-65lbs not including the inter-coolers but this supercharger may not have been big enough to supply more than 1000hp or so worth of air at 19,000ft. It all has to go behind the engine.


 
No doubt it would be tight. I think the superchargers could fit relatively easy and cooling air supply could be provided by enlarging the air intake. Fitting the intercoolers would be a challenge. But the P-39 needed this to be a real player in Europe and the Pacific.


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## gjs238 (Oct 6, 2011)

davparlr said:


> But the P-39 needed this to be a real player in Europe and the Pacific.


It depends on your expectations.
I was hoping to come up with something universally accepted to be superior to the P-40.
Altitude issues aside, I'm having a hard time understanding why the P-39 "concept" should not be able to achieve that.
I understand that the execution left a lot to be desired.
But why can't the general concept be made to work?
Does the driveshaft add so much weight to make the concept ineffective?


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## Ratsel (Oct 6, 2011)

tomo pauk said:


> Disclaimer: I did not rectified discrepancy PS vs. HP here. The lines for V-1710 are not set in stone: for any pixel of those lines that might be wrong, I am to blame.


Intresting how the GM-1 system wasn't included.


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## tomo pauk (Oct 6, 2011)

Meaning (and yes, I know what is GM-1)?


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## Ratsel (Oct 6, 2011)

Meaning to show that besides engine development, the Germans were unique in their ability to think 'outside' the box when it comes to power development. Would be interesting to show the comparitive results to the normal DB 601 DB 605 engines ran with B4 / C3 fuels. And maybe apply that to a theoretical P-39 build.


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## Shortround6 (Oct 6, 2011)

gjs238 said:


> But why can't the general concept be made to work?
> Does the driveshaft add so much weight to make the concept ineffective?



the drive shaft added about 100lbs. 50lbs for the drive-shaft itself and about 50lbs for the more rigid fuselage needed to keep the propeller in alignment with the engine. The initial "concept" did not include either selfsealing tanks/armor or the wing guns. Bell was also off by a good 10% on weight even not including guns. the plane was "sized" for a much smaller weight than it turned out. The prototype did have tankage for 200US gallons but increasing weight and the change to self sealing tanks cut the fuel capacity to 120 gallons. Long thin tanks suffer more from thick walls than fat tanks. Bell didn't like or want the wing guns which were not in the initial proposals. throw in 245-265lbs of armor and BP-Glass and the planes weight was really spiraling. 
Part of the "concept" is that the engine goes on or close to the center of gravity which is supposed to help maneuverability, maybe it does but it also means that there is no room left (or darn little) to put consumable items like fuel and ammo on the center of gravity to avoid trim changes as the load is used up. 
Building the smallest airframe they thought they could get away with also cut down on the volume to add things later. 

P-63 shows what was needed to fit in what was wanted and even it is tight. Trying to get P-63 performance (load carrying ability and flight characteristics) from the smaller airframe doesn't seem likely or Bell probably would have done it.


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## tomo pauk (Oct 6, 2011)

Hi, Ratsel,
That was the only chart providing graphs for 601/605 that I've managed to find, but if you have another one or more, than please, post them  And some data about usage of GM-1, other than occasional instances?
As one can see, I didn't bothered with turboed Allison either - fellow member asked about real P-39s,109s Spits.


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## Shortround6 (Oct 6, 2011)

Ratsel said:


> Meaning to show that besides engine development, the Germans were unique in their ability to think 'outside' the box when it comes to power development. Would be interesting to show the comparitive results to the normal DB 601 DB 605 engines ran with B4 / C3 fuels. And maybe apply that to a theoretical P-39 build.



Germans didn't think 'outside' the box when it came to power development. they chose a different approach to begin with and not all that different from their early V-12 engine ( the big BMW) or the Hispano V-12. Large displacement, low rpm, low boost, low weight for displacement engines. Once on that path a lot of the development becomes rather predictable and just like the Allison and Merlin, not easy to change in midstream.


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## Ratsel (Oct 6, 2011)

sure they did with the GM-1 / MW-50 and on the rare occasion, both at the same time. DB 605DAM = 2000hp would have been much higher with the N2o.

Kindest regards.


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## Shortround6 (Oct 6, 2011)

Ratsel said:


> sure they did with the GM-1 / MW-50 and on the rare occasion, both at the same time. DB 605DAM = 2000hp would have been much higher with the N2o.
> 
> Kindest regards.



Add how was the German use of MW-50 any more outside the box than the allied use of water injection? Tested by the USAAC in the mid/late 30s on a P&W wasp engine. 

How much would a DB 605DAM that used N2O to get more than 2000hp have weighed? these aren't drag car engines, the pilots expect not only to be able to use the power but actually get back to base after doing so. The US would not approve a WER power level until a single engine had racked up 7 1/2 hours at that rating (5 min at time) I imagine that most other countries did something the same.


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## MikeGazdik (Oct 7, 2011)

Just found this on Youtube, it is a film about the Russian P-39 found in a lake, the same story as posted previously in this thread. The underwater footage of it at the bottom of the lake raised a few hairs on the back of my neck, knowing the pilot was still entombed inside the cocpit. 
_View: https://www.youtube.com/watch?v=hGjkQL1w6Vc_


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## Ratsel (Oct 7, 2011)

Shortround6 said:


> How much would a DB 605DAM that used N2O to get more than 2000hp have weighed?


The exact same weight as the MW-50 setup as the GM-1 used the same delivery system as the MW-50. Tanks/lines/electrical/ everything more or less. Instead of methenol-water mix, it would use the n2o and would be in the 2150hp range. The cooling effects of the n2o has the exact same effect as the methenol-water mix.

As far as the weight goes for using them combined, as the Germans found out it was to much weight.. hence its rarity.


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## Shortround6 (Oct 7, 2011)

Ratsel said:


> The exact same weight as the MW-50 setup as the GM-1 used the same delivery system as the MW-50. Tanks/lines/electrical/ everything more or less. Instead of methenol-water mix, it would use the n2o and would be in the 2150hp range. The cooling effects of the n2o has the exact same effect as the methenol-water mix.
> 
> As far as the weight goes for using them combined, as the Germans found out it was to much weight.. hence its rarity.



A. the cooling effect is not exactly the same.
B. If you use N2O to boost power below the FTL of the engine you are using instead of extra supercharging to burn more fuel to create more heat. Over the FTL is is being used to maintain a certain level of power to a high altitude. 
C. Increasing power may require heavier crankcase, crankshaft, rods, pistons etc. 

TANSTAAFL

For any side or country.


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## Ratsel (Oct 7, 2011)

I'd be more worried about overboosting then a nitrous hit. The Germans used a wet system, ie: the fuel pressure goes up when the nitrous is injected. If an engine can handle a blower it will handle the n2o without any problems.

whats a TANSTAAFL?


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## fastmongrel (Oct 7, 2011)

Ratsel said:


> whats a TANSTAAFL?



There Aint No Such Thing As A Free Lunch


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## Shortround6 (Oct 7, 2011)

Ratsel said:


> I'd be more worried about overboosting then a nitrous hit. The Germans used a wet system, ie: the fuel pressure goes up when the nitrous is injected. If an engine can handle a blower it will handle the n2o without any problems.



The question is can it handle both at the same time, you are claiming they could have gone over 2000hp. The only way to get more than 2000hp is to increase the BMEP (Brake Mean Effective Pressure, or the mean pressure inside the cylinders) or to increase rpm. Mean pressure is sort of the average pressure. To raise the mean pressure you have to raise the peak pressure. Supercharged aircraft engines were already highly stressed at their limits. They are much more lightly built for their power than car engines. They do have a reserve strength but it is nowhere near what car engines have and on the DB605 you have to ask, after they boosted it from 1475hp to 2000hp, how much reserve was left? And BMEP is not IMEP that blower at 1.80 ata or 1.98 ata was taking more power to drive than a blower supplying 1.42 ata. IMEP (Indicated Mean Effective Pressure is the mean pressure needed to give the total power of the engine, propshaft power, internal friction, pumps and the supercharger) is more the actual limit but is harder to figure than BMEP.


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## Ratsel (Oct 7, 2011)

a 325 shot on a 2000hp engine should handle it no problem. the Allison V-1710 were handling a 30"hg increase without disasterous results. The internals were relativly the same. the DB 605A -> DB 605D had a cam timing change to handle the extra boost from the DB 603 supercharger.


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## Shortround6 (Oct 7, 2011)

A cam timing change doesn't make the pistons stronger,or the con rods, or the crankshaft or the crank case. At 2000hp the DB 605 was already making 35% more power than it started with. Now you are claiming it can do 57%???

Allison never sanctioned 30" more boost on it's early 1150hp engines ( the ones after the "C" series.) 

It may have been a field expedient. The Allisons that were factory rated for 75-76in of manifold pressure had a number of internal modifications. Over 2000 of the later ones had a new 12 counter weight crankshaft that was 27lb heavier than the older crankshaft. While it might have been no stronger the improved balancing significantly reduced the loads on the main bearings. A new design of piston ring cut down on blow by and the loss of oil through breathers at high power levels in addition to improved cylinder lubrication. 
Boosting power is easy. Getting the engine to last at the higher power levels is the hard part. It took until the 1960s for most racing car engines to reach the power to weight ratio that was common for aircraft engines at the start of WW II. That is Formula 1 or sprots racing cars like at Le mans, not 10 second drag engines.


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## Siegfried (Oct 8, 2011)

Shortround6 said:


> Add how was the German use of MW-50 any more outside the box than the allied use of water injection? Tested by the USAAC in the mid/late 30s on a P&W wasp engine.
> 
> How much would a DB 605DAM that used N2O to get more than 2000hp have weighed? these aren't drag car engines, the pilots expect not only to be able to use the power but actually get back to base after doing so. The US would not approve a WER power level until a single engine had racked up 7 1/2 hours at that rating (5 min at time) I imagine that most other countries did something the same.



Neither MW50 or the cryogenic N2O system added much weight. Both substances were added at about the same rate as the fuel flow rate (about 1/14th of the air mass flow rate).
A 110L insulated tank (about 25 gallons) whose fluid weight would add about 100kg (220lbs) could contain either cryogenic N2O (SG 1.2), or MW-50 or fuel depending on mission. Roughly MW-50 was used below the full pressure altitude ( 1 atmosphere manifold pressure) of the engine, N2O was used above that altitude. Due to plumbing supply issues the tanks usually were not '3 purpose' as described. If one imagines an DB605 engine with a sfc of 0.42lbs/hp then 1600hp would consume about 666lbs/hour i.e. that tank would have enough fluid for 20 minutes at full flow, perhaps a little less at WEP but note the DB engines had good fuel economy even at Military power since it didn't use rich mixtures. The Ta 152H1 had prodigious fuel volume in 6 wing tanks and one was used for MW50 while the N2O was in the fuselage. (from memmory). From the top of my head there were 3 settings for N2O flow rate of 130 grams, 80 grams or 50 grams per 100 grams of fuel. Ruediger Kosin (Ar 234 designer) in his book "The German fighter" gives a flow requirement of 360kg/hour of N2O in addition to 300kg/hour of fuel to restore a DB605 from 1000hp back to 1340hp at 9000m. I think the book "war prizes" mentions that on the Ta 152 the boost system could be used in two 20 minute bursts; they were quite long. Obviously the system was not for sustained high speed cruising but it should be considered a system good for only 3 or 5 minutes like some allied WEP systems. 

The Germans engines used direct in cylinder fuel injection so there was no charge cooling effect from injecting high aromatic fuel into the supercharger. The injection of MW-50 makes a lot more sense from this perspective. The Jumo 213A on the FW 190D9 did have a rich mixture injection system added just after it entered service, from my understanding of what I've read the system worked by bleeding the air line to the fuel injection systems air flow sensor to trick it into into lowering the injection of fuel simultaneously the the "lost" fuel and some extra was then added into the eye of the supercharger to cool the charge, this boosted power from 1769 to 1900hp. MW-50 was latter added as well with two systems in use: a field modification known as the "Oldenburg" system which used supercharger pressure to blow MW50 into the inlet and a more capable Junkers factory system which pumped in the MW50 at higher flows and atomization and required factory technicians to visit the squadron due to the control system modification required. I think this further boosted power to 2000 and 2100 respectively, probably as much as 2240 with C3 fuel.

Fuel injection means fuel doesn't displace air and it also allowed the German engines to run radical valve overlaps to thoroughly scavenge end gases without loosing fuel into the exhaust however it meant no charge cooling from rich mixture injection into the supercharger unless special mods were provided as described above.

The MW50 system became standard on Me 109G6AM and Me 109G6ASM around March 1944, standardised on the Me 109G14A and Me 109G14AS around May/June.

FW 190's rarely used MW50 as it cracked the cylinder heads, the BMW 801D2 produced about 1729hp standard and 1950hp with rich mixture injection of C3 fuel. The MW-50 "Ribbentrop" system was added to the FW 190A9 in 1944 and I suspect brought the engine to 2200hp. I believe from readings on a newsgroup a small number of FW 190A5 "Jabos" used in hit and run raids had an MW-50 in use in 1943 but I think it was rare.

Amazingly Rudiger Kosin reports the minutes of a November 4th 1942 meeting which had been set up to study high altitude fighter development. The meeting reported that a two stage BMW 801 could be flying today but as it was expected to eliminate this engine from the fighter production program by the turn of the next year work was suspended in view of BMW's workload. Obviously they were expecting to move on to other fighter types and engines that never came about!

The allies also used Water Injection on the P-47, the system was unpopular in winter as the ethanol anti-freeze was inadequate (this is the reason the Germans used methanol) in a European winter and could blow up the engine. Some nitrous oxide was added to P-51's assigned to attack Me 262, to give them a better chance. Mostly the availability of 100/130 fuel is how the allies boosted their engine. The first number is the octane rating (octane to heptane ratio running stoichiometric ie 14:1 air fuel) while the second figure is the PN or performance number the 130 meaning 30% extra power is available if the mixture is run about 12:1 and over boosting the engine by adding 30% more pressure (the test engine however adjusted compression ratio but its a good rule of thumb. Much of the German fighter and bomber force had to make use of B4 which was rated at only 87 octane lean.


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## Siegfried (Oct 8, 2011)

Shortround6 said:


> Germans didn't think 'outside' the box when it came to power development. they chose a different approach to begin with and not all that different from their early V-12 engine ( the big BMW) or the Hispano V-12. Large displacement, low rpm, low boost, low weight for displacement engines. Once on that path a lot of the development becomes rather predictable and just like the Allison and Merlin, not easy to change in midstream.



The DB600 series was highly innovative in some areas.

The DB605 ran a compression ratio of: 7.5/7.3 (B4 87 octane optimized engines) or 8.5/8.3 (C3 96 octane optimized engines). This compression ratio is much higher than the
6.2 approx used on the Merlin, Griffin and Alison even without considering the 100/130 fuel these engines used. For the same air mass flow the power losses from suction are about the same but the proportionately longer stroke allows recovery of more of the energy of the burnt gases. This means a much better power to weight ratio and much better fuel efficiency. This high compression ratio explains why the DB605 could get away with a modest single stage supercharger and low boost levels. It's supercharger was used primarily to compensate for altitude whereas the allies used their superchargers to squeeze in more air to obtain higher power levels. The highest boost level the DB605 used was 1.98 ata which is equal to 14psi in British terms (probably used by 1942) and 60 inches of mercury in US terminology. By the end of the war Merlins were operating at 28 psi/ 90 inches which is 3 ata in German terminology. It also explains why the DB603L and DB605L (with two stage superchargers) didn't need inter-coolers.

Secondly the DB600 series engines from 1941 onwards (starting with the DB601E) used a radical 105 degree valve overlap (both inlet and exhaust valves open simultaneously) which allowed tuned resonance scavenging of the end-gasses. This allowed more air into the cylinder and got rid of the end-gas species that tend to cause pre-detonation. In order to ensure good idling and low RPM opperation the inlet manifold was made variable length to re-tune the inlet like a trumpet. This is only possible because of the 3rd innovation which is direct in cylinder fuel injection which ensured the fuel was not lost during scavenging since the fuel was only injected on the in the upstroke. The 4th innovation was the over head cams, engines like the Griffin, Merlin and Allison used push rods. The rocker covers probably needed to be bigger but this does not matter as the engines were only installed inverted.

When the DB600 series had problems it was with the bearing lubrication system design, which had to run at high pressure which lead to poorly understood frothing at high altitude, this was dealt with by with a de-aerator but took a long time to discover. German spark plug development also lagged for a while, though they placed less demands on their spark plugs.

The DB series didn't use 4 bolts to tie down the cylinder head, they used (I believe) a single large nut that used the threaded cylinder sleeve as a bolt. This ensured even stresses and allowed a thinner cylinder wall and the engine to have a high volume in compact light dimensions. The Hispano-Suiza 12Y I believe bolted the head to the block and had an integral head.

The use of direct in cylinder fuel injection was partially motivated by the need to avoid patent issues with British and American companies, however it should be noted there are many technical advantages and two problems with the Allison and CW R-3350 would have gone away had the US used direct fuel injection. The P-38's fuel in European conditions fractionated in the inlet manifold leading to low octane fuel getting to some cylinders which then pre-detonated while the R-3350 had uneven fuel distribution which lead to burned cylinder (the top 5 in the second row).


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## fastmongrel (Oct 8, 2011)

The Merlin had overhead camshafts unless by pushrods you mean valve rockers


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## wuzak (Oct 8, 2011)

The Allison also had an OHC with 4 valves per cylinder and pent roof combustion chamber.

All Rolls-Royce poppet valve piston engines from the Kestrel, save the early Merlins, had the same style combustion chamber shape and 4v ohc. Kestrel, Buzzrd (and the R derivative), Goshawk, Peregrine, Merlin, Griffon and Vulture had the style of head shown in the picture fastmongrel posted.

Most had single piece head and blocks, with wet liners - something that the German engines did not do, IIRC (wet liner has the piston and combustion chamber on the inside, and is directly exposed to the coolant on the outside. Dry liner sleeves fit into the block and do not have direct contact with the coolant.) . The Merlin started out with the ramp head, which required 2 piece construction, but when they proved to be disappointing they went back to the single piece Kestrel style head/block with the valve layout as above. Then there were problems with coolant leakage, and the Merlin was redesigned with two piece heads and blocks. I believe that the Grifon was designed from the outset with 2 piece head and blocks.


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## Shortround6 (Oct 8, 2011)

The German engines used a different approach to getting power than the Allison or Merlin. Airplane designers do not care what the HP per cu in or cu liter is. They do care what the horsepower per pound of engine weight is. You can build a large displacement slow turning engine for the same weight as a small displacement high rpm engine. Picking either approach is not "thinking out side the box" as this choice had been faced by engine designers for may years. The availability of materials such as good bearings at a given time can influence the designer one way or another. As could previous experience (in the late 20s the internal combustion engine had been around for just 30 years) or a designers own ideas/ prejudices, as in air cooled or liquid cooled. designers or even companies often picked one or the other and stuck with it. 
Once a designer/team starts down a certain path certain developments follow. Some of these developments over improvements in one area or aspect (or more) of performance but have limits or drawbacks in others. The Germans were very interested in fuel efficiency, as well they should be given their chronic shortage of fuel. High compression helps here, but high compression limits the amount of boost that can be used with a given grade of fuel. The Merlin used a 6:1 compression ratio while all but a few Allison's used a 6.68:1 if memory serves. It was estimated that the Allison got 5-10% better fuel economy than the Merlin but couldn't use as a high a boost pressure. At the end of the war (and post war--the P-82 twin Mustangs) some Allisons were fitted with 6:1 pistons to allow for higher boost and power levels. 

Airplane engines are very lightly built. The stresses on the Crankshaft and bearings go up with the square of the engine speed (rpm) increasing the max RPM of an aircraft engine (and keeping anywhere near the same engine life between overhauls) by even a few hundred rpm usually called for a new crankshaft and a strengthened crankcase. Cylinder size also plays a part in rpm limits. The gasoline mixture in the cylinder burns at a pretty constant rate, that is if ignited on one side of the cylinder the flame front travels across the cylinder at pretty much the same speed in all engines. Too wide a cylinder at high rpm means that the burn isn't completed when the piston reaches bottom dead center or the exhaust valve opens, which ever is first. Big heavy pistons also don't reverse direction very well. Too long a stroke for a given bore size goes the other way, combustion is finished (for the most part) before the piston gets near bottom. While the gases are still expanding some what peak pressures are long gone. It might be good for fuel economy but not for high power output. These are generalizations and don't really apply to the the engines we are discussing because their designers knew about this to begin with. It does help to explain why the Russian AM 35-42 series was the largest V-12 used in WW II though and it used the same bore and stroke as the BMW V-12s used in many of the Luftwaffe's early airplanes. See: 

BMW VI - Wikipedia, the free encyclopedia

Notice the weight. Large, light,slow turning engines were not new, they were not "outside the box". In fact they actually represented the normal in aircraft engine design when you consider the Hispano engines and the Italian V-12s and W-18s. 

In the 1930s with 80 and then 87 octane fuel every bodies superchargers were pretty much the same. The fuel just wasn't going to support much more than 6:1 to 7:1 compression ratios and 4-6lbs of boost. The German move to fuel injectors may have been an attempt to push that boundary at bit. It also offered less chances of icing in the intake, freedom from backfires and a few other advantages. When better fuel came along and superchargers could offer higher boost (Early Merlins and Allisons got a good part of their power from RPM not boost) the British and Americans were able to use the evaporation of the fuel in the supercharger as a low level charge coolant. I would guess that this was more by chance than a long term plan. Once noticed however it was a strike against going to the German style injectors if other problems could be solved. 

Since it can take 4-6 years (or longer) from start of pencil on paper to squadron service of an aircraft engine decisions made in the mid 30s affected engine choices and design till the end of the war if not beyond. 

I am not trying to say one side was better than the other or smarter.

Choices were made early on that lead to other choices later or that limited options in one way or another later on. There was little that was "NEW" in engines as far as ideas went. Turbo chargers had been used (experimented with) since WW I in a several countries. Diesel aircraft engines had been worked on by the Americans, British, French and Russians as well as the Germans (maybe the Italians too?) Fuel injection systems had been worked on by a number of countries/companies. The USAAC wanted Allison to work on fuel injection in the mid 30's, Allison turned down the request because they didn't have engineers to take on the project. Bristol in England had several fuel injected prototypes in the early 30s. Water injection had been tested in the Mid 30s. exotic fuel blends had been a staple of both auto racing and speed record aircraft flights for years. 
The Germans were not "thinking outside the box" any more than anybody else was to end up where they did. They may have devoted more resources to a particular area than other people did while other countries/companies devoted their resources to a different aspect/area of engine development. 

As an aside the P-38 troubles with fuel distribution were known, predicted and a solution was in the works even as it happened. The Specifications for 100/130 fuel was changed allowing for a higher amount of heavier compounds which would separate out at low temperatures. This could potentially affect ALL Allison engines, not just P-38s although due to the altitudes at which they flew they were the most affected. A new intake manifold was designed to solve the problem and was being fitted to engines at the factory in Nov/Dec of 1943. Hundreds were shipped overseas for refitting to exiting engines. Most of the problems occurred in those few months while the change over was taking place. Work had started on the Manifold back in the summer of 1943. This is part of the problem the US and to some extent the British faced. It could take weeks if not 2-3 months to get a factory modification to units in the field.


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## Ratsel (Oct 8, 2011)

Shortround6 said:


> A cam timing change doesn't make the pistons stronger,or the con rods, or the crankshaft or the crank case. At 2000hp the DB 605 was already making 35% more power than it started with. Now you are claiming it can do 57%???


No, it dosn't make it stronger but cam timing will change the Dymamic compression ratio (not the static c/r). 

Siegfried explained everything else very well.


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## Shortround6 (Oct 8, 2011)

Ratsel said:


> Siegfried explained everything else very well.



Siegfried explained a lot of things but there is more than one error in his explanations, but then nobody is perfect and it is only through such explanations that we can arrive at a valid conclusion and stop the "German stuff was better because it was German" or "British stuff was better because it was British" or " >insert country of choice< was better........) nonsense.


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## Ratsel (Oct 8, 2011)

wuzak said:


> , *with wet liners - something that the German engines did not do, IIRC *(wet liner has the piston and combustion chamber on the inside, and is directly exposed to the coolant on the outside. Dry liner sleeves fit into the block and do not have direct contact with the coolant.) ..








This DB 605 seems to have bolt-on heads (kinda sorta), wet sleeves, 4-valve/cyl, OH camshafts. If it leaked tons of oil.. its running right


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## Shortround6 (Oct 8, 2011)

It was not uncommon for the cylinder block ( which is different from the engine block) to be detachable from the crankcase. In the world of cars crankcase, engine block, and cylinder block are often used interchangeably. In many cases it makes no difference because they are all referring to the same casting in many cases. 
In the case of the DB engines and many other aircraft engines (and few car engines, mostly race cars) making a one piece casting that big was too hard. SO they split it up. the crankcase holds the crankshaft. The cylinders are cast in rows of six for a v-12 and TWO cylinder blocks are fastened to the crankcase. Want to do a valve job? pull the cylinder block off the pistons and have at it. 

The picture actually shows the the dry liners. The serrated rings at the lock nuts that hold the cylinder block in place, the shiny tube they are threaded onto is the cylinder liner which fits into the cylinder block casting. the green colored serface is where the coolant hits the cylinder block. A tight fit is needed on a dry liner to transfer the heat from the cylinder liner (wall) to the cylinder block and then to the coolant. an air gap would act as an insulator and and cause hot spots or cooling failure. In a wet liner engine parts of the cylinder block would be cut away to expose the liner to the coolant.


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## fastmongrel (Oct 8, 2011)

Interesting stuff guys keep it coming.


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## Ratsel (Oct 8, 2011)

Shortround6 said:


> The picture actually shows the the dry liners..


actually it shows the top half of the cylinders being the conventional 'wet' design.. as you know, thats where 90% of the heat exchange takes place, scratch that, the top 10% of the cylinder does 90% of the cooling. the lower parts, only bearing 10% or so of the heat, the oil does more then an adequite job in cooling, as its designed to too. dunno why you keep refering to car engines, but on some of our car V8 engines we use to fill the lower 1/2 of the cooling passages around the sleeves with readyset, effectively reducing the cooling capacity by 1/2.. with never a cooling issue, and a stiffer bottom end to boot  good for about 15lbs of boost, a 300hp shot. street driven.


edit for the post below,

same difference, just a different way of implementing it.


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## wuzak (Oct 8, 2011)

I think you'll find that the green bit you refer to as the sleeve is in fact the aluminium casting into which the iron or steel liner is pressed. The advantage doing it that way is, providing that the casting is good, is little chance of coolant leaks. Which plagued the Merlin with its one piece block and heads.


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## Ratsel (Oct 8, 2011)




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## Shortround6 (Oct 8, 2011)

Ok, what does the second picture show? 

we know that the cylinder blocks are made in piece with the heads. We know the crankcase was a separate piece. We know that teh crankcase was full depth and not split horizontally like an Allison. Bith Allison and the Early Merlins used one piece heads and cylinder blocks with separate crankcases.


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## wuzak (Oct 8, 2011)

I thought that the Allison was always a two piece block/head design.


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## Ratsel (Oct 8, 2011)

Shortround6 said:


> Ok, what does the second picture show?


its show the condition after being pulled from underground after 65 years, + the word "Achtung" and a different firing order. They changed it to squeeze more ps out of the DB 605D.


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## Siegfried (Oct 9, 2011)

wuzak said:


> The Allison also had an OHC with 4 valves per cylinder and pent roof combustion chamber.
> SNIP.



Thanks everybody, I was clearly wrong about the Allison Merlin not also having an OH camshaft.

There is a brief description of the DB600 cylinder block setup. I assume the engine has an integral head.
Daimler-Benz DB 601 - Germany

"Daimler-Benz DB 600 series engines was one of those designs, that turned out to be right from the very start. It was an inverted vee, and the original engine displaced 2069 cu in (33.9L). It had three main features; it had roller bearing connector rods, it used dry cylinder liners and had a unique system of attaching the cylinders to the crankcase."

"CONSTRUCTION DB 605: Cylinder barrels of steel are screwed and shrunk into the cast Silium-Gamma-alloy cylinder blocks. These dry liners project beyond block providing attachment by means of threaded rings which pull the liners against the finished face of the crankcase. This feature helped to save the weight of the studs and avoided the possibility of distortion."


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## fastmongrel (Oct 9, 2011)

Siegfried said:


> This feature helped to save the weight of the studs and avoided the possibility of distortion."



However it meant you needed extra weight for lockrings and thicker walled liners to take the threads. To restate Shortround6s point you save weight in one place and gain it somwhere else.


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## Siegfried (Oct 10, 2011)

fastmongrel said:


> However it meant you needed extra weight for lockrings and thicker walled liners to take the threads. To restate Shortround6s point you save weight in one place and gain it somwhere else.



I'm not so sure I'd agree; 16 small bolts are better than 4 large ones from a stress distribution point of view and the ultimate is then just a huge sleeve. The key 'accomplishment' of the DB600 series was that it produced competitively high levels of power from B4 (87 octane fuel). The idea of the Db600 series seems to have been to produce a large swept volume in a small space through use of think walls between cyclinders suitable for use high power to wieght ratio outputs using lower grade fuels. The Merlin jumped from about 1030 to 1280 or 1310hp when 100/130 became available and allowed a jump in boost pressure from 6psi to about 12psi, an increase of almost 30%. Without that fuel I suspect the usefullness of the Merlin was limited or Rolls-Royce would have needed to start using water injection.

The production of C3 fuel required so much additional input an additional 30% of ordinary B4 could have been made. (though I believe the introduction of the process of alkylation and hydroforming improved this from sometime in 1943 onwards).

The Reich had available about 20 billion tons of fuel per year while the USA had at least 10 times as much: 200 billion. There is not way that the Germans could have afforded to throw away that much energy and so most of the bomber and transport fleet as well as the half of the fighter fleet had to run of B4.

For Instance the DB605ASB and DB605DBM (both with ovesized superchargers) managed 1850hp using B4 (87 octane) and MW50, the DB605DCM managed this power level of C3 (96/130) without water injection. 
The Luftwaffe page , Daimler-Benz DB 605
Note allied 100/130 was really 104/130. The Merlin 66 using 100/130 managed 1720hp on 100/130 at 18 psi boost. Greater power levels used 100/150.

Both engines weight was 745kg and 744kg respectively.

The two stage Merlin 66 running 100/130 was a superior engine to the DB605A1a running B4, at altitude, from its introduction to the appearance 1 year and 8 months latter of the oversized supercharger variants of the DB605; at this point the Daimler Benz engine caught up. The delay seems to have been related to lack of production of the supercharger rather than the engine itself and a failure to anticipate the need for high altitude engines.

I am aware that the Merlin 100 series (Packard V-1650-9) as developed for the P-51H was able to produce around 2100-2200 using 100/150 and water injection combined but it looks like the DB605 with suitable fuel could have also run at higher pressures. Kurfurst claimed it was benching 2.4 ata which means 2400hp.


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## Siegfried (Oct 10, 2011)

Getting back on topic: one reason the P-39 turbo-charger installation didn't work was because of the bulky external side mounted intercooler and the drag it created. This was attributable to the fact that it didn't use a water cooled intercooler (like the Merlin). Had a water cooled itercooler have been used I assume only an enlarged or deepened radiator would have been needed.

It seems odd that a water cooled engine was compelled to not use a water cooled radiator.


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## tomo pauk (Oct 10, 2011)

I'm told on this forum that radiators have net zero drag? Plus, that XP-39's intercoler was undersized, even for 1150HP?


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## riacrato (Oct 10, 2011)

Net zero drag (or better) under the most ideal of circumstances. Realistically most radiator installations will produce some drag most of the time.


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## tomo pauk (Oct 10, 2011)

Than (if the radiator was producing _some_ drag), wasn't that in a function to get more power, and even more power as we fly at high altitude (12,000 and above)?


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## Shortround6 (Oct 10, 2011)

Radiator drag is all over the map. The absolute best, the P-51, may have had zero drag, it may have had positive thrust, it may have had a small amount of drag, anybody that really knows doesn't seem to have talked 

Using the Meredith to actually get to zero drag or positive was a lot harder than than just saying "we will use the Meredith effect", It is bit like saying "we will just streamline the airplane to get drag co-efficient XX" 

And yes the original XP-39 inter-cooler was undersized. And either the engine radiator and oil cooler were undersized or there were severe air flow problems though them as the Prototype had overheating problems on short flights and ground running. Many planes do not like ground running but on the XP-39 they were doing modifications to the ducting before it flew, while it was doing initial flight tests and before handing over to the air corp and as a result of the NACA wind tunnel tests.


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## Siegfried (Oct 11, 2011)

riacrato said:


> Net zero drag (or better) under the most ideal of circumstances. Realistically most radiator installations will produce some drag most of the time.



It's basically a ramjet so the faster the airflow (the more 'compression') and also the hotter the heat exchanger then the more thrust can be created. At the speeds of most piston engined aircraft the effect was limited. I wonder if anyone ever tried to add burners after the radiator to augment the thrust?


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## Shortround6 (Oct 11, 2011)

Siegfried said:


> It's basically a ramjet so the faster the airflow (the more 'compression') and also the hotter the heat exchanger then the more thrust can be created. At the speeds of most piston engined aircraft the effect was limited. I wonder if anyone ever tried to add burners after the radiator to augment the thrust?



Not really but some people did try routing the engine exhaust into the radiator exit duct in order to raise the temperature/pressure of the outgoing air.


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## tomo pauk (Oct 12, 2011)

While trying to learn more about the consequences of ram effect to the different planes, something popped out. 
The V-1710s with (static) FTH at cca 12,000 ft (found in early P-40s, -39s, 51s) act different with regard to the dynamic (= ram in effect) FTH. The P-40E achieved max speed at 15,000ft, P-51 maxed 13,000+, and P-39D at (surprise, surprise) 12,000 ft.
The later 1710s, with static FTH at cca 15,000ft: P-40N maxed at 16,000, P-51A at 18,000 (static FTH 14,6kft), while P-39Q maxed at mere 13,500 ft (!! - static FTH was at 15,5kft).
We can argue that faster (P-51) can better harvest ram air, but, the P-39 was faster than P-40, yet it cannot excel there...

Another weak point of P-39 ( P-63) - inability to take the advantage of ram effect in a measure other were able to (because intake scoop was both too short and 'in shadow' of the canopy?)?


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## Shortround6 (Oct 12, 2011)

You are on to something Tomo. To get maximum "RAM" the actual intake has to be in a high pressure area and not just facing forward. And it helps if there is enough space for the duct to change shape/direction to convert the speed of the air into pressure. 

This is also why the engine maker almost never quotes FTH using "RAM" as it varies from airframe to airframe.


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## krieghund (Oct 14, 2011)

Here is a graphic of the P-39 inter-cooler setup. NACA suggested to move it into the radiator ducting to reduce drag.


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## tomo pauk (Oct 14, 2011)

The bottom picture shows an 'ordinary' P-39, not the turbo one.
With intercoolers relocated into (glycol?) radiator's ducting, what should feed the radiator itself? 
Were the waste gates inter-cooler really such draggy items, compared perhaps with U/C of P-40, bulges of many Me-109s, added inter-cooler for later Spits etc? IIRC waste gates were faired with sheet metal? 
Not to mention that such parts were the ones (among other) that helped creating extra HP, at twice the altitude - eg. nobody was arguing that Spit IX should have it's inter-cooler deleted.


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## davparlr (Oct 14, 2011)

tomo pauk said:


> Another weak point of P-39 ( P-63) - inability to take the advantage of ram effect in a measure other were able to (because intake scoop was both too short and 'in shadow' of the canopy?)?





Shortround6 said:


> You are on to something Tomo. To get maximum "RAM" the actual intake has to be in a high pressure area and not just facing forward. And it helps if there is enough space for the duct to change shape/direction to convert the speed of the air into pressure.


I thought this the first time I saw it. However, I then remembered a program where there was a desire to place the pitot tube inside the engine inlet duct. I thought this was stupid since the inlet often controls inlet pressure, but then ole Bernoulli stuck his nose into the issue. His comment about total pressure being a constant came into play, and since pitot tubes measure total pressure, the placement was just fine. So, unless there is turbulent airflow into the air intake caused by the canopy, and since total pressure is a constant, the higher velocity, lower static pressure, airflow can be easily converted back to the correct parameters by an adequately designed divergent intake.

Space to do this is unknown but Bell surely took the divergent requirement into the original design and it in itself doesn't take much room. The losses incurred in changing the routing to feed the engine is another issue.


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## riacrato (Oct 14, 2011)

I'll be the a-hole: Wasn't the whole point behind the P-39 layout that it could mount the T9 cannon? Since that turned out to be not a good weapon, I see no point in trying to make the P-39 work (better) unless you have another (better) weapon that dictates such an arrangement.


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## tomo pauk (Oct 14, 2011)

With belt-fed Hispano in lieu of 37mm, plus 2 x HMGs, the layout looks OK. The firepower comparable to 3-gun Bf-109G-6, La-5, US six-gun fighters, while trumping all the Yaks (not the Yak-9T), 4-gun US fighters, MiG-1/3, Bf-109G-2 (no gondolas) and earlier, many of Japanese fighters, most of the Italian stuff...
Alas, the belt-fed Hispanos were never installed there.


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## Shortround6 (Oct 14, 2011)

davparlr said:


> I thought this the first time I saw it. However, I then remembered a program where there was a desire to place the pitot tube inside the engine inlet duct. I thought this was stupid since the inlet often controls inlet pressure, but then ole Bernoulli stuck his nose into the issue. His comment about total pressure being a constant came into play, and since pitot tubes measure total pressure, the placement was just fine. So, unless there is turbulent airflow into the air intake caused by the canopy, and since total pressure is a constant, the higher velocity, lower static pressure, airflow can be easily converted back to the correct parameters by an adequately designed divergent intake.
> 
> Space to do this is unknown but Bell surely took the divergent requirement into the original design and it in itself doesn't take much room. The losses incurred in changing the routing to feed the engine is another issue.



Air flow can be a funny thing. back in the late 60s General Motors had a number of divisions competing with various "super" cars. Mid sized cars (for the time) with big engines and every one had a "ram" air option of some sort. Pontiac and Buick has two small 'scoops' about half way out the hood. Chevrolet had a _rear facing_ trap door inlet at the base of the windshield. Oldsmobile had two BIG scoops (14 in X 2in If memory serves) under the front bumper. The Oldsmobile set up worked best (biggest scoops and in a high pressure area) the Chevrolet worked next best, the base of the wind shield being a high pressure area. Pontiac setup worked hardly at all while the Buick 'scoops" being very low and in the middle of the hood were actually in a low pressure area and sucked air out from under the hood/intake area. granted these are at much less than aircraft speeds but the air flow along a fuselage is not constant. I would think that putting the intake behind the canopy has got to affect the airflow into it somewhat. 

http://www.fiddlersgreen.net/aircraft/Bell-P39-Airacobra/IMAGES/Bell-P-39-Airacobra-Cutaway.jpg


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## riacrato (Oct 14, 2011)

tomo pauk said:


> With belt-fed Hispano in lieu of 37mm, plus 2 x HMGs, the layout looks OK. The firepower comparable to 3-gun Bf-109G-6, La-5, US six-gun fighters, while trumping all the Yaks (not the Yak-9T), 4-gun US fighters, MiG-1/3, Bf-109G-2 (no gondolas) and earlier, many of Japanese fighters, most of the Italian stuff...
> Alas, the belt-fed Hispanos were never installed there.


 
True, but you can have two Hispanos and 2 HMGs in the wings of any allied single engine fighter, so why bother with such a complex and ultimately troublesome layout. Sure convergence is an issue, but the double cannon should make up for that.


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## gjs238 (Oct 14, 2011)

Question: If the 37mm cannon was so bad, were the Soviets removing it and/or substituting something else in the space?


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## wuzak (Oct 14, 2011)

gjs238 said:


> Question: If the 37mm cannon was so bad, were the Soviets removing it and/or substituting something else in the space?



I believe they did, yes. Not for every P-39, but for quite a few.


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## krieghund (Oct 14, 2011)

tomo pauk said:


> The bottom picture shows an 'ordinary' P-39, not the turbo one.
> With intercoolers relocated into (glycol?) radiator's ducting, what should feed the radiator itself?
> Were the waste gates inter-cooler really such draggy items, compared perhaps with U/C of P-40, bulges of many Me-109s, added inter-cooler for later Spits etc? IIRC waste gates were faired with sheet metal?
> Not to mention that such parts were the ones (among other) that helped creating extra HP, at twice the altitude - eg. nobody was arguing that Spit IX should have it's inter-cooler deleted.



The NACA report stated that the ducting for cooling the oil and glycol radiator be redesigned to incorporate the intercooler. I put in the P-39 drawing as reference to how the stock version looks.


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## tomo pauk (Oct 15, 2011)

Okay, think I get NACA's idea now 
An issue of how would anyone succeed to mount intercooler there, is a new ball game.



riacrato said:


> True, but you can have two Hispanos and 2 HMGs in the wings of any allied single engine fighter, so why bother with such a complex and ultimately troublesome layout. Sure convergence is an issue, but the double cannon should make up for that.



While 2 cannons 4 HMGs as a SE fighter armament do sound like a good idea, the proposal has some shortcomings IMO. Ammo count, for example - we can stick to, say 100-120 shells or 250-300 HMG rounds per barrel. That way our SE fighter lugs around about same armament ammo weight as P-38, P-47, Typhoon, Tempest. Nice if one has at least 2000 HP on board, not that nice if there is only 1200 HP. Or, we can reduce the ammo count to save weight - US pilots would've hated us, and the weight of armament is still there.
We can delete a pair of HMGs, and the armament ammo weight is like at P-51D, Hellcat, Corsair - but our plane still lacks 300-400 HP if we want it to be competitive.
There are other minor things to consider, like need to purchase another 10000 cannons from UK, 3 guns (= cannon + 2 guns) are easier to install maintain than 6, less drag (3 openings less), single gun heater can heat all armament, almost no impact to accuracy if one gun jams, central battery is not susceptible to wing flex, less inertia - better roll...

My proposal cuts weight (P-39 was able to out-climb any contemporary US single-engined fighter anyway), while enabling all 3 guns to be fired simultaneously (similar bullet paths).


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## krieghund (Oct 15, 2011)

tomo pauk said:


> Okay, think I get NACA's idea now
> An issue of how would anyone succeed to mount intercooler there, is a new ball game.
> 
> 
> ...




I think 5 x .50 cal in the nose would be sufficient.


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## tomo pauk (Oct 15, 2011)

Sufficient indeed.
The minor issue would've been the change of CoG.
5 HMGs with at least 200 rpg yields 300 lbs to be expanded some 2-3 ft away from CoG, vs. 180 lbs that real P-39s had for 37mm 2 HMGs. It's 210 lbs for belt fed Hisso (120 rds) + HMGs.


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## krieghund (Oct 15, 2011)

tomo pauk said:


> Sufficient indeed.
> The minor issue would've been the change of CoG.
> 5 HMGs with at least 200 rpg yields 300 lbs to be expanded some 2-3 ft away from CoG, vs. 180 lbs that real P-39s had for 37mm 2 HMGs. It's 210 lbs for belt fed Hisso (120 rds) + HMGs.



What was the weight of the P39C with 1x37, 2x.50 2 x.30?


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## tomo pauk (Oct 15, 2011)

Don't know; I was referring to the expandable weight - ie. ammo weight in this case.


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## davparlr (Oct 16, 2011)

Shortround6 said:


> Air flow can be a funny thing. back in the late 60s General Motors had a number of divisions competing with various "super" cars. Mid sized cars (for the time) with big engines and every one had a "ram" air option of some sort. Pontiac and Buick has two small 'scoops' about half way out the hood. Chevrolet had a _rear facing_ trap door inlet at the base of the windshield. Oldsmobile had two BIG scoops (14 in X 2in If memory serves) under the front bumper. The Oldsmobile set up worked best (biggest scoops and in a high pressure area) the Chevrolet worked next best, the base of the wind shield being a high pressure area. Pontiac setup worked hardly at all while the Buick 'scoops" being very low and in the middle of the hood were actually in a low pressure area and sucked air out from under the hood/intake area. granted these are at much less than aircraft speeds but the air flow along a fuselage is not constant.


 I had to laugh when I thought of the aerodynamics of the the late 60s early 70s cars. While I love those cars and I still think they are some of the best looking cars ever (I had a 71 Cutlass S), I suspect there is turbulent airflow all over the place. All aerodynamic theories go out the door, Olds with the inlet closer to clean, laminar flow would be best. Chevy had all the turbulent airflow pile up at where the hood meets the windshield, which is also where cars took air in for the air vents since the mid 50s. The hood definitely had low pressure, and also boundary layer issues (low). If they had stuck up like a supercharged engine, it would probably had worked. 


> I would think that putting the intake behind the canopy has got to affect the airflow into it somewhat.


If the airflow is laminar, there should be no problem. However, the airflow could be perturbed by gaps and protrusions and possibly by stall caused by the cockpit, all causing turbulent airflow. I suspect stall would not be problem at the low speed it would occur. I am sure Bell understood this and tested the inlet for the conditions needed. They did appear to have a raised the inlet out of the boundary layer conditions, surprisingly, something Lockheed failed to do on the XP-80. The P-63, which updated issues with the P-39, appears to have a similar inlet, so apparently they uncovered no problems with this particular design.


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## Shortround6 (Oct 16, 2011)

I think the P-39 made the best of a bad situation. Stick the air intake up behind the canopy and take what you can get or try sticking it out to the side and then looping it up and over in the down draft carburetor (which might have more drag than the behind the canopy intake) or get really tricky and try to duct the intake air from the wing roots or nose past the pilot and engine. 

If essentially the same engine installed in a P-39 gives a different (lower) FTH than the when installed in a P-40 under ram conditions there has to be an explanation somewhere. If both planes are going roughly the same speed (with in a few %) then one is doing a better job of managing the intake air (RAM) than the other.


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## tyrodtom (Oct 16, 2011)

What way did they have to determine exactly how much horsepower a engine was developing installed in a aircraft at altitude ?

Was it just calculation from the dyno results, with corrections throw in for alltitute, and a little bit of swag thrown in as to how much ram air and popeller efficiency added to or took away from the total ?


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## Shortround6 (Oct 16, 2011)

Some radial engines had torque meters built into the reduction gear box so direct readings of actual torque could be made and since HP = Torque x RPM ÷ 5252 it wasn't hard to know what those engines were doing. As far as inline engines ( and radials without torque meters) went they had the dyno results from the test chambers on the ground. They Knew the power at a certain RPM and manifold pressure. AS long as the RPM and manifold pressure were on spec then the engine put out the correct power. If the engine wasn't getting the correct or calculated manifold pressure at a given altitude for the prescribed rpm then something in the inlet system wasn't working as it was supposed to and needed investigation. The test chambers on the ground had air supplies that could be varied in both pressure and temperature to simulate higher altitude flight so there was a lot less WAG going on that you might think. Ram was easy to measure, If the engine without RAM (ground test) needed and got 44in of manifold pressure at certain air pressure that corresponded to say 12,000ft, just fly the plane at 12,000ft and then do a series of slow climbs until you no longer get 44in on the manifold pressure gauge. That is the altitude that ram will let you pull full power at in high speed flight. Ranm was not used to increase the rated power of an engine, it was used to get the rated power higher in altitude. At least until WER came along.


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## vanir (Oct 16, 2011)

davebender said:


> We want a Army fighter aircraft that works right now. (i.e. around 1940)
> 
> 1. Turborchargers are still experimental for fighter aircraft during 1940.


Not exactly, metallurgy for impellers that can cope with more than 30,000rpm inside a hot exhaust pipe are still experimental in 1940, the issue is technological and/or industrial limitations and not a failure to come up with ideas to solve problems, no shortage of ideas ever because everybody wants to be king of the world.



> 2. The Allison engine is a dog at all but low altitude.


The Allison is a better engine than the Merlin from the F-series. The blower sizing was a doctrinal influence and not a technological one, and Allison performance under 11,000 feet is exactly the same as a low alt Merlin right down to the +15lbs two-minute WER. At this particular point the two engines are pretty much identical but the Allison is better built and tolerates lower fuel grades better. The medium alt Merlin is an interceptor engine and pursuit engines aren't interceptor engines, a Merlin 60-series should be compared with the P-38 turbocharged allisons and in this case the multiple stage blower is more reliable, but doesn't have the same development potential. When Packard started making Merlins a RR engineer toured the factory and decided to implement new bearing compositions and machining that Packard was doing with their Merlin 25, it was introduced into all British RR Merlin production because they improved the engine.



> 3. The RR Merlin engine is state of the art and arrangements are already being made for mass production in the USA. Britain is investing heavily into further performance improvments for this engine.


State of the art in 1936 mate. In 1941 state of the art is complicated multistage, multiple speed blowers, including your turbos.



> Seems like a no brainer to me. Pay Packard to expand production capacity so they can produce RR Merlins for the P-39 as well as for the Lancaster bomber.


It might be interesting to put turbo Allisons in a Lanc but why would you put Merlin-25s in the P-39? You'd be better off putting a contemporary two-stage F7R or E9 motor, either will handle plenty of overboost easily and I see no problem with these motors.


Dave the real differences between the Merlin and Allison are very circumstantial and other than that they're just about the same motor, an inline V-12 of the second generation (the curtiss and kestrel are 1st generation, merlin, allison, jumo and hispano are the 2nd and all the others like klimov copied). Everybody was using the same tech, it was just nation-specific industry.


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## Shortround6 (Oct 17, 2011)

One might say that the Merlin, Allison, Jumo and DB engines were 3rd generation even. Hispano, Curtiss, Kestrel being second generation. All the WW I and WW I leftovers being 1st generation. But you are quite right, in the 1938-1940 period there was little to choose between the 4 engines themselves. And even later it was a mater of superchargers and not basic engine. 
The USAAC did order the P-39 and P-40 without turbos (actually they didn't take the time to engineer installations that worked) because they estimated that such planes would be ready 1 year sooner than turbo equipped planes. The Army did have 13 YP-37s to play with. A P-36 wing and rear fuselage with a turbo Allison so they had some idea of the both the problems and capabilities of a turbo fighter.


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## tomo pauk (Oct 17, 2011)

Shortround6 said:


> I think the P-39 made the best of a bad situation. Stick the air intake up behind the canopy and take what you can get or try sticking it out to the side and then looping it up and over in the down draft carburetor (which might have more drag than the behind the canopy intake) or get really tricky and try to duct the intake air from the wing roots or nose past the pilot and engine.
> 
> ...



Perhaps something like this; the intake should enter the hull approximately at the same place it entered from inter-cooler of the XP-39?


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## davparlr (Oct 17, 2011)

Shortround6 said:


> I think the P-39 made the best of a bad situation. Stick the air intake up behind the canopy and take what you can get or try sticking it out to the side and then looping it up and over in the down draft carburetor (which might have more drag than the behind the canopy intake) or get really tricky and try to duct the intake air from the wing roots or nose past the pilot and engine.



I think you are kinda right and kinda wrong. I do not think the inlet location is aerodynamically wrong. As you can see, both the B-2 and Tacit Blue both have engine inlet located in classically low pressure locations, on top of the wing and on top of a lifting body type fuselage. Both of these inlets are very efficient because they deal in a laminar flow slipstream. However the ducting could be problematic, being so short and sharply turned.



> If essentially the same engine installed in a P-39 gives a different (lower) FTH than the when installed in a P-40 under ram conditions there has to be an explanation somewhere. If both planes are going roughly the same speed (with in a few %) then one is doing a better job of managing the intake air (RAM) than the other.


If the difference here is in the ram air design, I suspect the difference is in the ducting. The P-40 duct appears quite straight and long before it enters the turn to the carburetor. While this may increase friction loss, the divergent compression may be more efficient. The P-39 duct is short with a quick turn to the carburetor. This may cause losses. Also, duct design is complex and sophisticated aerodynamics which may have been in its infancy at this time, which could caused losses. It is really complex for supersonic airflow. The advent of stealth and the need for hidden engines and heat suppressed exhaust has driven ducting to an entirely new level of design.


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