# XP-39: pros cons



## tomo pauk (Mar 31, 2011)

The prototype of future Aircobras was featuring a turbocharger, providing an excellent all-altitude performance for the era. The turbo installation was deleted shortly after XP-39 flew, so serial-produced P-39 were lacking performance, above 12 kft especially.

If someone has a more detailed info re. XP-39 pros* cons (esp. connected to the turbo installation), please post here 

* not counting already-covered high performance


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## Shortround6 (Mar 31, 2011)

Tomo, this has been gone over in other threads, but basically, The XP-39 did not meet the performance numbers promised.

There is no real evidence that it ever came close to the numbers given for in many books/web sites. And there is evidence that it couldn't have done what is claimed during the time period it is claimed it flew those numbers. 
1. There was a potential problem with drive-shaft vibration that called for a redesigned heavier drive shaft to be fitted, This was not done until after the NACA wind tunnel tests and until fitted the engine was restricted to 2600rpm. No where near full power.
2. XP-39 was plagued with both oil and coolant overheating problems which called for redesign of both the oil cooler and radiator ducts. 
3. General Arnold was making arrangements less than a month after it's flight to have the XP-39 put into the full size wind tunnel at Langley. 
4. Contract weight was 5,550lbs, when weighed at Wright field during "expedited" acceptance trials it weighed 6,104lbs. about 10% over weight. 

Many people claim that army generals and the NACA "ruined" the P-39. They never go into any details except to say they removed the turboPlease note the above was before the NACA got their hands on the P-39. 

In their report they (the NACA) claim the XP-39, as they received it was good for 340mph at 20,000ft and just under 280mph at sea level. 
There were severe problems with the airflow for the radiator, oil cooler and intercooler. 
For the last, US practice of the time was that the intercooler should remove 1/2 of the heat added by the turbo-supercharger to the intake air before it entered the engine carburetor. In the XP-39 the NACA estimated the intercooler (based on airflows) was removing only 25% in high speed flight and about 12% during climb. This significantly affected power output. The inter cooler was small in size to fit the airframe and had a high pressure drop across it. this meant high drag. The XP-39 needed a much larger intercooler to perform properly and this larger intercooler would not fit in the airframe.
Bell did at least two mock-ups of turbo/intercooler units in 1941 to be 'added' to the P-39. the extra drag of these units caused 40-45mph speed loss at the lower altitudes.

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## tomo pauk (Mar 31, 2011)

Thanks for clearing it up; I've read (P-39 Detail Scale; Americas 100K  - guess they believed Bell ) the figure of 390 @ 20K. 
Quite a disappointment re advertised vs. achieved :\


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## tomo pauk (Mar 31, 2011)

After reading that power @ military rating was just 1150 HP @ 20kft for the XP-39, the 390mph figure looks achievable only in shallow dive


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## tomo pauk (Apr 4, 2011)

FWIW, here is the turbo installation of our plane:


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## davebender (Apr 4, 2011)

I don't doubt the GE turbocharger salesman made such a claim. It's unfortunate the U.S. Army Air Corps believed this claim even though nobody had yet produced a successful turbocharged fighter aircraft.


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## Jabberwocky (Apr 4, 2011)

davebender said:


> I don't doubt the GE turbocharger salesman made such a claim. It's unfortunate the U.S. Army Air Corps believed this claim even though nobody had yet produced a successful turbocharged fighter aircraft.


 
Depends on how you define the term "successful turbocharged fighter".

I'd call the P-30A a minor success, proving that a turbosupercharged engine gave significant performance gains at altitude. Despite being built in only small numbers, and being somewhat heavy and ungainly due to the two person requirement, the fighter's altitude performance was much better than the P-35 and P-36 which followed it.


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## Shortround6 (Apr 5, 2011)

davebender said:


> I don't doubt the GE turbocharger salesman made such a claim. It's unfortunate the U.S. Army Air Corps believed this claim even though nobody had yet produced a successful turbocharged fighter aircraft.



Are you perhaps referring to Dr. Sanford Moss?
Try defining successful please. 
In addition to the 50 P-30s built there were small batches (1,2,5) of turbocharged Biplane fighters from about 1925/26. These were single stage systems (no engine supercharger) but they certainly gave the Army a rather good idea of what was possible even though the early installations had some practical problems. Experience going back 10 years with 60-70 (more?) airframes before the the Airacuda and P-37 make the scene? 
Turbos might not have been ready for squadron service in wartime until 1942 but the Army had a rather good idea of what they were getting, both in benefits and costs (weight, bulk, cost of the equipment.

That is unless you can show documents that say otherwise?
edit>
And what other "salesmen" should the Army have believed?

The one with the two speed supercharger drive?
First used on the Armstrong-Siddley Tiger VIII in the mid 30s it doesn't make the US until about 1937 on a few Wright Cyclones, P&W soon follows but most (all?) P-36s with P&W engines have single speed superchargers.

The one with the mechanical drive two-stage supercharger? That would be P&W With two, count them-two, fighters equipped with them in the 1939 fighter trials. It wasn't really ready for squadron service at the time either.
<edit


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## Shortround6 (Apr 5, 2011)

tomo pauk said:


> FWIW, here is the turbo installation of our plane:



Thank you Tomo. 

I have no idea why (not enough cooling airflow front to back-not enough cooling area for the bottom to top charge air?) but this set up provided only about 1/2 the needed intercooling for the performance that was needed. that is for an 1150hp engine, as power grew to 1325hp or even 1425hp (P-38 engine) the intercooler would have had to get even bigger. 
The turbo installation of the P-39 (turbine hanging out the bottom for cooling and the four exhaust pipes) also left something to be desired drag wise.


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## davebender (Apr 5, 2011)

The manufacturers of the two most successful V12 fighter engines during the late 1930s. Daimler-Benz and Rolls-Royce.


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## tomo pauk (Apr 5, 2011)

About low cooling performance of the inter-cooler on XP-39: perhaps it was the position, combined with small size of the intercooler (compared with what, even early, P-38 were equipped with) that was hampering it? My take is that during the climb the inter-cooler was in 'aerodynamic shade' of the wing, since the plane experiences increase of angle-of-attack. Mounting of intercooler of greater capacity* to a position akin of D.520/Hurricane would've been better IMO. The relocation of 4 exhaust pipes' exits to the back side of wing fairing would've been cool too.

Having a P-39 with such changes with 1325HP @ 25kft makes a nice asset for second half of 1942.

*we pay the drag penalty/loss of speed under cca 12kft altitude, P-40s non-turbo P-39 cater for that during 1942


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## Milosh (Apr 5, 2011)

davebender said:


> The manufacturers of the two most successful V12 fighter engines during the late 1930s. Daimler-Benz and Rolls-Royce.



D-B and R-R were trying to sell engines to the Americans?


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## Shortround6 (Apr 5, 2011)

tomo pauk said:


> About low cooling performance of the inter-cooler on XP-39: perhaps it was the position, combined with small size of the intercooler (compared with what, even early, P-38 were equipped with) that was hampering it? My take is that during the climb the inter-cooler was in 'aerodynamic shade' of the wing, since the plane experiences increase of angle-of-attack. Mounting of intercooler of greater capacity* to a position akin of D.520/Hurricane would've been better IMO. The relocation of 4 exhaust pipes' exits to the back side of wing fairing would've been cool too.
> 
> Having a P-39 with such changes with 1325HP @ 25kft makes a nice asset for second half of 1942.



Tomo, when climbing many planes flew at a speed of 160-200mph. with top speeds of 320-400mph you are going to get twice the airflow through the intercooler per minute. 


tomo pauk said:


> *we pay the drag penalty/loss of speed under cca 12kft altitude, P-40s non-turbo P-39 cater for that during 1942



cater how? provide more airplanes? Non-turbo P-39s PLUS turbo P-39s? how many more P-39s do you plan to build and where do the pilots and ground crew come from?


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## tomo pauk (Apr 5, 2011)

Thanks for pointing to the speed difference. 

I don't plane to build more P-39s; the idea is that, as number of turboed P-39s produced increases, the number of non-turboed decreases. Eventually in some time in 1943 P-39 is built as turboed exclusivelly, along with other changes as stated in other thread.


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## Shortround6 (Apr 5, 2011)

Milosh said:


> D-B and R-R were trying to sell engines to the Americans?



Rolls -Royce and Diamler-Benz Superchargers were better than Allison super chargers in 1937-39?


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## Shortround6 (Apr 5, 2011)

tomo pauk said:


> Thanks for pointing to the speed difference.
> 
> I don't plane to build more P-39s; the idea is that, as number of turboed P-39s produced increases, the number of non-turboed decreases. Eventually in some time in 1943 P-39 is built as turboed exclusivelly, along with other changes as stated in other thread.



No matter how we gloss over it the P-39 was a plane with definite limitations. Unless you can figure out how to get 1 1/2 liters in a 1 liter bottle it was never going to do what was originally promised. Bell had a rather long history of innovative planes that never lived up to their initial promise or advertising.


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## tomo pauk (Apr 5, 2011)

I've described what things could've been done to address main shortcomings of P-39 (performance beyond 12kft combat/ferry range) here:

http://www.ww2aircraft.net/forum/aviation/fighters-made-usa-mid-1943-how-would-you-do-28623.html#post776073


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## Shortround6 (Apr 5, 2011)

I know you have made a number of proposals. I don't believe most will work or will require such major redesign you might as well build a new plane. For instance you say just move the radiator and oil cooler to the wings leaving center section free for fuel tanks. Fuel tanks are already in wings just out board of the air intakes. They are between the two wing spars. behind the the second wing spar is the landing gear. You can either move the radiator/s oilcooler even further out board or hang them under the wing like a Spitfire but that means more drag. That or all you have done is change the fuel tanks from the wings to the center section for not much change.
Oil coolers and radiators were in the wings on the XP-39 but suffered very bad airflow from ducts trying to get through the landing gear area and the spars. XP-39 had cronic and sever cooling problems which were solved by putting the cooling systems in the center section. Under wing loads require beefed up wing structure. 

The P-39 was the smallest US fighter and had the lowest drag next to the Mustang. There is a reason for that and a price, people want to take the benefit of the small size and low drag but don't want to pay the price, less room for fuel and systems. Especially with the engine taking up a lot of room at or near the CG. 

One book on Aircraft powerplants published in 1943 estimates a 1000hp powerplant needs about 10 cu ft of space for a turbo and inter cooler installation. A more powerful engine will need more volume.

The Bell engineers tried to improve the P-39 themselves. It was called the P-39E and was going to be produced as the P-76 because it had so little in common with the P-39. It helped a lot with the other "improved" P-39, the P-63. I am not an engineer but I figure that even with the mistakes the Bell guys made with the P-39 they still knew more than I ever will and had reasons for making the successor aircraft bigger to solve the problems rather than just rearranging some items in the existing airframe.


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## krieghund (Apr 6, 2011)

This my conception of what the armed turbo version would look like with NACA's proposed revised ducts.


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## tomo pauk (Apr 6, 2011)

1st picture shows that 2 air intakes were in each wing, other 2 (feeding glycol cooler) were in wing central section. 
2nd picture shows the wing of P-39. Note the wing part of oil cooler duct in front of wheel well. Also note that at least 3 fuel cells might have been installed outboard of U/C leg attachment point - a simple upgrade that would've increased internal fuel tankage by some 40-50%. The idea never dawned at Bell designers (not trying to be harsh on people).
The reduction of armament into hull-only guns can add another 40-50% of fuel. Never tried.

edit: Neither of those two modifications increases drag, second one reduces it. (end edit)

Installation of wing racks for drop tanks, for historical P-39s, was also never accomplished, unlike for P-40.


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## Shortround6 (Apr 6, 2011)

tomo pauk said:


> 1st picture shows that 2 air intakes were in each wing, other 2 (feeding glycol cooler) were in wing central section.


And on the XP-39 the oil cooler and radiator were in the wing root area ahead of the wheel, but to have good airflow you also need an exhaust duct behind the the radiators/coolers. 


tomo pauk said:


> Also note that at least 3 fuel cells might have been installed outboard of U/C leg attachment point - a simple upgrade that would've increased internal fuel tankage by some 40-50%. The idea never dawned at Bell designers (not trying to be harsh on people).


Maybe it was never tried because that location is behind the CG. P-39 already changed handling just by having empty ammo magazines in the nose. 
The P-39 was loved by good pilots but was considered tricky to fly by average or below average pilots. Making it tricker to fly probably is not a good idea. Granted you can use the rear wing tanks first (just like a P-38 ) but take-off, climb-out accidents may increase. Then there is the question of weight. The fuel system in a 120 gal P-39 went about 290lbs. Let us say you can get 40 gallons behind the wing spar (wing is getting thinner) and that your fuel cells only add another 100lbs (they won't, fuel cell/tank weight varies with the surface area of the tank, not it's capacity) empty. Another 240lbs for fuel. P-39s, while small, were heavy. One reason some of the Q's had the fuel capacity cut to 87 gals, increase in performance wasn't worth the loss of range.


tomo pauk said:


> The reduction of armament into hull-only guns can add another 40-50% of fuel. Never tried.


It was, sort of, post war. Some of the Thompson Trophy racers put fuel tanks in the wing gun/ammo bays. They managed 100 gal but were using thin wall fuel bladders. They had also replaced the self sealing tanks with thin wall fuel cells/bladders. Two planes were prepared by a group of Bell engineers/employees in their off hours. 

While a break down of the CG isn't available the list of changes and weight differences is. 

They did install a 25 gallon fuel tank in the rear of the fuselage but considering the amount of weight they took out of the plane and the amount of weight they added that doesn't really mean much. Not only did guns, ammo and armor go but so did 109 lbs of radio, 70 lbs of communications equipment, 10lbs worth of window winders, 40lbs worth of instruments, 47lbs worth of engine starter and other items. 
Things added include a P-63 25gallon water injection system in the wing, a 75 gallon water tank in the nose, a 4 bladed propeller from a P-63 at 110lbs more than the stock propeller and more.
When they got done the racer/s weighed 47.2lbs more for take-off (clean) than the P-39Q-10 they started with.


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## tomo pauk (Apr 6, 2011)

> Maybe it was never tried because that location is behind the CG. P-39 already changed handling just by having empty ammo magazines in the nose.
> The P-39 was loved by good pilots but was considered tricky to fly by average or below average pilots. Making it tricker to fly probably is not a good idea. Granted you can use the rear wing tanks first (just like a P-38 ) but take-off, climb-out accidents may increase.



The hull fuel tanks on P-40, P-51 and some Spitfire variants were located even more away from CoG than ones I propose. Despite that, benefits out-weighted the shortcomings.
For P-39, the decrease of frontal weight by expanding the hull ammo is nicely canceled out by fuel burned from additional tanks.



> Then there is the question of weight. The fuel system in a 120 gal P-39 went about 290lbs. Let us say you can get 40 gallons behind the wing spar (wing is getting thinner) and that your fuel cells only add another 100lbs (they won't, fuel cell/tank weight varies with the surface area of the tank, not it's capacity) empty. Another 240lbs for fuel.



Too bad we haven't continued this discussion at the thread about possible US fighters for mid 1943. I've proposed there that P-39's have either 5 x .50 cals, or cannon + 2 x .50 cals; we save 100-150 lbs on empty/equipped weight.



> P-39s, while small, were heavy. One reason some of the Q's had the fuel capacity cut to 87 gals, increase in performance wasn't worth the loss of range.



Basic weights, from US 100K, in lbs, rounded to nearest 10lbs:
P-39D1: 6290 ; D2: 6420; 39Q-1: 6420
P-40B: 5990; P-40E: 6700; P-40N-25: 6720
P-51A: 6890
Apart from light-weight P-40B, P-39 is lightest.



> It was, sort of, post war. Some of the Thompson Trophy racers put fuel tanks in the wing gun/ammo bays. They managed 100 gal but were using thin wall fuel bladders. They had also replaced the self sealing tanks with thin wall fuel cells/bladders. Two planes were prepared by a group of Bell engineers/employees in their off hours.
> 
> While a break down of the CG isn't available the list of changes and weight differences is.
> 
> ...



Thanks about the info re. racers


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## Shortround6 (Apr 6, 2011)

tomo pauk said:


> The hull fuel tanks on P-40, P-51 and some Spitfire variants were located even more away from CoG than ones I propose.



Why did I know you were going to bring those planes up 

Please note that by the time the Mustang and Spitfire got rear fuselage tanks they also had engines that were over 300lbs heavier than the engines they were first built with, they had propellers that were 80-100lbs heavier than they were first built with (in the case of the Spitfire it could be 300lbs heavier, MK I Spits needed about 70lbs of ballast in the tail when they switched from the 2 blade wood prop to the 3 blade constant speed prop).
And a fair amount of other equipment added or moved around. Trying to stick a rear fuselage tank in a MKI or II Spitfire with no other modifications could well lead to disaster. It will fit in the space though 

P-40 was designed from the start to have that tank, it wasn't added later. Most planes have a CG range in which they are safe to fly. The CG depending on load can move a bit fore or aft. once you get outside that range things get squirrelly in a hurry. Think of some 4 passenger light planes, some have a baggage compartment aft of the cabin with a certain weight limit. You are not allowed to put more weight in that compartment even if you are only carrying 3 people and your total gross weight is within limits. 
P-40 was designed to be safe to fly with that tank full even if some maneuvers were restricted. 
P-39s were operating closer to the aft CG limit than some other planes to begin with. They had a no warning stall, bad spin characteristics and very effective elevator control. Not much input needed for a lot of results, great for really good pilots. Not so good for the more ham fisted. a little to much up elevator in certain flight conditions and the plane stalls and goes into a flat spin. 



tomo pauk said:


> Basic weights, from US 100K, in lbs, rounded to nearest 10lbs:
> P-39D1: 6290 ; D2: 6420; 39Q-1: 6420
> P-40B: 5990; P-40E: 6700; P-40N-25: 6720
> P-51A: 6890
> Apart from light-weight P-40B, P-39 is lightest.



Great, now figure that the P-39 also had an almost 10% smaller wing than the P-40 and P-51 and that you aren't trying to beat the P-40 or P-51 but Me 109s or Zeros or Tony's and figure out the power to weight ratios. 

The drive shaft on the P-39 added at least 50lbs to the weight of the plane while the stiffer fuselage required another 50lbs or more. 

Birch Mathews book "Cobra" says that it wasn't possible to add rear fuselage tanks to the P-39 or P-63 and while you aren't proposing rear fuselage tanks both planes had CG problems. The P-63 water injection system used a 25 gallon tank in leading edge of the left wing and not out board of the landing gear like you propose.


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## tomo pauk (Apr 6, 2011)

Shortround6 said:


> Why did I know you were going to bring those planes up







> Please note that by the time the Mustang and Spitfire got rear fuselage tanks they also had engines that were over 300lbs heavier than the engines they were first built with, they had propellers that were 80-100lbs heavier than they were first built with (in the case of the Spitfire it could be 300lbs heavier, MK I Spits needed about 70lbs of ballast in the tail when they switched from the 2 blade wood prop to the 3 blade constant speed prop).
> And a fair amount of other equipment added or moved around. Trying to stick a rear fuselage tank in a MKI or II Spitfire with no other modifications could well lead to disaster. It will fit in the space though



Oil tank (full) weights some 70-80lbs. We can move it adjacent to engine (for non-turboed P-39), or under pilot's seat (for turboed ones). The oil armor plate can be moved forward, too. The pic attached shows plenty of room between angine and oil tank it's armor plate. Also plenty of room under cockpit.
Addition of fuel tanks 1-2 ft away from CoG makes less CoG issues than adding it 3-5 ft away.



> P-40 was designed from the start to have that tank, it wasn't added later.
> Most planes have a CG range in which they are safe to fly. The CG depending on load can move a bit fore or aft. once you get outside that range things get squirrelly in a hurry. Think of some 4 passenger light planes, some have a baggage compartment aft of the cabin with a certain weight limit. You are not allowed to put more weight in that compartment even if you are only carrying 3 people and your total gross weight is within limits.
> P-40 was designed to be safe to fly with that tank full even if some maneuvers were restricted.



As said, benefits overweighted the issues.




> P-39s were operating closer to the aft CG limit than some other planes to begin with. They had a no warning stall, bad spin characteristics and very effective elevator control. Not much input needed for a lot of results, great for really good pilots. Not so good for the more ham fisted. a little to much up elevator in certain flight conditions and the plane stalls and goes into a flat spin.



P-39 had issues, main ones being performance at altitude range. CoG issues were present, allocation of P-39s for more experienced pilots seem like remedy.



> Great, now figure that the P-39 also had an almost 10% smaller wing than the P-40 and P-51 and that you aren't trying to beat the P-40 or P-51 but Me 109s or Zeros or Tony's and figure out the power to weight ratios.



If I tried to propose US Zero, I'd say: 'delete the armor, install non-protected fuel tanks, go with 3 HMGs all the time; good, we've saved 1000 lbs'. But I'm not. The proposal is US-way: plenty of armament, fuel, armor, power; travel fast, make fast attack, avoid turning battle. Worked well historically.
Glad you've mentioned power to weight ratio. 
The non-turbo P-39 I've proposed has same weight with same fuel aboard, as historical ones had. 
The turboed one is heavier 5% for take off, little more drag, while having 50% more power already at 15K. 



> The drive shaft on the P-39 added at least 50lbs to the weight of the plane while the stiffer fuselage required another 50lbs or more.



Already counted in 'basic weight' figures.



> Birch Mathews book "Cobra" says that it wasn't possible to add rear fuselage tanks to the P-39 or P-63 and while you aren't proposing rear fuselage tanks both planes had CG problems.



As above: comparing CoG issues with fuel tank 1-2ft away from CoG with the one 3-5 ft away is apples oranges.



> The P-63 water injection system used a 25 gallon tank in leading edge of the left wing and not out board of the landing gear like you propose.



Allocation of fuel tanks to leading edge was beyond of designer's scope, unfortunately 

BTW, P-63 have had fuel tanks at location similar to my proposed additional tanks for P-39, aft central spar. Since it was okay for P-63, I'd reckon they would've worked just fine for P-39


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## Shortround6 (Apr 10, 2011)

tomo pauk said:


> Oil tank (full) weights some 70-80lbs. We can move it adjacent to engine (for non-turboed P-39), or under pilot's seat (for turboed ones). The oil armor plate can be moved forward, too. The pic attached shows plenty of room between angine and oil tank it's armor plate. Also plenty of room under cockpit.
> Addition of fuel tanks 1-2 ft away from CoG makes less CoG issues than adding it 3-5 ft away.


They usually tried to keep the top of the oil tank and the coolant header tank above the top of the engine to help get air bubbles out. They may have tried to space the oil tank from the engine for temperature reasons. You have attached a schematic diagram and not a photograph, it does not show all pipe connections, brackets and small pieces of equipment in the area. I don't have a picture either so I don't know what is in that area. I do know that in a similar discussion concerning the P-40 somebody had a similar diagram that showed space behind the engine and claimed that something could go there. In the real plane that space was partially occupied by the sidesway brace of the engine mount. That is probably not the case here but these diagrams should not be take as the last word on available space. You may also want to leave room for the mechanics to work on the back of the engine, carburetor, starter, generator, gun synchronizers and any other accessories are on the back.
Some planes were more tolerant of CG issues than others, either they weren't flying normally with their CG quite as far aft as the P-39 or they had different stall characteristics or they had different spin recovery characteristics or combinations of the these.



tomo pauk said:


> As said, benefits overweighted the issues.
> 
> P-39 had issues, main ones being performance at altitude range. CoG issues were present, allocation of P-39s for more experienced pilots seem like remedy.



Increasing the likelihood of spin/crashes for extra range calls for a very careful balance. There perhaps were ways to gain a slight performance increase for the P-39 that did not involve making the flight characteristics anymore dangerous. In the early part of the war the P-39 was the second most produced fighter. By the end of 1942 over 2800 had been built. That is a lot of "experienced pilots" to come up with by then and it tends to leave the P-38 and P-40 squadrons with more than a fair share of "green" pilots.




tomo pauk said:


> If I tried to propose US Zero, I'd say: 'delete the armor, install non-protected fuel tanks, go with 3 HMGs all the time; good, we've saved 1000 lbs'. But I'm not. The proposal is US-way: plenty of armament, fuel, armor, power; travel fast, make fast attack, avoid turning battle. Worked well historically.



It didn't work historically for the P-39 and that is why the P-39 has it's bad reputation. Pilots, group commanders and theater commanders knew it couldn't do the all round fighter job although it could be useful in some roles. I am not saying turn it into a Zero but that extra weight is one reason it can't perform like other fighters using similar powered engines. Over coming a 15-30% weight handicap is going to take more than minor tweaking. Making a fast attack is a little difficult if the enemy is 5,000ft higher than you are. 

Something to consider when using book performance figures to evaluate combat is that book performance charts are established using standard atmospheric conditions. 59 degrees F or 15 degrees C and standard sea level pressure. Planes operating in colder areas got a bit of a boost in performance (denser air) while planes operating in the tropics/desert took a performance hit from the less dense air. It was a double wammy. The engines provided less power while the wing provided less lift. 
Because of Engine boost and wing loading not all aircraft suffered exactly the same performance loss per degree of temperature. 




tomo pauk said:


> Glad you've mentioned power to weight ratio.
> The non-turbo P-39 I've proposed has same weight with same fuel aboard, as historical ones had.
> The turboed one is heavier 5% for take off, little more drag, while having 50% more power already at 15K.


Well then, the non-turbo won't perform any better, but it might not perform worse at best? not exactly an endorsement.
Just 5% heavier? only a little more drag if you don't use an intercooler. While you may get 50% more power (using WER rating from a P-38 J engine? 1600 vs the 1125hp for a P-39Q engine is 42%) you might be getting only 13% more power 9000ft. 



tomo pauk said:


> As above: comparing CoG issues with fuel tank 1-2ft away from CoG with the one 3-5 ft away is apples oranges.
> 
> BTW, P-63 have had fuel tanks at location similar to my proposed additional tanks for P-39, aft central spar. Since it was okay for P-63, I'd reckon they would've worked just fine for P-39



Talk about apples and oranges  

P-63, aside from using a 37mm cannon and the Allison extended shaft engine set-up had no interchangeable parts with a P-39. A new fuselage, a new wing of different airfoil (laminar flow with maximum thickness much further back along the cord, which is why the fuel tanks were back there) different area and different construction, it used a main spar and a rear spar instead of the spar arrangement of the P-39. While it had a longer fuselage please note that the wing was further back on the fuselage than on the P-39 which means the engine was further forward in relation to the wing, center of lift. Cockpit was in front of the wing instead of above the leading edge. It's landing gear was wider tracked and it had 10 more inches between the nose gear and the main wheels. 
Where they could put fuel on the P-63 has very little to do with where they could put fuel on the P-39.

However, with both the P-39E and the P-63 as attempts to improve the concept of the P-39 (37mm gun firing through prop hub) they are indications that more "stuff" could not be crammed into the existing P-39 airframe. 
While we "know" that P-39 was flown with rear fuselage/wing tanks we do not "know" one way or the other that the idea wasn't thought of, sketches made with preliminary calculations and given up. Considering some of the other stuff that shows up on paper sketches (Merlin powered P-63s with massive airscoops in the wing center section) it doesn't seem likily that this was totally over looked.


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## tomo pauk (Apr 10, 2011)

Shortround6 said:


> They usually tried to keep the top of the oil tank and the coolant header tank above the top of the engine to help get air bubbles out.



Okay, then we'll rotate it 80 deg upwards, so it's parallel to the prop shaft; it's own CoG mowes forward too. Under it goes the turbocharger. We will locate inter-cooler just aft the front wheel well, while also moving radio forward. The oil-tank armor can loose it's lower half to address CoG issues, too. 



> They may have tried to space the oil tank from the engine for temperature reasons. You have attached a schematic diagram and not a photograph, it does not show all pipe connections, brackets and small pieces of equipment in the area. I don't have a picture either so I don't know what is in that area.



You can check the schematics in The Book. 
Plenty of space between engine oil tank, with couple of pipes to connect the tank engine, plus some parts of starter system. 



> I do know that in a similar discussion concerning the P-40 somebody had a similar diagram that showed space behind the engine and claimed that something could go there. In the real plane that space was partially occupied by the sidesway brace of the engine mount.



Yep, looking at P-40 diagram the turbo is a non-starter with current set up, aft the engine.
(soon in theaters near you: P-40 with turbo intercooler mounted in chin, with glycol oil coolers relocated in front of wings)


> That is probably not the case here but these diagrams should not be take as the last word on available space. You may also want to leave room for the mechanics to work on the back of the engine, carburetor, starter, generator, gun synchronizers and any other accessories are on the back.



Not very maintenace friendly, I agree. But perhaps comparable with P-47, or twice as good as P-38 (maintenace-wise)?


> Some planes were more tolerant of CG issues than others, either they weren't flying normally with their CG quite as far aft as the P-39 or they had different stall characteristics or they had different spin recovery characteristics or combinations of the these.
> Increasing the likelihood of spin/crashes for extra range calls for a very careful balance. There perhaps were ways to gain a slight performance increase for the P-39 that did not involve making the flight characteristics anymore dangerous. In the early part of the war the P-39 was the second most produced fighter. By the end of 1942 over 2800 had been built. That is a lot of "experienced pilots" to come up with by then and it tends to leave the P-38 and P-40 squadrons with more than a fair share of "green" pilots.


 
You have a good point there.
Would you check out the sketch of wing for P-39 in thread about hypothetical US fighters for P-39. I've tossed some ideas above, that should've helped to keep CoG at it's place. 



> It didn't work historically for the P-39 and that is why the P-39 has it's bad reputation. Pilots, group commanders and theater commanders knew it couldn't do the all round fighter job although it could be useful in some roles. I am not saying turn it into a Zero but that extra weight is one reason it can't perform like other fighters using similar powered engines. Over coming a 15-30% weight handicap is going to take more than minor tweaking. Making a fast attack is a little difficult if the enemy is 5,000ft higher than you are.



If we have turbo on board, the P-39 is the one 5000ft above.



> Something to consider when using book performance figures to evaluate combat is that book performance charts are established using standard atmospheric conditions. 59 degrees F or 15 degrees C and standard sea level pressure. Planes operating in colder areas got a bit of a boost in performance (denser air) while planes operating in the tropics/desert took a performance hit from the less dense air. It was a double wammy. The engines provided less power while the wing provided less lift. Because of Engine boost and wing loading not all aircraft suffered exactly the same performance loss per degree of temperature.



I'm trying all the time to ram more air in engine, so we can have more power 



> Well then, the non-turbo won't perform any better, but it might not perform worse at best? not exactly an endorsement.



The non-turbo (main changes are in reduced weight of armament [either 37 + 2x .50 cal, or 5 x .50 cal] to cancel out the weight of new tanks) will have the same weight. If on e wants range, he can cram more fuel in it. Or attach a bomb, and have decent range. So it's better performer. 


> Just 5% heavier? only a little more drag if you don't use an intercooler. While you may get 50% more power (using WER rating from a P-38 J engine? 1600 vs the 1125hp for a P-39Q engine is 42%) you might be getting only 13% more power 9000ft.



300 lbs for "supercharging with accesories" - half of what is stated for same item(s) of P-38J in The Book. The drag is price to pay, Supermarine paid it when developing Spit IX from Mk.V. The comparison between two V-1710s makes all the point: we want a plane for USAAC that can do hi-alt work as early as mid-42, while being available affordable.



> Talk about apples and oranges
> 
> P-63, aside from using a 37mm cannon and the Allison extended shaft engine set-up had no interchangeable parts with a P-39. A new fuselage, a new wing of different airfoil (laminar flow with maximum thickness much further back along the cord, which is why the fuel tanks were back there) different area and different construction, it used a main spar and a rear spar instead of the spar arrangement of the P-39. While it had a longer fuselage please note that the wing was further back on the fuselage than on the P-39 which means the engine was further forward in relation to the wing, center of lift. Cockpit was in front of the wing instead of above the leading edge. It's landing gear was wider tracked and it had 10 more inches between the nose gear and the main wheels.
> Where they could put fuel on the P-63 has very little to do with where they could put fuel on the P-39.



No apples and oranges.
If it's bad to have plenty of expandable stuff aft the CoG, it doesn't make any difference what plane is subject. 
If you point at impossibility to install fuel tanks in front of main spar, because of lack of space, that's not likely. The wing thickness in front of main spar, but inboard, was greater than where fuel tanks were located. The P-38 have had fuel tanks in leading edge - a space much less generous than front half of P-63's wing.



> However, with both the P-39E and the P-63 as attempts to improve the concept of the P-39 (37mm gun firing through prop hub) they are indications that more "stuff" could not be crammed into the existing P-39 airframe.



The only 'stuff' P-63 was carrying extra was auxiliary blower (not even intercooler). Despite designing new wing, it was inferior to, older, P-51's (less fuel, no internal HMGs, ammo for only for 1 HMG). P-63 can hardly be used as a proof how some people are good designers/engineers, dismissing a premise that only P-63 is a way to make better P-39.



> While we "know" that P-39 was flown with rear fuselage/wing tanks we do not "know" one way or the other that the idea wasn't thought of, sketches made with preliminary calculations and given up. Considering some of the other stuff that shows up on paper sketches (Merlin powered P-63s with massive airscoops in the wing center section) it doesn't seem likily that this was totally over looked.



Well put.
Is the part "with rear fuselage/*wing tanks*" referring to drop tanks, among other, or to additional inner wing tanks? I'd like to know more anyway


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## Shortround6 (Apr 10, 2011)

Tomo, First you have my apologizes for the last sentence in my post. 

"While we "know" that P-39 was flown with rear fuselage/wing tanks we do not "know" one way or the other that the idea wasn't thought of,...."

It should have read "While we "know" that P-39 was _NOT_ flown with rear fuselage/wing tanks we do not "know" one way or the other that the idea wasn't thought of,...."

Sorry for the confusion. 



something to consider while thinking of all of these "let's just squish it here" plans, sometimes "empty" space on some of these systems drawings isn't empty. 

Why no leading edge tanks on the P-39? Maybe that is because that is where the Aileron control runs were. 

Why is there "nothing" in the space in the wing center section under cockpit and between the radiator intake ducts? Maybe because that is where the the bottom of the control stick was with the aileron and elevator connections. the "stick" had a hole in it and rotated around the drive shaft but it went through the floor of the cabin and and made it's connections in the space below. 

Why was the oil tank in the rear fuselage? In addition to the possible reasons I have already given maybe there wasn't room for it in the engine compartment. The space available between the fuselage beams may not have been great enough without redesigning the fuselage beams. 

Considerations for turbo location include the distance from the engine, space was needed to allow the exhaust to cool somewhat before it hit the turbine buckets (blades). Blade life was significantly shortened if incoming exhaust gas was too hot. 
Allison was experimenting with turbo compounding in 1944. they found that even using massive amounts of ADI they could still exceed safe inlet temperatures on the turbine. They resorted to injecting ADI fluid straight into the exhaust manifolds. 

Short, sharp bends in internal ducts increase drag even if they save space. Usually not a good trade. 
Oil coolers, radiators and intercoolers need careful designing. There are many trade offs and sometimes they do get it wrong. But consider, the drag of the airflow through an oil cooler, radiator or intercooler goes up with the square of the speed of the air going though it. Doubling the size of a radiator can reduce the airflow speed for the same cooling power ( same mass of air) such that the bigger radiator has 1/8th the drag of the smaller radiator. Against this you have to balance the weight of the bigger radiator and the bigger duct/s needed. 

I have been looking for the source and can't find it at the moment but I remember reading that a rule of thumb for a second stage super changer system (turbo?) was on the order of 10 cu ft for a 1000hp engine. Thar is 60cm X 60cm by 75cm. Even if this is a bit off and you can get the system for 1500hp into 12cu ft. that is 60cm X 60 Cm by 90cm. Granted it doesn't have to be all in one place (it is better if it isn't) but the more spread out the more ducting and volume needed. 

As for a few specific points. 
On the 1600hp turbo Allison (50% more power than the regular P-39).
"we want a plane for USAAC that can do hi-alt work as early as mid-42, while being available affordable"

The Engine with 1600hp potential doesn't show up until March of 1943 with the P-38H, It was severely limited by the intercoolers which you can possibly fix earlier but the basic engine may not be quite 1600hp. We may want it but available and affordable may not be there. The Us Army did not approve WER settings for just about all of 1942. An earlier acceptence of such power ratings may have helped US planes in combat even if it doesn't do anything for altitude performance. 

The apples and oranges does apply. With the different wing, different wing location and different center of gravity relationships, the P-63 was a different airplane not only in construction but to fly. It provided more stall warning, it didn't enter a spin as easy as a P-39 and it was easier to get out of a spin. The different location of the wing and equipment (the engine may be a foot or so further forward in relation to the wing than a P-39) make it hard to judge weights and balances with the information we have available. I would suggest you to read "The Book" a bit more, like top of page 209 

"It has been stated the P-39 was the single exception to the rule that all US World War II fighters had good safe characteristics." Granted that may be just a few peoples opinion but read the section on the P-63.

There are a number of comments about how improved the P-63 was in general handling compared to a P-39.

I am OK with the P-63 being not as good as the P-51 but that in no way proves that the engineers were incompetents who could have "fixed" the P-39 with a few simple, cheap tweaks if only they had thought of them.
The 37mm cannon and midship engine layout imposed penalties that the P-51 didn't have to put up with. With North American guys may have had a better idea to begin with or executed it better but they didn't have to deal with the 37mm cannon and the 10 ft extension shaft.


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## tomo pauk (Apr 11, 2011)

I can certainly agree that slapping parts around don't make an eagle from turkey; I've felt that P-39 was feasible to upgrade with proper functioning turbo, and tried to find the possible 'solutions' for space CoG issues. While I agree that you've made many fair points, in the same time many of mine were not off the mark, too 

I was not inquiring about absence of fuel tanks in front of main/central spar of P-39, but of P-63 (tomo pauk: _The P-38 have had fuel tanks in leading edge - a space much less generous than front half of P-63's wing._

About the power ratio (1600 hp vs. 1125): I agree that we are almost in 1944 here.
Let's then compare 1325 hp @ 25K (MIL for P-38F) with 1150 hp 12K (MIL for P-39s of same era). The MIL value @ 25K for earlier P-39s should've been 900-950Hp (100 % of power is at 12 kft, zero HP at 55-56Kft, meaning a 10% loss for each 5.5kft above those 12kft) - only 60% of what was possible in same time frame, but with turboed engine. Quite a difference. At 17.5kft we can talk about 30% more hp for turbo.


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## Zipper730 (Jan 16, 2017)

Shortround6 said:


> 1. There was a potential problem with drive-shaft vibration that called for a redesigned heavier drive shaft to be fitted


To start off, are you talking about the turbocharger or the crankshaft?


> This was not done until after the NACA wind tunnel tests and until fitted the engine was restricted to 2600rpm. No where near full power.


What is the full RPM?


> In their report they (the NACA) claim the XP-39, as they received it was good for 340mph at 20,000ft and just under 280mph at sea level.


Not very impressive, the Spitfire would perform better.


> US practice of the time was that the intercooler should remove 1/2 of the heat added by the turbo-supercharger to the intake air before it entered the engine carburetor. In the XP-39 the NACA estimated the intercooler (based on airflows) was removing only 25% in high speed flight and about 12% during climb.


So it would overheat?


> The XP-39 needed a much larger intercooler to perform properly and this larger intercooler would not fit in the airframe.


1. Would it have fit in a plane the size of a P-63? It was about two feet longer...

2. Would it have been possible to remove the door, and used a sliding canopy? It could allow the linkages to be reworked and placed on the side rather than underneath the pilot (I just contacted an other aviation buff who came up with the idea).



> Bell did at least two mock-ups of turbo/intercooler units in 1941 to be 'added' to the P-39. the extra drag of these units caused 40-45mph speed loss at the lower altitudes.


Just out of pure interest, do you have drawings of these layouts?


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## Zipper730 (Jan 16, 2017)

While a little off topic: If Bell had designed the P-39 different from the start, what would have allowed it to have met the baseline requirements that Saville and Kelsey dictated?

360 mph in level flight, preferably 400
Climb to 20,000 feet in 6 minutes or less
Carry 1000 pounds of armament including a cannon
Preferably have a nose-gear


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## wuzak (Jan 16, 2017)

Zipper730 said:


> To start off, are you talking about the turbocharger or the crankshaft?



The extension drive shaft to the propeller.




Zipper730 said:


> What is the full RPM?



It was in the region of 2800-3000rpm. I think for an early V-1710 as in teh XP-39 it may have been 2950rpm.




Zipper730 said:


> 1. Would it have fit in a plane the size of a P-63? It was about two feet longer...



The P-63 used the 2 stage V-1710 and still no intercooler (at least for production versions). Instead it used ADI.




Zipper730 said:


> 2. Would it have been possible to remove the door, and used a sliding canopy? It could allow the linkages to be reworked and placed on the side rather than underneath the pilot (I just contacted an other aviation buff who came up with the idea).



Yes, since later P-63s did.


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## wuzak (Jan 16, 2017)

Zipper730 said:


> While a little off topic: If Bell had designed the P-39 different from the start, what would have allowed it to have met the baseline requirements that Saville and Kelsey dictated?
> 
> 360 mph in level flight, preferably 400
> Climb to 20,000 feet in 6 minutes or less
> ...



Lighter weight and better aero.

Which is what the P-39 had over the XP-39. The P-39 certainly met 1 and 4, maybe 3 and I'm not sure about 2.


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## Zipper730 (Jan 16, 2017)

wuzak said:


> The extension drive shaft to the propeller.


Gotcha...


> It was in the region of 2800-3000rpm.


Okay


> The P-63 used the 2 stage V-1710 and still no intercooler (at least for production versions). Instead it used ADI.


That was to avoid the intercooler?


> Yes, since later P-63s did.


Didn't know that...


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## wuzak (Jan 16, 2017)

Zipper730 said:


> That was to avoid the intercooler?



Yes.


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## Zipper730 (Jan 17, 2017)

wuzak said:


> Yes


I take it the intercooler was preferred over ADI?


> Lighter weight and better aero.


Let's start with strength and ruggedness

How would you propose increasing overall strength?
How did the P-39 compare in strength to the F4F, F6F, F4U, Fw190, Hurricane, P-40, P-47, P-51, and Me 109?


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## GregP (Jan 17, 2017)

The P-63 wasn't inferior to the Mustang. It would give a P-51 all it could handle at 25,000 feet. It certainly rolled better and the climb should have been comparable if not a bit better, too. It did come up some 15 mph slower, but that's nothing in combat. It makes a big difference in a protracted race, maybe. But if you were fighting with someone, you'd not notice 15 mph since neither of you is at top speed or anywhere even close.

Go ahead and make that high-speed 5 g break. You won't be at top speed when you come out of it.

It also wan't designed as a long-range fighter. And I get the idea that a long-range fighter is what they were looking for when the P-63 was ready.

I am under the distinct impression that the P-63 was not bought for one primary reason. And that is because the P-63 was about as good as a P-51 and the P-51 was already in production. If you are not getting a demonstrably better plane, there is really no compelling reason to start a production line that also requires training and a logistics chain. Also the P-63 did not have long range, and if they weren't getting SOME benefit from tooling up for it, there was no compelling reason to do it.

That is was produced for the Soviet Union was probably due to the USAAF wanting all the P-51s it could get for the U.S.A. and UK, so they let Bell Aircraft make the P-63, and only actually procured a handful themselves, mostly for evaluation purposes and experiments. Perhaps they didn't want Bell to make P-51s because they didn't want the Soviet Union to get P-51s in quantity. I bet we never really find out if we haven't to date.

I would like to have seen a 2-stage Merlin-powered P-63, but that apparently wasn't in the cards.

I like the P-63, but probably would not have bought it either, unless there was an issue with P-51 production, had it been my choice. As it turned out, there was no issue with P-51 production, and they'd have been better off having Bell build P-51s rather than chase the P-39 concept except for possibly not wanting to give Mustangs to the USSR in quantity.

I'm with Shortround when he says the P-39 airframe was too small to have a lot of improvements worked on it, and that it really wasn't big enough for a turbo system.

They also couldn't get the P-39 to tumble in a wind tunnel during the war. After the war was over, around 1949 or so, they did a wind tunnel test with the model ballasted as if the ammunition were empty ... and the CG was a bit more rearward, as when flying it in combat and most of your ammunition is gone. It tumbled beautifully in the wind tunnel. During the war they always tested it as though it had full ammunition, and the CG was more favorable, causing it to not even be ABLE to tumble. That sort of hints at a possible design weakness, and that is very limited CG range.

That only proves that you need to test at both forward AND aft CG limits for full aerodynamic results to be obtained, which any good test engineer SHOULD have been able to tell them anyway.


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## tomo pauk (Jan 17, 2017)

Zipper730 said:


> While a little off topic: If Bell had designed the P-39 different from the start, what would have allowed it to have met the baseline requirements that Saville and Kelsey dictated?
> 
> 360 mph in level flight, preferably 400
> Climb to 20,000 feet in 6 minutes or less
> ...



The 2 and 3 are in conflict, no single V12 before 1942 will satisfy both requirements. Since it is USA we're talking about, the R-2800 is needed to meet all the criteria, or go with two V-1710s. Lockheed managed it with P-38.

In the old continent, the IMAM Ro.58 and Westland Whirlwind should fit the requirement, bar the tricycle gear.


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## Zipper730 (Jan 18, 2017)

tomo pauk said:


> The 2 and 3 are in conflict, no single V12 before 1942 will satisfy both requirements.


Maybe true, but at least it would be decent to try and bring them into the closest degree of alignment


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## Shortround6 (Jan 18, 2017)

Zipper730 said:


> Maybe true, but at least it would be decent to try and bring them into the closest degree of alignment



MK I & II Spitfires carried under 500lbs of armament, so did Bf 109s (except gun boats) , As did most Russian fighters with V-12 engines.

You can't put 1000lbs of armament in a 5-6000lb aircraft. Not had have a powerful engine or a strong airframe.


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## Zipper730 (Jan 18, 2017)

Shortround6 said:


> MK I & II Spitfires carried under 500lbs of armament, so did Bf 109s (except gun boats) , As did most Russian fighters with V-12 engines.


How much did the P-39 actually carry?


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## Shortround6 (Jan 19, 2017)

Zipper730 said:


> How much did the P-39 actually carry?



The 37mm gun weighed about 238lbs (or about what a pair of 20mm Hispanos did) while the ammo was about 900 grams per round so 30 rounds is just about 60lbs. 298lbs?
The .50 cal guns go about 140lbs a pair. 400 rounds of linked .50 cal is about 120lbs or a bit more. 280lbs.
Four .30s (a later requirement than the original) are about 94 lbs and 4000 rounds (a little over kill) of .30 cal is 260lbs. 355lbs for around 933lbs.
Please note that this for guns and ammo only, No ammo bins/boxes, no chutes/feedways, no gun heaters, charging systems, firing systems. Also no gun mounts. Mounting things like 37mm cannon and .50cal guns requires more than few L brackets pop-riveted to a bulkhead  

Granted all this "stuff" would run up the weight of the fighters I mentioned earlier but things are usually in proportion although there are exceptions. Hispanos like substantial gun mounts as they were designed to be bolted to a V-12 engine block. Germans fastened their hub guns to the airplane and the gun barrel went through a tube in the engine. You could pull the engine and leave the gun behind. 

Early P-39s had no wing guns but had two .30s in cowl outboard of the .50s. of course this model didn't have armor, bullet proof glass or self-sealing fuel tanks.


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## Zipper730 (Feb 14, 2017)

Shortround6,

1. Was the ammo specification dictated around the weight of the guns, the weight of the ammo, both?
2. Guns and ammo amounted to around 578 to 698 pounds prior to the 0.303's being added on...
3. How much would armor, bullet proof glass and self-sealing tanks add?


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## Shortround6 (Feb 14, 2017)

Ammo was often a legacy requirement at this time. ALL US Fighters during the 30s being capable of being fitted with a .50 cal gun and 200 rounds of ammo in place of one of the .30 cal guns. The .30s had 500 or 600rpg in old the old fighters. The 37mm got 30 rounds becasue that is what they could fit in the "magazine"/feedsystem. On the P-39 they may not have had the room to increase the ammo supply for the .50s. 
The weight of Armour and bullet proof glass varied between the P-39D, the D-2 and the P-40 but 250-265lbs. the self-sealing tanks may have added over 100lbs, I don't have the weight of the unprotected tanks to compare to.


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## Zipper730 (Feb 15, 2017)

Shortround6 said:


> Ammo was often a legacy requirement at this time. ALL US Fighters during the 30s being capable of being fitted with a .50 cal gun and 200 rounds of ammo in place of one of the .30 cal guns. The .30s had 500 or 600rpg in old the old fighters. The 37mm got 30 rounds becasue that is what they could fit in the "magazine"/feedsystem.


I understand why the round numbers were lower... that's not complicated bigger bullets take up more space.


> On the P-39 they may not have had the room to increase the ammo supply for the .50s


200 rounds per gun seems to meet the requirements though. I'm surprised that the requirement for 1000 pounds of ammo never was an issue


> The weight of Armour and bullet proof glass varied between the P-39D, the D-2 and the P-40 but 250-265lbs. the self-sealing tanks may have added over 100lbs, I don't have the weight of the unprotected tanks to compare to.


That'll do.

So the plane would be 350-365 pounds more


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## GregP (Feb 23, 2017)

Probably shouldn't say this, but Zipper, you really need to do a little research!

You're asking other people to fill you in on data all the time that they spent years accumulating. Probably need to Google it and check out books in libraries. Think about it and come back with discussion on the subject. Continuous requests for data that isn't that hard to find will grow old. You can probably find the answer to your last question in online P-39 specifications that are likely in a pdf file of the POH (Pilot's Operating Handbook).

Anyway, if you don't check it out, how do you know they're right? Most are, admittedly, but there are backup facts abounding for those that look.

People are happier to share data when you show some initiative yourself to go find things out, and then share it.

Just saying, a LOT of the things you ask about were covered in older threads. Search them for an answer with keywords. Most have been covered before. Cheers.


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## tomo pauk (Feb 23, 2017)

Zipper730 said:


> ...
> While on it, how much ammo did the P-38 carry, and how much armor and self-sealing tank weight?



Several good sites offer reliable answers on questions like that (not that I'm against fellow forum members making straight answers):
- this forum, 'Techincal' section
- avialogs.com
- LiTOT: Content
- wwiiaircraftperformance.org
- kurfurst.org

Conten found on all of the web-sites listed would've lasted for a life time worth of reading.


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## Zipper730 (Apr 2, 2018)

GregP said:


> Probably shouldn't say this, but Zipper, you really need to do a little research!


No biggie, I already found the information. The P-38 information isn't really all that important, the weight figures listed were fine.



Shortround6 said:


> Contract weight was 5,550lbs


I've been looking for that figure for some time.


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## P-39 Expert (Apr 6, 2018)

Just my opinion, but that turbo in the XP-39 was never going into production. The biggest problem was the oil cooler and intercooler ducts were not adjustable. So basically the same volume of air was passing through both radiators whether the engine was operating at low power or full power. Too much cooling at low power and not enough at high power.

If the ducts could have been made adjustable with moveable trailing exits there were still too many problems. The turbo and exhaust were partially protruding into the airstream causing aerodynamic drag. The coolant radiator was in the leading edge of one wing and it's duct was not adjustable either. It needed to be enlarged and made adjustable. 

All these problems were somewhat solved with the deletion of the turbo (and intercooler) and moving the coolant and oil radiators into the wing center section formerly occupied by the turbo. They now had adjustable exit flaps so the volume of cooling air could be adjusted.


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## Zipper730 (Apr 7, 2018)

P-39 Expert said:


> Just my opinion, but that turbo in the XP-39 was never going into production.


It's just a hypothetical scenario


> If the ducts could have been made adjustable with moveable trailing exits there were still too many problems.


Could you make a guess as to the size of the necessary radiators if such flaps existed?


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## fubar57 (Apr 7, 2018)

So long thread.

Reactions: Like Like:
1 | Funny Funny:
3 | Like List reactions


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## P-39 Expert (Apr 9, 2018)

Zipper730 said:


> It's just a hypothetical scenario
> Could you make a guess as to the size of the necessary radiators if such flaps existed?


I have no idea on the size of the radiators. And Bell did put adjustable exit flaps on the production planes, so it could have been done.

Better (to me) to delete the turbo and get the plane into production concentrating on keeping the weight as low as possible for good performance. And more powerful V-1710s were coming, one even had a mechanical second stage supercharger (went into the P-63).


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## Zipper730 (Apr 9, 2018)

P-39 Expert said:


> I have no idea on the size of the radiators.


I've been told the typical radiator area for cooling is typically around 3 times the size of the carburetor intake. Would that be a good estimate point, to take the carburetor intake and triple the area?


> And Bell did put adjustable exit flaps on the production planes, so it could have been done.


That's good to know!


> Better (to me) to delete the turbo and get the plane into production concentrating on keeping the weight as low as possible for good performance.


Frankly, I'm honestly surprised they didn't just put a twin-staged supercharger on it more quickly (it would have been a simpler fix than a turbocharger).

Still, with the earlier turbocharger configuration: I'm curious why they mounted the turbocharger under the engine when there was enough room behind it, provided the cooling tank was moved ahead of the engine?


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## P-39 Expert (Apr 10, 2018)

Zipper730 said:


> I've been told the typical radiator area for cooling is typically around 3 times the size of the carburetor intake. Would that be a good estimate point, to take the carburetor intake and triple the area?
> That's good to know!
> Frankly, I'm honestly surprised they didn't just put a twin-staged supercharger on it more quickly (it would have been a simpler fix than a turbocharger).
> 
> Still, with the earlier turbocharger configuration: I'm curious why they mounted the turbocharger under the engine when there was enough room behind it, provided the cooling tank was moved ahead of the engine?


That's the big question, why did it take so long to develop the mechanical two stage supercharger? Started in 1940, it finally made production in April '43. It's just a simple impeller in a diffuser driven from the engine by a jackshaft. Three full years to develop what should consist of off-the-shelf parts. High altitude performance without all the ducting of the turbo (14' per engine on the P-38 Lightning) and it didn't need an intercooler except for WEP. If you can figure out why this took so long to develop please tell me.

A turbo in the rear fuselage (instead of under the engine) like the P-47 would have caused (more) balance problems.


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## wuzak (Apr 10, 2018)

Why would they be "off the shelf parts"?

Allison had a 9.5" supercharger for the V-1710 and a 10.0" supercharger for the V-3420. The auxiliary supercharger was 12.1875" - not something they had lying on a shelf.

As pointed out by SR on another thread, Allison was so busy being all things to all people that they didn't have the resources to develop a 2 stage engine as well as all the other crap.


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## Shortround6 (Apr 10, 2018)

P-39 Expert said:


> That's the big question, why did it take so long to develop the mechanical two stage supercharger? Started in 1940, it finally made production in April '43. It's just a simple impeller in a diffuser driven from the engine by a jackshaft. Three full years to develop what should consist of off-the-shelf parts. High altitude performance without all the ducting of the turbo (14' per engine on the P-38 Lightning) and it didn't need an intercooler except for WEP. If you can figure out why this took so long to develop please tell me.
> 
> A turbo in the rear fuselage (instead of under the engine) like the P-47 would have caused (more) balance problems.



Oh boy , oh boy, oh boy.

Allison first toyed with idea of a 2 stage supercharger in 1938 or 1939 but quickly gave up as they didn't have enough engineering staff to work on it plus all the "stuff" the army wanted at the same time (Like pusher engines for the Airacuda).

first example (or more than the first) did use "off the shelf parts". The tried using the same impeller as the engine supercharger, I don't know about the diffuser. They soon learned that far from the optimal set up, very far.

When dealing with these superchargers you have several things going on. What is wanted for making power in the engine is mass flow of air, that is to say pounds per minute. But that is hard to measure but pressure isn't. So they use pressure gauges but they are only indicators. Since the weight of the air per cubic foot varies with the temperature even at the same pressure they have to make some assumptions in aircraft (they have a lot more instruments on a test bench/test cell). ANd as you go higher the density of air changes, by the time you get to 20,000ft air is just over 1/2 the weight it is at sea level per cubic ft. You need a much bigger intake system (inlet duct/s and supercharger inlet) to handle the low density air. Once you have used a 1st stage supercharger to squeeze the air down to something like sea level air your 2nd stage impeller-diffuser can be about what it was for the single stage engine.
RR got "lucky" in that they had a large supercharger already "in house". They just grabbed the impeller (for sure) and housing (maybe) from the Vulture engine as it flowed about the mass of air they wanted for for the hi 20,000 ft to 30,000ft area. This was for the first test rig, later prototypes and production engines used purpose designed impellers and diffusers.
Allison had no such large supercharger or impellers hanging around and no good place to get them.

You need intercoolers to make high power at altitude. Otherwise you are just adding weight, bulk and cost to make a slightly better *low* altitude engine. The supercharger adds considerable heat to the intake charge. For instance the supercharger on a Merlin XX added 148 degrees celsius (over 260 degrees F) to the temperature of the incoming air when in high gear. using more compression in the supercharger/s just adds more heat and at some point you reach the detonation point. 

The higher temperature if using higher 'boost' without cooling means the air is less dense which means you get less power from the same 15lb of boost than if the air was "cooler" and at 15lbs boost. The heat load on the entire engine goes up. Raise the heat of the intake charge 100 degrees and peak temperature in the cylinder goes up 100 degrees and the heat of the exhaust goes up 100 degrees. 
And you wind up having to use lower intake pressure because of the higher temperatures. Say your engine can handle 15lb of boost with an intake temperature of 300 degrees (hypothetical figures) with an intercooler. take out the intercooler and lets say the temperature goes to 400 degrees. Now lets say that extra 100 degrees pushes the mixture to a point where, as the piston rises and further compresses the mixture, it raises the temperature to the flash point and the mixture either ignites on it's own before the spark plugs fire or the when the spark plugs fire the added pressure of the flame front cause all the mixture to explode at once rather than burn in an advancing flame front across the cylinder as the cylinder goes past top dead center and starts to descend. 

While using two stages gets you less temperature rise than using one stage for a given level of compression trying for high boost (and a Merlin 61 was compressing the air over 5 times in the superchargers at altitude) without using an intercooler runs into diminishing returns real quick.

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## P-39 Expert (Apr 10, 2018)

Shortround6 said:


> Oh boy , oh boy, oh boy.
> 
> Allison first toyed with idea of a 2 stage supercharger in 1938 or 1939 but quickly gave up as they didn't have enough engineering staff to work on it plus all the "stuff" the army wanted at the same time (Like pusher engines for the Airacuda).
> 
> ...


Still, it's just an impeller in a diffuser. All that time just to come up with the perfect size impeller? 

Big question is, why did they put the carb on the second stage? Why take so long to move the carb back to the engine for more efficiency like the P-38 and P-47?


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## Shortround6 (Apr 10, 2018)

Early F4Fs had problems with mis-matched impellers or speeds, they got rumbling in the ducts which is an indication of break down of air flow, not just an annoying noise. 
The combination has to work at a variety of altitudes and speeds/power outputs.
Allison first tried using a single speed drive to the auxiliary supercharger then replaced it with the variable hydraulic drive. 

and you are ingoring the part about intercoolers. A number of the test rigs on the ground used intercoolers.


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## P-39 Expert (Apr 10, 2018)

Shortround6 said:


> Early F4Fs had problems with mis-matched impellers or speeds, they got rumbling in the ducts which is an indication of break down of air flow, not just an annoying noise.
> The combination has to work at a variety of altitudes and speeds/power outputs.
> Allison first tried using a single speed drive to the auxiliary supercharger then replaced it with the variable hydraulic drive.
> 
> and you are ingoring the part about intercoolers. A number of the test rigs on the ground used intercoolers.


Sorry about ignoring the intercoolers. Do you view the water injection system employed on the P-63 as being inferior?


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## Shortround6 (Apr 10, 2018)

Of course. While intercoolers are large, heavy and bulky they are of unlimited duration. 
And water injection is NOT light if it is to be used for long periods of time or as a substitute for intercoolers.
The F6F and F4Us used both but the water injection was to supplement the intercoolers, not replace them. 
The P-47s used both and since the Turbo could supply more air than the mechanical drive superchargers it would up with a 30 gallon tank.
P-63s used a 25 gallon tank (actually two tanks) , 186.5 lbs of fluid. Compact but hardly light. This was good for 15 minutes at max boost. 

Then we have the total engine cooling problem. The auxiliary stage supercharger is NOT free.

Our P-39 that can make 1150 hp at 15,000ft is making 1450-1500hp in the cylinders? (about 100hp for friction in the engine and 200-250hp to drive the supercharger). 
The P-63 making 1150hp at 25,000ft (or lower?) needed to make another 250hp or so in the cylinders to power the auxiliary supercharger.
Granted it wasn't using quite as much power to drive the engine supercharger but the cooling load has gone up and bigger radiators are going to be needed. Not to mention bigger oil coolers to handle the heat generated by the hydraulic drive. 

The auxiliary supercharger weighed about 125lbs and added about 21 in to the length of the engine. Two weights are given. 175lbs for the one ones with the carb on the auxiliary supercharger and 125lbs for the ones with the carb still on the engine. It could be that the difference is the weight of the carb, and that it is counted as part of the auxiliary supercharger when attached? 

Superchargers got to be much more than just an impeller in a diffuser. Say you had two superchargers, both supplied the same amount of air at the same pressure but one was 65% efficient and the other was 70% efficient. Lets say that the first require 100hp to the input shaft to give you the required airflow and pressure. That means that only 65hp worth of work is actually being done and that 35hp is not only going to waste, it is going to waste as heat, unnecessarily heating up the intake charge. 
The 2nd supercharger only needs 93hp driving the input shaft, leaving an additional 7 hp for the propeller, but has 7hp (5220watts) LESS heating up the intake charge. about a 20% reduction in _waste _heat. There is a considerable amount of heat generated in compressing air even at a 100% theoretical efficiency so practical difference is not as bad as these figures show. But supercharger designers were trying to make things as good as possible and not just slam a few parts together and then live with the consequences.

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## P-39 Expert (Apr 10, 2018)

Shortround6 said:


> Of course. While intercoolers are large, heavy and bulky they are of unlimited duration.
> And water injection is NOT light if it is to be used for long periods of time or as a substitute for intercoolers.
> The F6F and F4Us used both but the water injection was to supplement the intercoolers, not replace them.
> The P-47s used both and since the Turbo could supply more air than the mechanical drive superchargers it would up with a 30 gallon tank.
> ...


All P-39s had cooling problems on the ground, but this was a 1325HP engine and the cooling was okay on the V-1710-63 that was also 1325HP.


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## tomo pauk (Apr 10, 2018)

Shortround6 said:


> ...
> You need intercoolers to make high power at altitude. Otherwise you are just adding weight, bulk and cost to make a slightly better *low* altitude engine. The supercharger adds considerable heat to the intake charge. For instance the supercharger on a Merlin XX added 148 degrees celsius (over 260 degrees F) to the temperature of the incoming air when in high gear. using more compression in the supercharger/s just adds more heat and at some point you reach the detonation point.
> ...



Excellent post, however I'd say that intercooler was not mandatory for 2-stage engines to work well at high altitude. The gain vs. 1-stage versions of same engine was about 60% on the DB 605L vs. 605A at 35000 ft, similar for the best 1-stage V-1710 vs. the 2-stage versions. Both DB-603L (or was it LA?) and Jumo 213F were 2-stage engines without intercoolers. 
Cooling drag of the Jumo 213E was about 35% greater than of the 213E, so we might take that factor in a consideration. One of things that made the otherwise unremarkable XP-40Q2 and P-63C as fast, if not faster than Spitfire IX/VIII.


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## wuzak (Apr 10, 2018)

Shortround6 said:


> The auxiliary supercharger weighed about 125lbs and added about 21 in to the length of the engine. Two weights are given. 175lbs for the one ones with the carb on the auxiliary supercharger and 125lbs for the ones with the carb still on the engine. It could be that the difference is the weight of the carb, and that it is counted as part of the auxiliary supercharger when attached?



It could be that the one with teh carb on the auxiliary supercharger also had a liquid to air intercooler.


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## Zipper730 (Apr 10, 2018)

P-39 Expert said:


> That's the big question, why did it take so long to develop the mechanical two stage supercharger?


If I was to make a guess, the USAAF seemed to prefer the turbocharger, whereas the Navy preferred twin-stage supercharging.


> A turbo in the rear fuselage (instead of under the engine) like the P-47 would have caused (more) balance problems.


From what you said (regarding the P-63), the supercharger was added behind the engine, with the cooler moved forward...



Shortround6 said:


> Allison first toyed with idea of a 2 stage supercharger in 1938 or 1939 but quickly gave up as they didn't have enough engineering staff to work on it plus all the "stuff" the army wanted at the same time (Like pusher engines for the Airacuda).


Why was the pusher so desirable for the YFM-1


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## Shortround6 (Apr 10, 2018)

Zipper730 said:


> Why was the pusher so desirable for the YFM-1



Because if you made it with tractor propellers the gunners would be shooting backwards 





The 37mm guns were flexible 




and could be aimed over a cone independent of the aircrafts direction and in fact cold be aimed at different targets.

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## Zipper730 (Apr 10, 2018)

Shortround6 said:


> Because if you made it with tractor propellers the gunners would be shooting backwards


What was the motive for such a weird design? As I understand it, I twas supposed to be a patrol-interceptor that could be an escort-fighter.


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## wuzak (Apr 10, 2018)

Zipper730 said:


> What was the motive for such a weird design? As I understand it, I twas supposed to be a patrol-interceptor that could be an escort-fighter.



To blow enemy bombers out of the sky.

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## Dimlee (Apr 11, 2018)

wuzak said:


> To blow enemy bombers out of the sky.



And it still blows many minds. What a cool "steampunk" machine.

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## J.A.W. (Apr 18, 2018)

Dimlee said:


> And it still blows many minds. What a cool "steampunk" machine.



Yeah that thing is a real-life 'Astounding SF'/'Sky Captain & the World of Tomorrow' - type of machine..

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## Zipper730 (Dec 19, 2018)

Shortround6 said:


> In their report they (the NACA) claim the XP-39, as they received it was good for 340mph at 20,000ft and just under 280mph at sea level.


Was this with or without the turbocharger?


> There were severe problems with the airflow for the radiator, oil cooler and intercooler.


The production P-39 would redesign the radiator & oil-cooler to cover the left & right inboard wing for this reason?


> For the last, US practice of the time was that the intercooler should remove 1/2 of the heat added by the turbo-supercharger to the intake air before it entered the engine carburetor. In the XP-39 the NACA estimated the intercooler (based on airflows) was removing only 25% in high speed flight and about 12% during climb. This significantly affected power output. The inter cooler was small in size to fit the airframe and had a high pressure drop across it. this meant high drag. The XP-39 needed a much larger intercooler to perform properly and this larger intercooler would not fit in the airframe.


Since this is kind of a 'what-if' exercise: Would it be fair to say that the intercooler was affected by the following issues

The intercooler lacked a splitter and, as a result, more turbulent flow entered, reducing the effectiveness of the intercooler for a given area?
The front of the intercooler duct was not shaped to correctly reduce the velocity and achieved desired pressure drop?
The rear of the intercooler duct lacked a flap to allow sufficiently high area at low speeds and sufficiently low area at high speeds?
The area of the intercooler was inadequate even if 1-3 were optimal?



> Bell did at least two mock-ups of turbo/intercooler units in 1941 to be 'added' to the P-39. the extra drag of these units caused 40-45mph speed loss at the lower altitudes.


So if 340 was achieved at higher altitudes, you'd be down to 295-300?

Regarding the YFM-1


> Because if you made it with tractor propellers the gunners would be shooting backwards


Early on, I thought they simply had the guns able to move because of the fact that the ballistic arc was substantial. Still, I'm surprised they didn't just put the guns in the fuselage below the optical sight, or in the wing-roots




wuzak said:


> The P-63 used the 2 stage V-1710 and still no intercooler (at least for production versions). Instead it used ADI.


This came up in a parallel thread on the P-39/P-63 -- it had a liquid to air intercooler.


> Allison had a 9.5" supercharger for the V-1710 and a 10.0" supercharger for the V-3420. The auxiliary supercharger was 12.1875"


I thought GE was making most all superchargers at first?


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## Zipper730 (Dec 19, 2018)

P-39 Expert


It's kind of a little off topic but I have three questions that you might have answers for

How much weight would be saved/added by removing the car-doors and going with a sliding canopy?
Other than the intercoolers and things of that sort: Why was the turbocharger harder to mount on the P-39 than the YP-37 (despite the nose being absurd -- the turbocharger was mounted decently under the engine without any real trouble
The redesign of the tail-cone on the P-63: Was this information learned from experience on the P-63, or did it come from some other source?
While I am in agreement that from a practical standpoint that a twin-stage supercharger would be better, Allison didn't have a workable system at the time, but did have a turbocharger. All of this is clearly an intellectual exercise


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## P-39 Expert (Dec 19, 2018)

Zipper730 said:


> P-39 Expert
> 
> 
> It's kind of a little off topic but I have three questions that you might have answers for
> ...


Thanks for the questions.
1. Probably a little weight would have been saved by replacing the car doors with a sliding canopy, but I bet more weight would have been saved by the accompanying deletion of the turnover bulkhead that wouldn't be needed with a sliding canopy. Save a little weight and have much better visibility. Replace the armored glass and small armor plates in the turnover bulkhead with normal armor plate behind the pilot. To me the biggest quick opportunities for weight reduction would be the deletion of the 4x.30 caliber peashooter guns in the wings along with their mounts, chargers, heaters and ammunition boxes (about 200# with 300rpg, closer to 400# with 1000rpg) and deleting the piece of armor plate in the nose (about 100#) and moving the radios from the tail cone to above the engine right behind the pilot to retain balance. These items would have helped the earlier P-39D/F/K/L with the 8.8 geared engines climb a lot faster, about 1.2fpm per every pound of weight saved. Reduce weight by 300# and increase climb rate by 360fpm, a worthwhile endeavor IMO. 
2. Regarding the turbo, if the YP-37 turbo was without any real trouble then it would have been produced in volume. The turbo at that time was still unreliable, it took lots of internal space, and the P-39 radiators (coolant, oil and intercooler) were a real Rube Goldberg arrangement with poor airflow and no adjustable exit flaps. In other words maybe 25% functional. The only other place to put the turbo was the rear fuselage, but then you've grown into a P-47.
3. The tail cone was lengthened to compensate for moving the wing back about a foot. Moving the wing back reduces the distance from the CG to the tail, so the tail needed to be lengthened to compensate for stability. The tail wasn't lengthened to provide space for the mechanical second stage, there was already room there, just move the coolant tank up right behind the pilot and put the second stage where the coolant tank was.

In summary, the turbo wasn't ready and the promised mechanical second stage offered almost as much horsepower with a much simpler and cleaner installation. Now, the two stage mechanical V-1710-93 didn't start production until May 1943 but it's installation into the P-39 and P-40 would have transformed them into superb high altitude planes. Hope this helps.


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## Shortround6 (Dec 19, 2018)

Zipper730 said:


> Was this with or without the turbocharger?


 With, the turbo charger. 


> The production P-39 would redesign the radiator & oil-cooler to cover the left & right inboard wing for this reason?


The air inlets were in the wing roots, the actual radiator and oil coolers were in the belly of the plane where the turbo had been. 




It is a model but gives the best idea as to what they changed. Radiator in the center, oil cooler on each side. 


> Since this is kind of a 'what-if' exercise: Would it be fair to say that the intercooler was affected by the following issues
> 
> The intercooler lacked a splitter and, as a result, more turbulent flow entered, reducing the effectiveness of the intercooler for a given area?
> The front of the intercooler duct was not shaped to correctly reduce the velocity and achieved desired pressure drop?
> The rear of the intercooler duct lacked a flap to allow sufficiently high area at low speeds and sufficiently low area at high speeds?



The area of the intercooler was inadequate even if 1-3 were optimal?[/QUOTE]
No 4 is the primary, they didn't have enough airflow by a factor of 4 when climbing, a splitter and.or modified air intake is not going to solve the problem of too small aaaaan intercooler to begin with. 




> So if 340 was achieved at higher altitudes, you'd be down to 295-300?



No, the turbo installation cost 40-45 mph off the speed of of an unmodified P-39. A "D" I believe? 

In other words if the standard D could do 336mph at 5,000ft it was down to 300mph or under with the add on turbo installations




Much like trying to fly/fight with a large drop tank under the plane as far as drag goes. This was a mock up which is why the exhaust doesn't connect.


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## Zipper730 (Dec 20, 2018)

P-39 Expert said:


> Thanks for the questions.


No problem


> 1. Probably a little weight would have been saved by replacing the car doors with a sliding canopy, but I bet more weight would have been saved by the accompanying deletion of the turnover bulkhead that wouldn't be needed with a sliding canopy.


I thought most aircraft had roll-bars up top?


> To me the biggest quick opportunities for weight reduction would be the deletion of the 4x.30 caliber peashooter guns in the wings along with their mounts, chargers, heaters and ammunition boxes (about 200# with 300rpg, closer to 400# with 1000rpg) and deleting the piece of armor plate in the nose (about 100#) and moving the radios from the tail cone to above the engine right behind the pilot to retain balance.


Was it a standard practice among any aviation service to mount the radio above the engine?


> 2. Regarding the turbo, if the YP-37 turbo was without any real trouble then it would have been produced in volume. The turbo at that time was still unreliable, it took lots of internal space, and the P-39 radiators (coolant, oil and intercooler) were a real Rube Goldberg arrangement with poor airflow and no adjustable exit flaps. In other words maybe 25% functional.


Firstly: I was under the impression the problems the P-37 were the fact that the design's long nose and cockpit position would basically provide nearly nonexistent visibility over the nose, particularly when taxiing or pulling-g for deflection shots (if you thought the F4U was bad, you ain't seen nothin' yet!).


> 3. The tail cone was lengthened to compensate for moving the wing back about a foot. Moving the wing back reduces the distance from the CG to the tail, so the tail needed to be lengthened to compensate for stability. The tail wasn't lengthened to provide space for the mechanical second stage, there was already room there, just move the coolant tank up right behind the pilot and put the second stage where the coolant tank was.


Ok


> Now, the two stage mechanical V-1710-93 didn't start production until May 1943 but it's installation into the P-39 and P-40 would have transformed them into superb high altitude planes.


Yeah, it definitely would have -- I'm curious why we didn't procure the P-63?



Shortround6 said:


> With, the turbo charger.


Okay, I understand


> The air inlets were in the wing roots, the actual radiator and oil coolers were in the belly of the plane where the turbo had been.
> View attachment 522424
> 
> It is a model but gives the best idea as to what they changed. Radiator in the center, oil cooler on each side.


Ok, so the modified arrangement actually made it impossible to stuff the turbocharger in?


> No 4 is the primary, they didn't have enough airflow by a factor of 4 when climbing, a splitter and.or modified air intake is not going to solve the problem of too small an intercooler to begin with.


Would having two scoops on either side fix it?


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## P-39 Expert (Dec 20, 2018)

Zipper730 said:


> No problem
> I thought most aircraft had roll-bars up top?
> Was it a standard practice among any aviation service to mount the radio above the engine?
> Firstly: I was under the impression the problems the P-37 were the fact that the design's long nose and cockpit position would basically provide nearly nonexistent visibility over the nose, particularly when taxiing or pulling-g for deflection shots (if you thought the F4U was bad, you ain't seen nothin' yet!).
> ...


Usually not a rollbar with a bubble canopy. I think the normal back/head plate armor may have acted as a rollbar.

P-39 radios were often mounted above the engine right behind the pilot. Look for it in P-39 photos.

I'm really no advocate of the turbo in either the P-39 or P-40. Allison mechanical two stage would have been almost as powerful and much simpler/more compact. And an intercooler isn't needed except for WEP, so don't have WEP. Only helps under the critical altitude anyway.

We did procure the P-63, they began trickling out in October '43 with only 28 completed by the end of that year and another 1800 by the end of '44. Interesting how the engine had been in production since May but the first production P-63 rolled out in October. My plan (hindsight) would have been to just put the two stage in the P-39 and skip the P-63, but... 

By the way, the V-1710-93 two stage engine wasn't really a new engine, it was the same constantly updated engine that was going into contemporary P-38s, P-40s and P-51s. Only thing new was the separate mechanical second stage.


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## Zipper730 (Dec 20, 2018)

P-39 Expert said:


> Usually not a rollbar with a bubble canopy. I think the normal back/head plate armor may have acted as a rollbar.


How much weight would come out of removing the roll-bar and doors, and replacing with sliding canopy and adding normal armor?


> P-39 radios were often mounted above the engine right behind the pilot. Look for it in P-39 photos.


Okay, so this would be simply taking it from the XP-39 to it's natural evolution?


> I'm really no advocate of the turbo in either the P-39 or P-40.


Of course, as I said it's an intellectual exercise only based on what existed at the time.


> We did procure the P-63


Why didn't we approve them for combat?


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## wuzak (Dec 20, 2018)

Zipper730 said:


> We'd discussed the idea of moving the coolant tank forward and positioning the turbocharger rearward in another thread, but the differences designs of the mechanical secondary stage (P-63) and the turbocharger (XP-39) in terms of weight and airflow pathway differences would effectively make the aircraft considerably too tail-heavy. Suffice it to say, I don't know if there was inherently any problem with having the turbo underneath the engine (it was used on the XB-38), or if it was the devil in the details so to speak. Would it be feasible to angle the turbo in a position that avoided having it pointed rearwards or straight down but pointed backwards to some extent at an angle? It would avoid the extreme CG of having it pointed dead aft like the P-63's supercharger layout, and it would avoid pointing it down?



The turbo was not under the engine one the XB-38. In fact they were in the same place as on the B-17 - under the nacelle, behind the firewall.


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## Shortround6 (Dec 20, 2018)

P-39 Expert said:


> I'm really no advocate of the turbo in either the P-39 or P-40. Allison mechanical two stage would have been almost as powerful and much simpler/more compact. *And an intercooler isn't needed except for WEP, so don't have WEP. Only helps under the critical altitude anyway.*



Oh boy, that is a major misconception. Please see the P-38 and it's history of a too small intercooler affecting not only WEP but the altitude at which military power could be used as the military power was increased (big hint, the critical altitude/FTH dropped). You might want to tell P & W that the R-1830 in the F4F didn't need intercoolers or the R-2800 used the F4U, F6F and P-61 didn't need intercoolers if they didn't use WEP. 

And in actual fact, an intercooler can _INCREASE the Critical Altitude _of a single stage engine if you have the volume/space to install one ( One or two versions of the JUmo 211 used intercoolers on bombers) 

If you are not interested in increasing the critical altitude of the engine/plane just drop the whole 2nd stage, mechanical or turbo. 



> We did procure the P-63, they began trickling out in October '43 with only 28 completed by the end of that year and another 1800 by the end of '44. *Interesting how the engine had been in production since May but the first production P-63 rolled out in October*. My plan (hindsight) would have been to just put the two stage in the P-39 and skip the P-63, but...



The *minor fact* that the V-1710-93 two stage engine didn't complete it's official type test until *Nov 27th 1943* didn't have anything to do with this delay did it? 
P-63s completed before that date with V-1710-93 engines were flying with restrictions. Not really suitable for combat until the restrictions were lifted with completion of the test. 
f you aren't going to use WEP or use the second stage to improve altitude performance what is the point? 



> By the way, the V-1710-93 two stage engine wasn't really a new engine, it was the same constantly updated engine that was going into contemporary P-38s, P-40s and P-51s. Only thing new was the separate mechanical second stage.


Part of the delay in testing was the time needed to develop the water injection and the improved pistons and piston rings needed to support the higher power. Late model P-38 engines benefited from these parts. Allison powered P-51s had stopped being built months before these developments were put in production. Likewise late model P-40s had no real need of these parts. 

BTW the WER test wasn't done until Dec 1943


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## P-39 Expert (Dec 20, 2018)

Zipper730 said:


> How much weight would come out of removing the roll-bar and doors, and replacing with sliding canopy and adding normal armor?
> Okay, so this would be simply taking it from the XP-39 to it's natural evolution?
> Of course, as I said it's an intellectual exercise only based on what existed at the time.
> But we didn't approve them for combat...


No idea on the weight savings on removing the turnover bulkhead and replacing the doors with a sliding canopy. We'd lose the doors, turnover bulkhead, it's rear armor glass and couple of small pieces of integral armor, then add the sliding canopy and head/body armor plate. Regarding weight I think it's largely a wash but the better visibility would be worth it.

I think the XP-39s natural evolution should have been lighter P-39D/F/K/L, then the M/N/Q with the more powerful V-1710-85 with 9.6 gears, then the -93 with the mechanical second stage which instead became the P-63. Seems like Allison was thwarted at every turn. The 9.6 supercharger step up gears were planned for introduction around new years '42, but tests showed they couldn't stand the increased MP so the gears had to be redesigned wider and that took until about August/October of '43 (8-10 months) to get the engine into the P-39M in November. Then the second stage took seemingly forever, originally scheduled around new year '43 but not actually in production until May. But so goes wartime production of new engines. Ideally the AAF would have had the N model around the beginning of '42 and the two stage around the beginning of '43. But actually they had the N at the end of '42 and the two stage P-63 just starting production the end of '43.


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## P-39 Expert (Dec 20, 2018)

Shortround6 said:


> Oh boy, that is a major misconception. Please see the P-38 and it's history of a too small intercooler affecting not only WEP but the altitude at which military power could be used as the military power was increased (big hint, the critical altitude/FTH dropped). You might want to tell P & W that the R-1830 in the F4F didn't need intercoolers or the R-2800 used the F4U, F6F and P-61 didn't need intercoolers if they didn't use WEP.
> 
> And in actual fact, an intercooler can _INCREASE the Critical Altitude _of a single stage engine if you have the volume/space to install one ( One or two versions of the JUmo 211 used intercoolers on bombers)
> 
> ...


Yes, an intercooler would be slightly better, but the P-63 didn't have one and used water injection instead. The purpose of the second stage is to provide high altitude performance. Intercoolers or water injection were used for WEP which was used below the critical altitude where the P-39 already had excellent performance. The P-39 could have had WEP with water injection like the P-63 if considered necessary. 

The official test was completed in November '43 but less than 28 P-63s had been completed by then. No big deal.

If you don't use WEP for the two stage engine then you don't need the extra time to develop water injection or the improved pistons/rings, but these items benefited the other Allison engines also.


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## Zipper730 (Jan 1, 2019)

Shortround6 said:


> Much like trying to fly/fight with a large drop tank under the plane as far as drag goes. This was a mock up which is why the exhaust doesn't connect.


And that was a proposed intercooler? I'm curious if a belly radiator of some sort or two ventral radiators like the Spitfire would be workable



wuzak said:


> The turbo was not under the engine one the XB-38. In fact they were in the same place as on the B-17 - under the nacelle, behind the firewall.


So it was further back? On a fighter it'd be under the pilot more or less?



P-39 Expert said:


> No idea on the weight savings on removing the turnover bulkhead and replacing the doors with a sliding canopy. We'd lose the doors, turnover bulkhead, it's rear armor glass and couple of small pieces of integral armor, then add the sliding canopy and head/body armor plate. Regarding weight I think it's largely a wash but the better visibility would be worth it.


What about the fact that the control linkages would be easier to lay out because of the fact that you don't have any interruptions in the fuselage because of the doors?

There was supposedly a proposal for some kind of higher altitude V-1710 that got cancelled, I'd almost swear I remember hearing that with the P-39. Was this a single stage supercharger with a different gear ratio or a twin-speed set-up? Or was it twin-speed?


BTW: While I'm beating a dead horse, I remembered something you stated about the P-63 -- it's wing was moved back by a foot. The changes needed to mount the twin-stage supercharger added some weight and that's why some tankage was moved around and, while a turbocharger would be bulkier and heavier -- if the wings stayed forward by their normal amount, that would mean the space between the wing and the elevator would be much larger, and the wing seems to bear a significant amount of the aircraft's weight, and while I could definitely be wrong, would you be able to have enough leverage to make everything work?


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## P-39 Expert (Jan 1, 2019)

Zipper730 said:


> And that was a proposed intercooler? I'm curious if a belly radiator of some sort or two ventral radiators like the Spitfire would be workable
> 
> So it was further back? On a fighter it'd be under the pilot more or less?
> 
> ...


 *If there was no balance issue with the wing forward then there would be more leverage since the tail was farther away from the wing. But I believe the wing was moved back for balance with the extended tail. Hope this helps.*

*See above.*


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## Zipper730 (Jan 2, 2019)

P-39 Expert


1. Would the control cables be lighter if they didn't have to be run under the doors? I figure the more changes in direction would make the cables longer and that would add some weight. Frankly -- I'm not even sure why they added the doors to the design.

2. The V-1710-59 and -83: Were they single-speed or twin-speed? What full-throttle height were they expected to reach? Also, was there anyway for them to have known, reasonably speaking, that the gears would wear-down more easily and would need to be beefed up to work?

3. I thought you said the P-63 was lengthened because the wing was moved back about a foot, and then to give the tail adequate leverage, the longer fuselage gave the tail adequate leverage?

I would have figured

P-63
If the wing was moved rear-words, the C/L would also go back accordingly and
This would produce a stronger nose-down tendency, and need more elevator to provide level flight and maneuvering
Making the plane longer and with the elevator further back and you'd get adequate control

P-39
The P-39 had the wing forward by a foot and thus these problems didn't exist
A turbocharger mounted behind the engine would be somewhat heavier and possibly further back than the P-63's set-up which would make the plane tail-heavy
Admittedly a longer tail would just make the plane twitcher...

Would it have been acceptable to use two smaller turbochargers in lieu of one? It might sound silly but from what I remember the F6F prototypes used such a set-up. I'm not sure if they would have viewed that as acceptable either as a fix or a start.


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## P-39 Expert (Jan 2, 2019)

Zipper730 said:


> P-39 Expert
> 
> 
> 1. Would the control cables be lighter if they didn't have to be run under the doors? I figure the more changes in direction would make the cables longer and that would add some weight. Frankly -- I'm not even sure why they added the doors to the design. *Me either, but they stayed with the plane even through the upgrade to the P-63.*
> ...


 *P-39 was too small for a turbo, much less two. See above for more answers.  Just my opinions. Thanks.*


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## wuzak (Jan 2, 2019)

Allison did develop 2 speed supercharged versions of the V-1710, but none of these went into production.

These include:
V-1710-E29 - number built not known, ratios 7.48/9.60 (not certain)
V-1710-F11R - 1 built, ratios 6.44/8.80
V-1710-F16 - 0 built, ratios 6.44/8.80
V-1710-G2R/L - number built not known, ratios 7.76/8.80
V-1710-G2RA - number built not known, ratios 7.76/8.80
V-1710-G3R - 8 built, ratios 7.48/8.80
V-1710-G4L - 0 built, ratios 7.76/8.80
V-1710-G4R - 0 built, ratios 7.76/8.80

Per Dan Whitney, Vees for Victory, Appendix 3.

Note that the 2 speed supercharged engines had (or would have had) a larger impeller of 10.25in, compared with 9.5in for the standard impeller. I believe that 10.25in also happens to be the impeller size used in most single stage Merlins.


The Grumman F6F never had 2 turbochargers.

The XF6F-2 used an experimental Birman type mixed flow turbocharger - which probably means it had axial and centrifugal compressors. Initially it was to have been fitted with the R-2600 (not sure if that ever flew), but was later fitted with the R-2800 connected to the turbocharger.

Looked at a few sources, including Grumman F6F Hellcat - Wikipedia, Graham White, _Allied Aircraft Piston Engines of World War II_, and Graham White, R-2800 _Pratt & Whitney's Dependable Masterpiece_ and am not really any closer to knowing if the XF6F-2 ever flew with the R-2600.


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## Zipper730 (Jan 2, 2019)

P-39 Expert said:


> Me either, but they stayed with the plane even through the upgrade to the P-63.


So I'm not the only one to think the car door idea was weird?


> Both single speed, Allison never develped a two speed. FTH was about 15000', up from 12000' with the earlier 8.8 geared engines. Allison was hoping for the step up gears to be interchangeable from 8.1 (for two stage engines) to 8.8 (earlier altitude rated engines) to 9.6 (later altitude rated engines) but the load on the gears was too high at 9.6. This was a big disappointment as the earlier -59 engines were tested in late '41 but the redesign pushed them back to mid-late '42, almost a year later.


The P-39 had a little ram compression effects if I recall right?


> Yes.


Okay, we're on the same page regarding length


> *P-39 was too small for a turbo, much less two.*


It might sound silly but two turbos running in parallel can be quite small compared to one normal turbo. It would allow the effect of one big turbo in a smaller place. That's why I mentioned it.



wuzak said:


> Allison did develop 2 speed supercharged versions of the V-1710, but none of these went into production.


Do you have any idea why?


> Note that the 2 speed supercharged engines had (or would have had) a larger impeller of 10.25in, compared with 9.5in for the standard impeller. I believe that 10.25in also happens to be the impeller size used in most single stage Merlins.


Nice touch...


> The XF6F-2 used an experimental Birman type mixed flow turbocharger - which probably means it had axial and centrifugal compressors. Initially it was to have been fitted with the R-2600 (not sure if that ever flew), but was later fitted with the R-2800 connected to the turbocharger.


The first supercharger I knew of to use axial-flow was Eastman Jakob's work. Do you know anymore about the Birman-type?


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## wuzak (Jan 2, 2019)

Zipper730 said:


> So I'm not the only one to think the car door idea was weird?



The Tornado and early Typhoons had a car door.

The Spitfire had a half door on one side.




Zipper730 said:


> It might sound silly but two turbos running in parallel can be quite small compared to one normal turbo. It would allow the effect of one big turbo in a smaller place. That's why I mentioned it.



The problem is what is available.

General Electric were producing B and C type superchargers, which were both too big to be used as a pair on the V-1710.

Two small turbos probably took up more space than one large turbo, though.

Where two turbos were used instead of one was for performance reasons. On C-series turbocharger may not have provided enough air for the R-3350 in the B-29, so they used two B-series turbos.




Zipper730 said:


> Do you have any idea why?



No orders.

One of the two speed engines was to be for the post-war DC-4M as an alternative replacement for the Merlins. But that didn't go ahead.





Zipper730 said:


> The first supercharger I knew of to use axial-flow was Eastman Jakob's work. Do you know anymore about the Birman-type?



A.A. Griffiths wrote a paper in 1927 about an axial flow compressor for a gas turbine. Over the next decade he, and his associates, designed and experimented with axial flow compressors, including a counter-rotating axial flow compressor.

Germany certainly did have people working on axial flow compressors in the late '30s.

As for the Birman type turbo, I don't have any specific information, other than it was tried on an XF6F-2, one of the XF4Us (can't recall dash number) and was to be fitted to one of the XP-60s powered by the V-1710 (not sure if it was XP-60A or XP-60B), that it didn't work well and was unreliable, and it was described as a "mixed flow" design.


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## P-39 Expert (Jan 3, 2019)

Anybody know anything about the Birman supercharger?


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## wuzak (Jan 3, 2019)

In White's book about the R-2800 he calls the unit used in the XF4U-3 a Turbo Engineering turbocharger and XF6F-2 as a Birman (Turbo Engineering) turbocharger.

This is what the installation looked like on the XF4U-3







From Category:XF4U-3 Corsair - Wikimedia Commons

and on the XF6F-2





from Wikimedia Commons

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## Shortround6 (Jan 3, 2019)

Zipper730 said:


> Would it have been acceptable to use two smaller turbochargers in lieu of one? It might sound silly but from what I remember the F6F prototypes used such a set-up. I'm not sure if they would have viewed that as acceptable either as a fix or a start.



The problem wasn't so much the turbo itself and if comparing turbos try to compare turbos of the same generation and pressure ratios. Car (or diesel truck) turbos are all over the place. 

The problem is so much the turbo itself but the ducts needed to get the exhaust to the turbo, the ducts needed to get the air to and from the turbo and the need for an intercooler (which can take up more volume than the turbo) and without the turbo much of the altitude advantage of using a turbo goes away.

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## Zipper730 (Jul 29, 2019)

wuzak said:


> No orders.


When were the superchargers you listed developed?


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## wuzak (Jul 29, 2019)

Zipper730 said:


> When were the superchargers you listed developed?



Late in the war.

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## Zipper730 (Jul 29, 2019)

wuzak said:


> Late in the war.


Like 1944 or 1945?


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## fubar57 (Jul 31, 2019)

Zipper730 said:


> Like 1944 or 1945?



I can't even......


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## tyrodtom (Jul 31, 2019)

Zipper730 said:


> P-39 Expert
> 
> 
> 1. Would the control cables be lighter if they didn't have to be run under the doors? I figure the more changes in direction would make the cables longer and that would add some weight. Frankly -- I'm not even sure why they added the doors to the design..


Why would you think the doors would require longer control cables ? The control stick yoke has connections in the floor right in front of the seat, like any other aircraft of that era, door or canopy hasn't got any effect on the length of the control cables. 
And would you really think the weight of the control cables is any significant weight ?


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## Zipper730 (Jul 31, 2019)

tyrodtom said:


> Why would you think the doors would require longer control cables?


Looking back on it, I don't actually know why I said what I did. I'm curious if any linkages ran along the sides? It seems that the aft sliding canopy would be more conventional and easier.


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## GrauGeist (Aug 1, 2019)

Zipper730 said:


> Looking back on it, I don't actually know why I said what I did. I'm curious if any linkages ran along the sides? It seems that the aft sliding canopy would be more conventional and easier.


The P-39/P-63 had car doors because the cockpit framing provided structural support to the airframe.

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## P-39 Expert (Aug 1, 2019)

GrauGeist said:


> The P-39/P-63 had car doors because the cockpit framing provided structural support to the airframe.


I think the car doors were part of the turnover structure which was part of the framing. In my opinion it didn't need the turnover structure, just added weight that most other fighters didn't have. Without the turnover structure a bubble canopy could have been used eliminating the car doors and improving visibility. The framing wasn't really necessary for structure, the whole forward fuselage was based on the two longitudinal beams that ran from the nose to aft of the engine providing a very robust "canoe" that also kept the drive shaft straight. Altogether a good structural solution even without the turnover structure or framed canopy. Just my 2 cents worth.

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## Zipper730 (Aug 1, 2019)

fubar57 said:


> I can't even......


I meant like 1944 late, or 1945 late. Depending on interpretation, either could be considered late.


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## Zipper730 (Aug 1, 2019)

My questions...

V-1710-35
What's the engine critical altitude (no ram)?
What's the aircraft critical altitude (ram) on the P-39D?

V-1710-59
What was the proposed engine critical altitude (no ram)?


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## P-39 Expert (Aug 2, 2019)

Zipper730 said:


> My questions...
> 
> V-1710-35
> What's the engine critical altitude (no ram)?
> ...


1. -35 was 11700'-12000' no ram without backfire screens. Critical altitude for the P-39D was 13800' with ram and no backfire screens per wwiiaircraftperformance.org.
2. -59 was 1100hp at 13800' (coincidence) with screens. Later 9.6 production models without screens was 1125hp at 15500'.


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## pbehn (Aug 2, 2019)

Zipper730 said:


> I meant like 1944 late, or 1945 late. Depending on interpretation, either could be considered late.


Well yes, since the USA formally entered the war days before 1942, you couldn't have 1942 or 1943 as late could you, it is nearer the beginning than the end. For many aspects of aviation the end was coming in mid 1944 with all sorts of people looking to the future.

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## Zipper730 (Aug 2, 2019)

P-39 Expert said:


> 1. -35 was 11700'-12000' no ram without backfire screens. Critical altitude for the P-39D was 13800' with ram and no backfire screens per wwiiaircraftperformance.org.


And horsepower was 1150? Do you have a chart that lists horsepower along with TAS along with Altitude. I'm trying to figure out how much speed would be added with the different altitudes.


> 2. -59 was 1100hp at 13800' (coincidence) with screens. Later 9.6 production models without screens was 1125hp at 15500'.


And without the screens, I would speculate it'd probably be less?


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## Zipper730 (Aug 2, 2019)

P
 pbehn

S
 Shortround6

W
 wuzak


What gearing would you guess the V-1710 would have needed to get 16200-16500' FTH without ram-compression?


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## P-39 Expert (Aug 2, 2019)

Zipper730 said:


> And horsepower was 1150? Do you have a chart that lists horsepower along with TAS along with Altitude. I'm trying to figure out how much speed would be added with the different altitudes.
> And without the screens, I would speculate it'd probably be less?


Yes horsepower was 1150 for the -35. I do have charts, but the wwiiaircraftperformance.org site has performance charts for the P-39D that show critical altitude of 13800' with about 370mph of ram. That's your difference in ram and no ram. Also the critical altitude for climb is 12400' and that would be with only about 170mph of ram. 
The backfire screens meant less power as they reduced manifold pressure, deleting the screens gained manifold pressure.

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## Zipper730 (Aug 2, 2019)

P-39 Expert said:


> I do have charts, but the wwiiaircraftperformance.org site has performance charts for the P-39D that show critical altitude of 13800' with about 370mph of ram.


The reason the charts can be useful is that the ones on WWII aircraft performance page don't list horsepower for speed/altitude. I was trying to compute what different critical altitude.


> The backfire screens meant less power as they reduced manifold pressure, deleting the screens gained manifold pressure.


Do you have any idea how much less?


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## pbehn (Aug 2, 2019)

Zipper730 said:


> P
> pbehn
> 
> S
> ...


Why would you ask me and why do you want to know. What use is a guess anyway?


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## Shortround6 (Aug 2, 2019)

Zipper730 said:


> P
> pbehn
> 
> S
> ...




It is getting self defeating. 

there are a chart and an explanation on Page 344 of "Vees for Victory" showing the power needed by the supercharger to flow various amounts of air using different gear ratios. 

The example goes something like this, Assume 10,000lbs of air flowing through the engine per hour, that is enough to make 1672hp in the cylinders (IHP) but the engine uses 147hp in internal friction, power internal drives and pumps. This cuts the power to about 1525hp and the 6.44 gears need about 110hp to supply that much air leaving you with 1415hp to the prop.
an engine with 9.60 gears needs about 240hp to drive the supercharger leaving you with 1285hp. 

You also have a 9.5in impeller spinning at 28,800rpm with the 9.6 gears. which means the tip speed of the impeller blades is 1193fps if I have done the math right. 
the speed of sound at sea level on a standard day is 1125fps. the higher pressure and temperature inside the supercharger allow for higher speeds before the tips break the sound barrier but obviously things are getting a bit tight. 

Then you have the heat problem, the faster you spin the impeller the more you heat the intake charge and the closer you get to the detonation limits which can cause the peak pressure to be reduced. 8.80 gear engines were allowed to use 60 in of MAP for WER while the 9.60 gear engines were limited to 57in. 

This also shows up with either (or both) reduced power for take-off or at low altitudes. The 8.80 gear engines in the P-40 made 1490 hp at 4300ft using 56in of boost (and 1580hp at 2500ft using 60in with the -73 engine). the 7.48 geared engine in the A-36 made 1500hp at 5400ft using 52in of boost. the 9.60 engines were rated at 1360hp at sea level using 57in. snf by opening the throttle made 1410hp at 9500ft at 57in. 

They paid for that 3-3,500ft of altitude by losing about 80-125hp at sea level. 

trying to go for 16,500ft is going to get you more of the same, less power down low, pushing closer to the detonation limits and pushing the engine harder for that last 1000ft of altitude.

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## Zipper730 (Aug 2, 2019)

pbehn said:


> Why would you ask me and why do you want to know. What use is a guess anyway?


Curiosity, plus you can always learn something. Shortround's response was pretty solid.


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## wuzak (Aug 3, 2019)

Zipper730 said:


> When were the superchargers you listed developed?





Zipper730 said:


> Like 1944 or 1945?



It was discussed at a meeting held in Indianapolis on Tuesday 17th October 1944, involving Allison, GM Executives, representatives of the USAAF and a couple of airframe manufacturers.

The meeting was set for 9am, but started at 8:56am because all attendees had arrived early, according to the minutes of the meeting.

The resolutions of the meeting included Allison agreeing to develop a 2 speed supercharger to maybe, possibly, suck a little bit less. The USAAF agreed to deliver a V-1650-1 to Allison so it could be copied!

The timetable was tight, the first one was to be on the dyno no more than one week of the delivery of the V-1650-1.

The USAAF delivered the V-1650-1 on Thursday 28th December 1944. Allison asked for an extension on their timeline, since their experimental shop gear cutter was out sick with the flu that week, the USAAF agreed because they didn't really give a shit about a 2 speed V-1710 at that stage.

The 2 speed V-1710 finally made it on the dyno on Sunday 4th March 1945. By that time no-one really cared, including Allison, because they were all about jet engines.





































Or not.

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## P-39 Expert (Aug 3, 2019)

Zipper730 said:


> The reason the charts can be useful is that the ones on WWII aircraft performance page don't list horsepower for speed/altitude. I was trying to compute what different critical altitude.
> Do you have any idea how much less?


The actual horsepower was listed on the performance memorandums in 5000' increments, but not on the charts. Hope that helps.

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## Zipper730 (Mar 6, 2020)

Sometimes I revisit old threads to ask questions I should have asked earlier. I also sometimes actually find answers to some degree that at least help out with the various discussions in the thread.



wuzak said:


> Lighter weight and better aero.


I should have asked this awhile back, but out of curiosity, what kind of aerodynamic improvements would have been do-able at the time?


> In White's book about the R-2800 he calls the unit used in the XF4U-3 a Turbo Engineering turbocharger and XF6F-2 as a Birman (Turbo Engineering) turbocharger.


I did some checking in _Warbird Tech Series: Vought F4U Corsair_ and it says the F4U-3 was to be fitted with a Birman type supercharger as well. From what I read, the goal was to increase the critical altitude a whole lot. They even managed to get up to altitudes of around 50,000 feet (I'm not sure if the cockpit was pressurized, but that seems to be the altitude where it'd be a very good idea!). The problem with the Birman was that it's reliability sucked, MTBF was something like one flight.



P-39 Expert said:


> That's the big question, why did it take so long to develop the mechanical two stage supercharger?


From what it would appear, the closest thing I can find is that Oliver Echols seemed to have a bias against twin-stage supercharging, and allocated all funding to turbochargers.

Often, from what I remember: There was a tendency to see the turbochargers designed in ways that reduced efficiency quite a bit. When Hooker came around, he actually identified numerous problems in the formulas that gave rise to the supercharger design. One thing that I remember being told was that (at least with twin-staged designs), the two stages were the same diameter (later on, it would be found that having the first stage larger and the second stage smaller would give better performance).

Interestingly, I'm not sure if there was enough money to have developed turbos for mass-production and twin-stage supercharging.


> -59 was 1100hp at 13800' (coincidence) with screens. Later 9.6 production models without screens was 1125hp at 15500'.


This assumes the intake was revised with more spacing to eliminate turbulent airflow, correct?

From what it appears, the problems with the 9.6 designs was that they seem to have miscalculated the amount of structural strength needed for the engine to hold together while driving a larger supercharger.


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## P-39 Expert (Mar 6, 2020)

Zipper730 said:


> Sometimes I revisit old threads to ask questions I should have asked earlier. I also sometimes actually find answers to some degree that at least help out with the various discussions in the thread.
> 
> I should have asked this awhile back, but out of curiosity, what kind of aerodynamic improvements would have been do-able at the time?
> I did some checking in _Warbird Tech Series: Vought F4U Corsair_ and it says the F4U-3 was to be fitted with a Birman type supercharger as well. From what I read, the goal was to increase the critical altitude a whole lot. They even managed to get up to altitudes of around 50,000 feet (I'm not sure if the cockpit was pressurized, but that seems to be the altitude where it'd be a very good idea!). The problem with the Birman was that it's reliability sucked, MTBF was something like one flight.
> ...


Regarding the 9.6 design, the engine held together just fine but the actual supercharger gears themselves couldn't take the strain of the step up from 8.8. The actual drive gear had one more tooth and the driven gear had one less tooth to go from 8.8 to 9.6. Same thing in reverse to go from 8.8 to 8.1 (for two stage engines). Allison was hoping to just substitute the 9.6 gears for the 8.8 gears and reap the benefit of more horsepower at higher altitude, but that was not to be without strengthening the gears by widening them which took about a year. Dang. The gear ratio would go all the way up to 11.6 without increasing the diameter of the gears, but Allison never made production engines higher than 9.6.


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## Zipper730 (Mar 6, 2020)

P-39 Expert said:


> Regarding the 9.6 design, the engine held together just fine but the actual supercharger gears themselves couldn't take the strain of the step up from 8.8.


So, they kind of were suffering a bit of wishful thinking, combined with miscalculation?


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## P-39 Expert (Mar 6, 2020)

Zipper730 said:


> So, they kind of were suffering a bit of wishful thinking, combined with miscalculation?


I don't know exactly, but you have to think that they weren't expecting the gears to wear that quickly. Cost them a year to redesign the accessories case to accommodate the wider gears. Could have had the new engine in early '42 instead of late '42. Of course they still had to deal with the backfire screens etc until mid '42 when they adopted the aluminum intake manifold and deleted the backfire screens. And raised the military power limit from 5 minutes to 15 minutes.


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## wuzak (Mar 6, 2020)

Zipper730 said:


> I should have asked this awhile back, but out of curiosity, what kind of aerodynamic improvements would have been do-able at the time?



You quoted me without a link to the original post so the question lacks context.


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## Shortround6 (Mar 6, 2020)

Zipper730 said:


> From what it appears, the problems with the 9.6 designs was that they seem to have miscalculated the amount of structural strength needed for the engine to hold together while driving a larger supercharger.



The supercharger was the same size. The engine would hold together, the supercharger gears would not.

As a rule of thumb a supercharger's required power to drive the impeller goes up the square of the speed of the impeller. 
So one might expect the Allison supercharger drive gears to need to handle abou 19% more power using the 9.6 ratio than the 8.80 ratio and indeed an Allison chart shows a bit over 210hp needed to flow 10,000lbs per hour with the 8.80 gears and just over 250hp to flow 10,000lbs per hour using the 9.60 gears. Allison may have decided not to make the same mistake twice and widened the gears to handle any foreseeable increase in required power.

There is an upper limit as to how fast you can spin an impeller. If the tip speed approaches the speed of sound the efficiency goes to pot in a hurry, The speed of sound is higher in the high pressure, high temperature environment inside the supercharger but the limit is there. 

One can understand why Allison designed a new supercharger for the later engines (the G series) rather than try to simply increase the gear ratio.


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## Zipper730 (Mar 7, 2020)

P-39 Expert said:


> I don't know exactly, but you have to think that they weren't expecting the gears to wear that quickly.


Ironically, had they done more work early on, they'd have saved themselves a lot of trouble later.


> Could have had the new engine in early '42 instead of late '42.


That would have had a significant effect on their usefulness in the war. As for the differences of ram compression by using a redesigned intake, as seen on the P-63 seems to increase critical altitude by around 900 feet over the P-39D (2000 vs 2900)



wuzak said:


> You quoted me without a link to the original post so the question lacks context.


Sorry about that.

Lighter weight and improved Aerodynamics: Page 2, Post 32
Use of the Birman supercharger: Page 5, Post 88



Shortround6 said:


> The engine would hold together, the supercharger gears would not.


I remember this discussion coming up regarding the amount of power related to the square of the supercharger RPM. I figure, this was already known prior to the decision to redesign the gear, so I figure the culprits would be...

The new supercharger gear would do better than it did: Miscalculation
The desire to make minimal design changes: Laziness/Over-ambition
An improperly calibrated and polished crystal-ball.


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## Shortround6 (Mar 7, 2020)

I would note the British ran into a somewhat related problem with the Merlin once higher boost levels were being put into use. The original supercharger drive (exact parts I don't know but I think it was a quill shaft?) was good for up to 15lbs of boost (perhaps 16lbs depending on gear ratio/altitude?) but had to modified for 18lb of boost, this is for the singel stage engines. 

Allison may have been using the RR development technique, flog it until it breaks, modify/fix it and flog it again.


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## wuzak (Mar 9, 2020)

Zipper730 said:


> I should have asked this awhile back, but out of curiosity, what kind of aerodynamic improvements would have been do-able at the time?





Zipper730 said:


> Sorry about that.
> 
> Lighter weight and improved Aerodynamics: Page 2, Post 32
> Use of the Birman supercharger: Page 5, Post 88




In terms of the of the Birman turbocharger, I don't think it ever made it into production and may not have been available for flight testing at the time of the XP-39.

In terms of aerodynamics there are many improvements that could have been made with the XP-39 given time and effort.

However, the turbo was not reliable at the time of the XP-39 (with issues on the XP-37 and YP-37) and reworking the aerodynamics was easier without the turbo.


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## Zipper730 (Mar 9, 2020)

Shortround6 said:


> I would note the British ran into a somewhat related problem with the Merlin once higher boost levels were being put into use. The original supercharger drive (exact parts I don't know but I think it was a quill shaft?) was good for up to 15lbs of boost (perhaps 16lbs depending on gear ratio/altitude?) but had to modified for 18lb of boost, this is for the singel stage engines.


But it seems that it took them a lot quicker to overcome their problems than Allison did with their higher geared superchargers...



wuzak said:


> In terms of the of the Birman turbocharger, I don't think it ever made it into production and may not have been available for flight testing at the time of the XP-39.


I'm pretty sure it didn't make production either. I'm curious what variables caused such a short service life. I'm curious where would one even look to find information on the Birman supercharger?


> In terms of aerodynamics there are many improvements that could have been made with the XP-39 given time and effort.


Do you have any examples?


> However, the turbo was not reliable at the time of the XP-39 (with issues on the XP-37 and YP-37) and reworking the aerodynamics was easier without the turbo.


That makes sense. There was this book I got recently which showed a bunch of aircraft designs, including proposals for the P-39. Interestingly, it actually showed one design with the turbo lying flat in the wing. It was a very unusual configuration.


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## wuzak (Mar 9, 2020)

Zipper730 said:


> Do you have any examples?



Canopy and cooling designs.


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## Zipper730 (Mar 9, 2020)

wuzak said:


> Canopy and cooling designs.


I thought the XP-39 was already streamlined by the time of the XP-39B (lowered canopy)? As for cooling changes, what would you have recommended for the production P-39 variants?


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## Zipper730 (Jul 7, 2020)

P-39 Expert said:


> I don't know exactly, but you have to think that they weren't expecting the gears to wear that quickly.


While I've touched on this before, how long did the 8.8 gears operate for? Do you know how much the 9.6 versions were shortened in comparison?


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## P-39 Expert (Jul 7, 2020)

Zipper730 said:


> While I've touched on this before, how long did the 8.8 gears operate for? Do you know how much the 9.6 versions were shortened in comparison?


The 8.8 gears were fine. The early 9.6 gears wouldn't pass the standard US 150 hour test. These were the -59 and -61. In one example the 9.6 gears completed the test but failed during inspection after the test. Redesign of the case to make the gears wider cured this problem with the first examples of the -83 completed in late '42.


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## Zipper730 (Jul 8, 2020)

P-39 Expert said:


> The 8.8 gears were fine. The early 9.6 gears wouldn't pass the standard US 150 hour test. These were the -59 and -61. In one example the 9.6 gears completed the test but failed during inspection after the test. Redesign of the case to make the gears wider cured this problem with the first examples of the -83 completed in late '42.


By wider, you mean the teeth right?


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## P-39 Expert (Jul 9, 2020)

Zipper730 said:


> By wider, you mean the teeth right?


I would think the whole gear would be wider, certainly the teeth.


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## Zipper730 (Jul 9, 2020)

P-39 Expert said:


> I would think the whole gear would be wider, certainly the teeth.


With the knowledge of the time, would there have been anyway to sidestep the problem (i.e. other than a well-maintained crystal ball)?

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## glennasher (Jul 9, 2020)

You folks sure are over-working your brains trying to make a silk purse out of a hunk of 'meh'. It didn't take the USAAC long to decide it wasn't worth the effort, I don't see why you don't do the same.

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## Zipper730 (Jul 9, 2020)

glennasher said:


> You folks sure are over-working your brains trying to make a silk purse out of a hunk of 'meh'.


We're fond of hypothetical situations.

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## P-39 Expert (Jul 9, 2020)

glennasher said:


> You folks sure are over-working your brains trying to make a silk purse out of a hunk of 'meh'. It didn't take the USAAC long to decide it wasn't worth the effort, I don't see why you don't do the same.


Hardly. Cost the AAF an extra 100 horsepower for the entire year of 1942. Doesn't sound like much, only added 800fpm climb and 20mph speed at 20000' in the P-39. Could have used that extra performance in New Guinea, Guadalcanal and North Africa.


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## P-39 Expert (Jul 9, 2020)

Zipper730 said:


> With the knowledge of the time, would there have been anyway to sidestep the problem (i.e. other than a well-maintained crystal ball)?


Vees for Victory blamed the excessive wear on manifold pressure. Until mid year 1942 the AAF was still clinging to the port backfire screens even though Allison told them repeatedly that they weren't needed with the new aluminum intake manifold that wouldn't catch fire like the magnesium intake manifold then in use. The port screens increased manifold pressure between the impeller and the screens but the screens reduced MP to the cylinders, effectively causing the supercharger gears to work harder than necessary. Earlier elimination of the screens and the freer airflow from the Impeller through the intake manifold to the cylinders would have reduced MP at the impeller and increased MP to the cylinders, which was the goal. The reduced MP at the impeller may have been enough to reduce wear on the 9.6 step up gears and allow the engine to pass it's 150 hour test. Spitballing here, but that's all I got.

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## Zipper730 (Jul 9, 2020)

P-39 Expert said:


> Vees for Victory blamed the excessive wear on manifold pressure.


What pressures were the 9.6 V-1710's to run at?


> Until mid year 1942 the AAF was still clinging to the port backfire screens


Why were they so insistent on the backfire screens? Why did they believe they were needed when Allison told them they weren't?


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## Shortround6 (Jul 9, 2020)

Zipper730 said:


> Why were they so insistent on the backfire screens? Why did they believe they were needed when Allison told them they weren't?



Let's just say that Allison's reputation in late 1941 and early 1942 wasn't quite they may have wished for. 
Several lost aircraft due to engine fires and back fires, a few more that needed rebuilding. About 170 engines in 1940 that needed new crankshafts and crankcases to meet the desired overhaul life. And a few other problems? 

Allison had built only a few dozen engines in up until mid/late 1939. Allison made good on the 170 engines, Allison introduced new parts and refitted some of the first engines. Allison engines rapidly improved in quality and engine life while Allison greatly increased production and became a first class supplier but in Dec 41/Jan 42 in the weeks after Pearl Harbor the Army may not wanted to take Allisons word that the problem/s had been fixed. Too much was riding on it.

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## P-39 Expert (Jul 10, 2020)

Zipper730 said:


> What pressures were the 9.6 V-1710's to run at?
> Why were they so insistent on the backfire screens? Why did they believe they were needed when Allison told them they weren't?


44.5 military and 57.0 WEP.

Good question, the Brits were running their V-1710s in P-40s with virtually no backfires by adjusting valve clearance. I believe a backfire into a magnesium intake manifold could cause a fire.


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## Peter Gunn (Jul 10, 2020)

glennasher said:


> You folks sure are over-working your brains trying to make a silk purse out of a hunk of 'meh'. It didn't take the USAAC long to decide it wasn't worth the effort, I don't see why you don't do the same.


I gave you bacon for the "hunk of 'meh'" phrase, man that got me laughing on what was starting out as a rather 'meh' morning, thanks. 

P.S. I agree with you, but the hypotheticals here make for very good reading.

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## Zipper730 (Jul 12, 2020)

Shortround6 said:


> Let's just say that Allison's reputation in late 1941 and early 1942 wasn't quite they may have wished for.
> Several lost aircraft due to engine fires and back fires, a few more that needed rebuilding. About 170 engines in 1940 that needed new crankshafts and crankcases to meet the desired overhaul life. And a few other problems?


A backfire is caused by fuel burning in the exhaust system right? What causes this?



P-39 Expert said:


> 44.5 military and 57.0 WEP.


Is this the same as the 8.8?


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## P-39 Expert (Jul 12, 2020)

Zipper730 said:


> A backfire is caused by fuel burning in the exhaust system right? What causes this?
> 
> Is this the same as the 8.8?


As I understand it a backfire is premature detonation of the fuel due to excessive MP or RPM. The backfire screens theoretically kept the detonation from getting back into the intake manifold which early on (pre mid '42) was made of magnesium and could actually catch fire.

8.8 geared Allisons were 51 for takeoff and 42 for military.

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## GregP (Jul 13, 2020)

Backfire screens were not needed once a pilot learned how to start an Allison. I've been present or started myself more than 40 Allisons and only have seen a backfire twice, both times when a new owner was learning to start it. There was no damage either time.

This is an Allison backfire screen from my collection:






This screen was inserted into the intake manifold right at the intake valve and, as you can imagine, interfered with the intake flow to a not insignificant degree. The outside envelope fit perfectly into the intake manifold.

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## P-39 Expert (Jul 13, 2020)

GregP said:


> Backfire screens were not needed once a pilot learned how to start an Allison. I've been present or started myself more than 40 Allisons and only have seen a backfire twice, both times when a new owner was learning to start it. There was no damage either time.
> 
> This is an Allison backfire screen from my collection:
> View attachment 587975
> ...


Thanks for the excellent photo. I understand they also easily became clogged and had to be cleaned periodically. Clogged screens further restricted airflow. They were finally done away with in mid-'42.


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## Zipper730 (Jul 13, 2020)

P-39 Expert said:


> 8.8 geared Allisons were 51 for takeoff and 42 for military.


That's actually higher. It seems that it would be less likely to backfire. If 
S
 Shortround6
is correct that it had to do with starting, I'm curious what conditions tended to cause it to backfire on starting.


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## GregP (Jul 13, 2020)

A backfire can be caused by either too rich or too lean a mixture. The mixture WANTS to be at about 14.7 : 1. Anything with a mixture less than 14.7 : 1 is rich. If it gets below somewhere around 11 - 12 : 1, it can backfire. Anything with more than 14.7 : 1 is lean. Again, if it gets above 17 - 18 : 1, it can backfire.

Most pilots in WWII wanted to cruise slightly rich, at about 13.5 : 1 or so. It has only been since digital fuel injection that we cruise lean. Without the computers, it is way too easy yo ruin an engine with lean cruising. You have to be alert to changes in altitude, altimeter setting, and cylinder head temperature. Generally, you lean as you go up and adjust to the richer side as you descend. You can do it with cylinder head temperature alone, but a deliberate descent or climb or a noticeable change in atmospheric pressure will make you look at mixture, too.

Once you as an operator learns the right position of the mixture lever for proper operation, you can do correctly it with your eyes closed.

Here is a page from an Allison engine manual that shows how to tell by looking at the exhaust if it is rich or lean.

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## Zipper730 (Jul 13, 2020)

GregP said:


> A backfire can be caused by either too rich or too lean a mixture.


With over-priming a problem, I assume on starting, rich would be more likely to cause trouble?


> Most pilots in WWII wanted to cruise slightly rich, at about 13.5 : 1 or so. It has only been since digital fuel injection that we cruise lean.


Wait, I remember hearing that later in the war (1944-45) you'd see auto-lean used at high altitudes?


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## GregP (Jul 13, 2020)

Usually rich from over-prime, as you say, when starting, but not necessarily.

As far as "auto" settings, yes ... you CAN let it run in auto-mixture. But, you can also adjust the mixture to suit yourself manually, just as you can in a typical light aircraft today.

Charles Lindberg was famous for coming home from a long flight in a P-38 with WAY more fuel than his mates. Using his leaning techniques, the range of the P-38 was considerably lengthened in the Pacific. But, it worked just as well anywhere else. He was cruising at higher manifold pressure and lower rpm. Up to a point, it was fine and did no harm. The trick is knowing when that point has been reached. At the time, it wasn't well-known. Today, it is and we let the run modern general aviation piston engines on the lean side of peak EGT with very accurate digital temp gauges. Back in the early 1940s, that was either a great recipe for winding up in the water or a good way to fly long over-ocean flights. Lindberg knew the difference.


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