Could you have designed a better P-39?

[Shortround6]

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 the 1400HP version was superseded by the 2000HP version.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

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

And, as in many things, timing is everything.

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

 

[davparlr]

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

I have no reason to argue this point.

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

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

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 the 1400HP version was superseded by the 2000HP version.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

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

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

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

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

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

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

 

[Hawk 75]

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

 

[Colin1]

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

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

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

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

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

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

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

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

 

[Shortround6]

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

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

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

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

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

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

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

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

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

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

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

 

[Markus]

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

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

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

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

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

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

 

[krieghund]

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

 

[drgondog]

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

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

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

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

 

[krieghund]

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

 

[Colin1]

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

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

 

[Clay_Allison]

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

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

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

 

[Colin1]

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

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

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

 

[gjs238]

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

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

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

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

The P-40 was never designed with a turbocharger.

 

[Clay_Allison]

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

The P-40 was never designed with a turbocharger.

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

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

 

[drgondog]

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

Dead on.

 

[Shortround6]

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

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

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

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

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

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

 

[Clay_Allison]

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

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

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



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

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

 

[Shortround6]

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

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

 

[Markus]

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

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

 

[krieghund]

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

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



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

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