GregP
Major
I have seen I said in here that there is no way the XP-39 went 390 mph, but have seen no proof of same except for some various quoting of poorly-documented wind tunnel testing done on the machine either before or after the turbocharger was deleted. The posters don't bother to say. There is, however, some considerable second-tier sources that say otherwise.
There is a digitized NACA report (Engineer in Charge: A History of Langley Aeronautical Laboratory 1917 – 1958) which states that the XP-39 DID achieve 390 mph using 1,150 hp @ 20,000 feet. But, it did so at a gross weight of 5,550 lbs at takeoff.
URL: Engineer in charge .
Look around page 199. Langley calculated that the normal P-39 with armament would come in about a ton heavier, meaning 7,550 lbs, and the maximum speed would be expected to be around 340 mph with the same powerplant setup. They achieved a wind tunnel drag reduction in a fully-faired model of 26%. This would mean a top speed of about 429 mph at 20,000 feet with an engine of 1,350 hp. The head of the FST team said that the additional air required to cool the extra HP would likely limit the maximum speed to 410 mph. This was with a "fully-faired" model that could not be achieved in real life. The reality was bound to be something less.
Bell incorporated enough changes to drop the drag by 16% instead of the theoretical 26%. But then, the engine had the turbocharger removed and the resulting engine only produce 1,090 HP. That is 60 less hp than the engine that had driven the XP-39 to 390 mph and 260 HP less than the 1,350 HP the FST team lead had assumed when he calculated 410 mph. Additionally, the 1,090 HP was achieved at 15,000 feet rather than 20,000 feet. Accounting for the 16% drag reduction, the loss of horsepower, and the added air density at the lower latitude, you get the result that the XP-39B achieved a maximum speed of 375 mph in the first flight trials.
The end result was that Bell and Air Corps expressed satisfaction with the results and asked Bell to produce the balance of the YP-39s without turbochargers. All this was done while Ben Kelsey was in Europe working on another project.
If you go to WWIIaircraftperformance.org and look at the P-39 flight test data for a YP-39 (No. 40-30), the rate of climb started out at 3,600 fpm and was still 2,260 fpm at 15,000 feet. Moving to a P-39C (No. 40-2988), the maximum speed was 379 mph at 16,100 feet and the rate of climb started out at 3,720 fpm and was still 2,360 fpm at 16,750 feet. If we move yet again to a P-39N (No. 42-4400 test dated 17 Oct 1942), we see a maximum speed of 398.5 mph at 9,700 feet (critical altitude). The climb data for this airplane shows 3,320 fpm at sea level and 3,920 fpm at 11,000 feet. It was still climbing at 3,340 fpm at 15,000 feet, but tapered off to 2,630 fpm at 20,000 feet. 20,000 feet was achieved in 5.83 minutes. I believe the P-39N was at WEP power for the test.
The performance above for test data located at wwiiaircraftperformance.org does NOT seem to me to indicate the "dog" everyone seems to think it was. Rather it indicates that the aircraft was a decent-performing aircraft when operated below 20,000 feet. I will not get into the C.G. issues here. I think we all know the aircraft could be made to tumble if stalled when the ammunition was expended. That was proved post-war. But it doesn't seem to me to have the bad performance often attributed to it, at least in flight test, when operated at the altitudes it was produced to address in decisions made before we entered the war. I know the climb rates at Military power will be less than rates at WEP, but WEP was used by many aircraft in the heat of combat for short durations.
I am NOT saying the P-39 was a great airplane, or even that it was a good choice for a general fighter, but I am saying the test data seems to indicate that it doesn't seem to be as bad as is commonly assumed when used within its envelope.
Please note: This is NOT intended to start a debate on the use of the P-39 in the ETO, which was a higher-altitude theater of war. Rather, it seems to show why the Soviets had such a good experience with the P-39 in their lower-altitude, ground support war.
There is a digitized NACA report (Engineer in Charge: A History of Langley Aeronautical Laboratory 1917 – 1958) which states that the XP-39 DID achieve 390 mph using 1,150 hp @ 20,000 feet. But, it did so at a gross weight of 5,550 lbs at takeoff.
URL: Engineer in charge .
Look around page 199. Langley calculated that the normal P-39 with armament would come in about a ton heavier, meaning 7,550 lbs, and the maximum speed would be expected to be around 340 mph with the same powerplant setup. They achieved a wind tunnel drag reduction in a fully-faired model of 26%. This would mean a top speed of about 429 mph at 20,000 feet with an engine of 1,350 hp. The head of the FST team said that the additional air required to cool the extra HP would likely limit the maximum speed to 410 mph. This was with a "fully-faired" model that could not be achieved in real life. The reality was bound to be something less.
Bell incorporated enough changes to drop the drag by 16% instead of the theoretical 26%. But then, the engine had the turbocharger removed and the resulting engine only produce 1,090 HP. That is 60 less hp than the engine that had driven the XP-39 to 390 mph and 260 HP less than the 1,350 HP the FST team lead had assumed when he calculated 410 mph. Additionally, the 1,090 HP was achieved at 15,000 feet rather than 20,000 feet. Accounting for the 16% drag reduction, the loss of horsepower, and the added air density at the lower latitude, you get the result that the XP-39B achieved a maximum speed of 375 mph in the first flight trials.
The end result was that Bell and Air Corps expressed satisfaction with the results and asked Bell to produce the balance of the YP-39s without turbochargers. All this was done while Ben Kelsey was in Europe working on another project.
If you go to WWIIaircraftperformance.org and look at the P-39 flight test data for a YP-39 (No. 40-30), the rate of climb started out at 3,600 fpm and was still 2,260 fpm at 15,000 feet. Moving to a P-39C (No. 40-2988), the maximum speed was 379 mph at 16,100 feet and the rate of climb started out at 3,720 fpm and was still 2,360 fpm at 16,750 feet. If we move yet again to a P-39N (No. 42-4400 test dated 17 Oct 1942), we see a maximum speed of 398.5 mph at 9,700 feet (critical altitude). The climb data for this airplane shows 3,320 fpm at sea level and 3,920 fpm at 11,000 feet. It was still climbing at 3,340 fpm at 15,000 feet, but tapered off to 2,630 fpm at 20,000 feet. 20,000 feet was achieved in 5.83 minutes. I believe the P-39N was at WEP power for the test.
The performance above for test data located at wwiiaircraftperformance.org does NOT seem to me to indicate the "dog" everyone seems to think it was. Rather it indicates that the aircraft was a decent-performing aircraft when operated below 20,000 feet. I will not get into the C.G. issues here. I think we all know the aircraft could be made to tumble if stalled when the ammunition was expended. That was proved post-war. But it doesn't seem to me to have the bad performance often attributed to it, at least in flight test, when operated at the altitudes it was produced to address in decisions made before we entered the war. I know the climb rates at Military power will be less than rates at WEP, but WEP was used by many aircraft in the heat of combat for short durations.
I am NOT saying the P-39 was a great airplane, or even that it was a good choice for a general fighter, but I am saying the test data seems to indicate that it doesn't seem to be as bad as is commonly assumed when used within its envelope.
Please note: This is NOT intended to start a debate on the use of the P-39 in the ETO, which was a higher-altitude theater of war. Rather, it seems to show why the Soviets had such a good experience with the P-39 in their lower-altitude, ground support war.