What about the p-39?

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That the USAAF neutered the P-39 to no small extent is true. However, you have to remember that in many ways the performance of the XP-39 couldn't really have been replicated on production machines and that the philisophy of the USAAF at the time didn't really favour high altitude interceptors.

The 390 mph and 5 minutes to 20,000 feet figures that the XP-39 achieved in testing were for an unarmed, polished prototype without any military equipment. The XP-39 weighted just 3,995 lbs empty, only 200lbs heavier than an empty A6M Zero and more than 1400lbs lighter than P-40 protoypes. If you add self sealing fuel tanks, military radios, antenna, armour, guns, gunsights, ammunition and all the other stuff necessary to make the plane militarily effective, performance is going to take a nose dive.

Just to give an idea of how much weight creep there would be, the empty weight of the YP-39 was 5,040 lbs, up over 1000lbs from the 3,995 lbs of the XP-39. That's an increase of just over 25% on the empty weight AFTER stripping out the turbosupercharger. Loaded weight shot from 5,550 lbs for the unarmed XP-39 prototype up to 7,070 lbs for the armed YP-39, and it was still without internal or cockpit armour and had no self sealing fuel tanks. When these, and more guns and ammunition were added, combat weight climbed by another 500lbs.

So, a fully loaded, combat capable P-39D circa September 1941, weighted around 7500 lbs, almost a full 2000lb heavier than its turbosupercharged XP-39 ancestor. Its empty weight was 1,400lbs higher than that of the XP-39 prototype. That increase would markedly effect handling, speed and climb performance.

Would the P-39 have been better a better fighter in the ETO and Pacific if the USAAF had left it alone? Yes. It would of been marginally slower and less manouverable down low, but progressively faster above 15,000 feet. However, it wasn't going to happen. To turn the P-39 into a fighter it needed all that equipment. So, the weight creep meant that the performace exhibited by the prototype wouldn't of been achievable in real life.

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