Quote:
Originally Posted by tcolvin
2 TAF's CAS aircraft, Typhoon and Spitfire, were both liquid cooled with the engine in the most exposed position.
The only air-cooled CAS plane IIRC was the P-47 Thunderbolt.
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Indeed, and this was their greatest weakness. The P-39 would have been no better, and of poorer performance. You are however forgetting the ground-attack missions carried out by the USN, USMC, and the Japanese Army and Navy. All used radial engines. As did the Russian Su 2, and the unsuccessful Su 6, regarded as superior to the Il 2 but by then there was no point in stopping production. The Italians also used radial engines for ground-attack missions, as did the French. And Swedish, Romanian...... The RAF used liquid-cooled engines in the role because they were the best engines it had available at the time of placing its wartime types into mass production. Had Fedden not spent so much time on the sleeve-valve, then perhaps a poppet-valve Hercules or Centaurus could have been available much sooner, and the story completely different.
You also say "This aircraft (P-39) was rejected by the RAF without reference to the Army because of an irrelevant lack of performance at altitude." But there as nothing irrelevant to the RAF about a lack of performance at high altitude. They already had two superior types with better low-altitude performance, the Typhoon and the Mustang. The RAF just didn't need the P-39, and at that stage wasn't desperate enough to take anything that could fly.
It is certainly interesting to consider just what it was about the Soviet operations that brought out the best points of the P-39. Partially it has to be a criticism of their own types, particularly their robustness and weak armament. One key point seems to be the lack of significant medium-altitude bomber operations, either as subjects of attack or requiring defense. If you only meet the enemy, or protect your friends, at low level then a lack of performance at higher levels becomes irrelevant. The agility of the P-39 would come into its own, and you don't need great speed to run down a Stuka or an Fw 189.