The aircraft\'s unusual design featured its Allison engine mounted in the middle of the fuselage, just behind the pilot, driving the propeller through a driveshaft passing between the pilot\'s feet. The purpose of this was to free up space for the heavy main armament, a 37 mm T9 cannon firing through the center of the propeller hub for optimum accuracy and stability when firing. The weight distribution necessitated a tricycle undercarriage, a first among American fighters. Entry to the cockpit was through a side door rather than a moving canopy. The weight distribution of the P-39 supposedly is the reason for its tendency to enter a dangerous flat spin — a characteristic Soviet tests proved to the then-skeptical manufacturer who had been unable to reproduce them.
The P-39\'s Allison V-1710 engine had a single-speed, single-stage supercharger, which brought about a decrease of performance compared to the promising prototype which had been fitted with an exhaust-driven turbo-supercharger. Due to the high weight of the P-39 and the poor high-altitude power of the mechanically supercharged Allison, the P-39\'s performance was markedly inferior to the contemporary European fighters, and as a result the first USAAF fighter units in the European Theater were equipped with the Spitfire V (which ironically featured a single-speed, single-stage supercharger, too).
http://en.wikipedia.org/wiki/P-39_Airacobra
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