Bell P-39D Airacobra

Bell Aircraft Corporation's P-39 Airacobra was a rather strange sight to behold. It had a cannon located in the propeller hub, tricycle landing gear (the first fighter with such a configuration), a car-type door to enter and exit the craft rather than a sliding canopy, and wings set considerably farther aft than normal. Oddity aside, it was a trim and handsome airplane in profile, but it proved notoriously poor in air to air combat. Japanese pilots found it particularly easy to shoot down. Although its dog fighting capability was limited, it enjoyed great success in the hands of Soviet aviators. They found it to be a superb ground attack aircraft, raining destruction upon German armored vehicles.

Designed by Larry Bell to meet requirements for an Army Air Corps fighter competition, the XP-39 first flew in April of 1938. This prototype sported a turbocharger, pushing the aircraft's speed to 390 mph. Its performance roughly matched that of the XP-38, especially at high altitudes. Unfortunately, the Army decided they had no need for high altitude interceptors at the time and both the XP-38 and XP-39 lost the contest to the XP-40.

Nevertheless, the Army expressed interest in the Airacobra, but insisted on a redesign to meet their specifications. They deleted the turbocharger to reduce cost and drag (replacing it with a single stage mechanical supercharger), and also chopped 2 ft off its wings. Their tinkering resulted in a drop in performance, especially at high altitudes. Larry Bell did not dare to protest because the company was facing bankruptcy and desperately needed the Army contract. By December 7, 1941 Bell had delivered 600 P-39s to the Army Air Corps.

The first main production model, the P-39D, was quite typical of the type. It was powered by a 1,150 hp Allison engine (V-1710-35). The engine was placed in the middle of the fuselage behind the cockpit in order to accommodate a 37mm cannon positioned in the nose of the aircraft. This required a very strong structure to keep the engine in place. The whole airplane was basically designed around this gun. The engine drove the propeller via a 10-foot shaft that passed between the pilot's feet. The weight of the amidships engine moved the center of gravity aft. Because of the shift in the center of gravity, Bell's designers were forced to use a then radical tricycle landing gear. In order balance the plane in flight, the wings also had to be moved aft, to a position beneath the engine. Placing the wings directly beneath the engine meant that most of the P-39's weight was concentrated in the middle of the airplane, and that made it a twitchy airplane to fly, one that tended to initiate directional changes with very little control input, roughly analogous to a car that over-steers. The engine was also apt to break loose in the event of a crash landing, with unfortunate results for the pilot sitting right in front of it. It was not a pilot's airplane.

Without a turbocharger, power from the Allison engine fell off severely even at medium altitude. The P-39s acceleration, climb rate, and service ceiling were so compromised that it simply was not competitive with the performance of the enemy fighters it would soon face. Add to that its lack of maneuverability, and in aerial combat the P-39 became "meat on the table" to Axis fighter pilots.

On the plus side, the pilot sat well forward in the fuselage where he could see quite well. The well-designed canopy provided a very good all-around view for its time. The big cannon in the nose was devastating to other aircraft at long range if the pilot could score hits with it. The plane's sleek nose and careful streamlining gave it a good top speed for its time. And the tricycle landing gear was definitely ahead of its time.

Armament consisted of the aforementioned Colt 37mm cannon firing through the propeller hub (30-60 rounds of ammunition), two 50 caliber guns in the nose (200-270 r.p.g.), and 4-.30 cal. wing mounted machine guns (1,000 r.p.g. max.). The preponderance of firepower in the nose made for an effective armament, although the mix of three different calibers with their different trajectories and points of convergence diluted that effectiveness somewhat. The Airacobra was also capable of carrying 500 lbs. of bombs.

The first mass-produced model was the P-39D of 1941. All P-39 models from the D forward were really quite similar. The D-1 temporarily replaced the 37mm cannon with a 20mm cannon. The D-2 model introduced a more powerful 1,325 hp. Allison V-1710-63 engine.

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