The Airacobra was one of the principal American fighters in service when the United States entered World War Two.
First flown in April 1938, the Airacobra was initially designed as a high-altitude interceptor, but the lack of an ‘efficient’ turbo-charger prevented it from performing its design goals.
When the Bell P-39 was introduced into service in 1941, two major design changes made this aircraft unique from standard fighter designs of that time:
- The first production fighter to have a mid-engine layout.
- The first fighter aircraft with tricycle landing gear.
The engine was installed in the centre fuselage, behind the pilot, driving a standard arrangement tractor propeller in the nose with a long shaft which ran underneath the cockpit between the pilots legs. The engine was the powerful Allison V-12 mounted in the middle of the fuselage, just behind the cockpit, and a propeller driven by a shaft passing beneath the pilot’s feet under the cockpit floor. There were no problems with propeller shaft failure.
The main purpose of this configuration was to free up space for the heavy main armament, a 37 mm Oldsmobile T9 cannon firing through the centre of the propeller hub for optimum accuracy and stability. This was unusual, because fighter design had previously been driven by the intended engine, not the weapon system. Although devastating when it worked, the T9 had very limited ammunition, a low rate of fire, and was unfortunately prone to jamming.
A secondary benefit of the mid-engine arrangement was that it created a smooth and streamlined nose profile. Entry to the cockpit was through side doors mounted on both sides of the cockpit
At a pivotal meeting with the USAAC and NACA in August 1939, Larry Bell proposed that the production P-39 aircraft be configured without the turbocharger. In later years, Benjamin S. Kelsey Project Officer for Fighters at the United States Army Air Corps (USAAC), expressed regret at not being present to override the decision to eliminate the turbo.
Production P-39’s retained a single-stage, single-speed supercharger with a critical altitude (above which performance declined) of about 12,000 ft. As a result, the aircraft was simpler to produce and maintain. However, the removal of the turbo destroyed any chance that the P-39 could serve as a high altitude front-line fighter.
The Airacobra was also the first fighter fitted with a tricycle undercarriage layout. This was chosen because it provided better stability for new pilots transitioning to fighters. By placing the centre of gravity in front of the main wheels enabled the aircraft to track straight during landing and takeoff phases, plus enabled the pilot to have a better view on the ground.
UNUSUAL FLYING CHARACTERISTICS
Weight distribution could result in the Airacobra entering a dangerous flat spin, a characteristic Soviet test pilots demonstrated to the sceptical manufacturer, which had been unable to reproduce the effect. It was determined the spin could only be induced if the aircraft was flown with no ammunition in the nose. The flight manual noted a need to ballast the front ammunition compartment to achieve a reasonable centre of gravity. High-speed controls were light; consequently high-speed turns and pull-outs were possible. The P-39 had to be held in a dive since it tended to level out and the recommended never-exceed dive speed limit (VNE) was 475 mph (764 km/h).
Soon after entering service, pilots began to report that “during flights of the P-39 in certain manoeuvres, it tumbled end over end.” Most of these events happened after the aircraft was stalled in a nose high attitude with considerable power applied. Bell pilots made 86 separate efforts to reproduce the reported tumbling characteristics. In no case were they able to tumble it. In his autobiography veteran test and airshow pilot R.A “Bob” Hoover provides an account of tumbling a P-39. He goes on to say that in hindsight, he was actually performing a Lomcovak, a now-common airshow manoeuvre. When the centre of gravity moved aft, the aircraft was found to tumble.
The P-39 was widely used by the Soviet Air Force, and enabled individual Soviet pilots to collect the highest number of kills attributed to any U.S. fighter type flown by any air force in any conflict. The USSR adopted the fighter because most of their air combat took place at medium and lower altitudes. Other major users of the type included the RAAF (Australia), the Free French, the Royal Air Force (RAF) and the Italian Co-Belligerent Air Force.