Bell P-39 Airacobra

The Bell P-39 Airacobra was one of the principal American fighter aircraft in service when the United States entered World War II. The P-39 was used by the Soviet Air Force, and scored the highest number of individual kills attributed to any U.S. fighter type in the Eastern European theatre. Other major users of the type included the Free French, the Royal Air Force, the United States Army Air Forces, and the Italian Co-Belligerent Air Force.
Designed by Bell Aircraft, it had an innovative layout, with the engine installed in the center fuselage, behind the pilot, and driving a tractor propeller via a long shaft. It was also the first fighter fitted with a tricycle undercarriage. Although its mid-engine placement was innovative, the P-39 design was handicapped by the absence of an efficient turbo-supercharger, preventing it from performing high-altitude work. As such it was rejected by the RAF for use over western Europe but adopted by the USSR where most air combat took place at medium and lower altitudes.
Together with the derivative P-63 Kingcobra, the P-39 was one of the most successful fixed-wing aircraft manufactured by Bell.
design and development
In February 1937, Lieutenant Benjamin S. Kelsey, Project Officer for Fighters at the United States Army Air Corps (USAAC), and Captain Gordon P. Saville, fighter tactics instructor at the Air Corps Tactical School, issued a specification for a new fighter via Circular Proposal X-609. It was a request for a single-engine high-altitude "interceptor" having "the tactical mission of interception and attack of hostile aircraft at high altitude". Despite being called an interceptor, the proposed aircraft's role was simply an extension of the traditional pursuit (fighter) role, using a heavier and more powerful aircraft at higher altitude. Specifications called for at least 1,000 lb of heavy armament including a cannon, a liquid-cooled Allison engine with a General Electric turbo-supercharger, tricycle landing gear, a level airspeed of at least 360 mph (580 km/h) at altitude, and a climb to 20,000 ft (6,100 m) within 6 minutes. This was the most demanding set of fighter specifications USAAC had presented to that date. Although Bell's limited fighter design work had previously resulted in the unusual Bell YFM-1 Airacuda, the Model 12[12] proposal adopted an equally original configuration with an Allison V-12 engine 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.

A secondary benefit of the mid-engine arrangement was that it created a smooth and streamlined nose profile. Much was made of the fact that this resulted in a configuration “with as trim and clean a fuselage nose as the snout of a high velocity bullet”. Entry to the cockpit was through side doors (mounted on both sides of the cockpit) rather than a sliding canopy. Its unusual engine location and the long drive shaft caused some concern to pilots at first, but experience showed this was no more of a hazard in a crash landing than with an engine located forward of the cockpit. 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 (1.46 in) Oldsmobile T9 cannon firing through the center of the propeller hub for optimum accuracy and stability when firing. This happened because H.M. Poyer, designer for project leader Robert Woods, was impressed by the power of this weapon and pressed for its incorporation. 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 prone to jamming.

The XP-39 made its maiden flight on 6 April 1938. at Wright Field, Ohio, achieving 390 mph (630 km/h) at 20,000 ft (6,100 m), reaching this altitude in only five minutes. However, the XP-39 was found to be short on performance at altitude. Flight testing had found its top speed at 20,000 feet to be lower than the 400 mph claimed in the original proposal.XP-39 developments

As originally specified by Kelsey and Saville, the XP-39 had a turbo-supercharger to augment its high-altitude performance. Bell cooled the turbo with a scoop on the left side of the fuselage. Kelsey wished to shepherd the XP-39 through its early engineering teething troubles, but he was ordered to England. The XP-39 project was handed over to others, and in June 1939 the prototype was ordered by General Henry H. Arnold to be evaluated in NACA wind tunnels to find ways of increasing its speed, by reducing parasitic drag. Tests were carried out, and Bell engineers followed the recommendations of NACA and the Army to reduce drag such that the top speed was increased 16%.NACA wrote, "it is imperative to enclose the supercharger within the airplane with an efficient duct system for cooling the rotor and discharging the cooling air and exhaust gases."In the very tightly planned XP-39, though, there was no internal space left over for the turbo. Using a drag-buildup scheme, a number of potential areas of drag reduction were found. NACA concluded that a top speed of 429 mph could be realized with the aerodynamic improvements they had developed and an uprated V-1710 with only a single-stage, single-speed supercharger.
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. Some historians have questioned Bell’s true motivation in reconfiguring the aircraft. The strongest hypothesis is that Bell’s factory didn’t have an active production program and he was desperate for cash flow. Other historians mention that wind tunnel tests made the designers believe the turbocharger installation was so aerodynamically cluttered that it had more disadvantages than advantages.
The Army ordered 12 YP-39s (with only a single-stage, single-speed supercharger) for service evaluation and one YP-39A. After these trials were complete, which resulted in detail changes including deletion of the external radiator, and on advice from NACA, the prototype was modified as the XP-39B; after demonstrating a performance improvement, the 13 YP-39s were completed to this standard, adding two 0.30 in (7.62 mm) machine guns to the two existing 0.50 in (12.7 mm) guns. Lacking armor or self-sealing fuel tanks, the prototype was one ton (900 kg) lighter than the production fighters.
The production P-39 retained a single-stage, single-speed supercharger with a critical altitude (above which performance declined) of about 12,000 feet (3,660 m). As a result, the aircraft was simpler to produce and maintain. However, the deletion of the turbo destroyed any chance that the P-39 could serve as a medium-high altitude front-line fighter. When deficiencies were noticed in 1940 and 1941, the lack of a turbo made it nearly impossible to improve upon the Airacobra's performance. The removal of the turbocharger and its drag inducing inlet cured the drag problem but reduced performance overall. In later years, Kelsey expressed regret at not being present to override the decision to eliminate the turbo.
After completing service trials, and originally designated P-45, a first order for 80 aircraft was placed 10 August 1939; the designation reverted to P-39C before deliveries began. After assessing aerial combat conditions in Europe, it was evident that without armor or self-sealing tanks, the 20 production P-39Cs were not suitable for operational use. The remaining 60 machines in the order were built as P-39Ds with armor, self-sealing tanks and enhanced armament. These P-39Ds were the first Airacobras to enter into service with the Army Air Corps units and would be the first ones to see action.
General characteristics
  • Crew: One
  • Length: 30 ft 2 in (9.2 m)
  • Wingspan: 34 ft 0 in (10.4 m)
  • Height: 12 ft 5 in (3.8 m)
  • Wing area: 213 sq ft (19.8 m²)
  • Empty weight: 6,516 lb (2,955 kg)
  • Loaded weight: 7,570 lb (3,433 kg)
  • Max. takeoff weight: 8,400 lb (3,800 kg)
  • Powerplant: 1 × Allison V-1710-85 liquid-cooled V-12, 1,200 hp (894 kW) at 9,000 ft (using emergency power)
Performance
  • Never exceed speed: 525 mph (845 km/h)
  • Maximum speed: 389 mph (626 km/h) at 10,000 ft (using emergency power)
  • Stall speed: 95 mph (152 km/h) Power off Flaps & undercarriage down
  • Range: 525 miles on internal fuel (840 km)
  • Service ceiling: 35,000 ft (10,700 m)
  • Rate of climb: 3805 ft/min (19,3 m/s) at 7,400 ft (using emergency power)
  • Wing loading: 34.6 lb/sq ft (169 kg/m²)
  • Power/mass: 0.16 hp/lb (0.27 kW/kg)
  • Time to climb: 15,000 in 4.5 min at 160 mph (260 km/h).
Armament
  • Guns:
    • 1 × 37 mm M4 cannon in nose (firing through the propeller hub) with 30 rounds of HE-T ammunition.
    • 2 × .50 cal (12.7 mm) synchronized Browning M2 machine guns, nose-mounted; 200 rounds per gun
    • 2 × .50 cal (12.7 mm) Browning M2 machine guns (one each wing), 300 rounds per gun
  • Bombs: Up to 500 lb (230 kg) of bombs under wings and belly

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