This article was written by Bryan R. Swopes and first published on This Day in Aviation. Read the original article here.


On August 9, 1939, after General Henry H. Arnold had ordered that the prototype Bell Aircraft Corporation XP-39 Airacobra be evaluated in the NACA Full-Scale Tunnel at the Langley Memorial Aeronautics Laboratory (Langley Field, Virginia), it was flown there from Wright Field. It was hoped that aerodynamic improvements would allow the prototype to exceed 400 miles per hour (644 km/h).

The National Advisory Committee for Aeronautics, or NACA, engineers placed the full-size airplane inside the large wind tunnel for testing. A number of specific areas for aerodynamic improvement were found. After those changes were made by Bell, the XP-39’s top speed had improved by 16%.



Bell XP-39 Airacobra 38-326 in the NACA Full Scale Wind Tunnel at Langley Field, Virginia, 9 August 1939. The man at the base of the supports shows scale. (NASA)

The Bell XP-39 Airacobra was a single-place, single-engine prototype fighter with a low wing and retractable tricycle landing gears. The airplane was primarily built of aluminum, though control surfaces were fabric covered.

As originally built, the XP-39 was 28 feet, 8 inches (8.738 m) long with a wingspan of 35 feet, 10 inches (10.922 m). The prototype had an empty weight of 3,995 pounds (1,812 kg) and gross weight of 5,550 pounds (2,517 kg).

Changes recommended by NACA resulted in a re-contoured canopy, lengthened the airplane to 29 feet, 9 inches (9.068 m) and reduced the wing span to 34 feet, 0 inches (10.362 m). Its empty weight increased to 4,530 pounds (2,055 kg) and gross weight to 5,834 pounds (2,646 kg).



Bell XP-39 Airacobra 38-326 in the NACA Langley Memorial Aeronautical Laboratory Full-Scale Wind Tunnel, Langley Field, Virginia, 9 August 1939. The fuselage has had all protrusions removed. Right profile. (National Aeronautics and Space Administration NACA 18423)

The XP-39 was unarmed, but it had been designed around the American Armament Corporation T9 37 mm autocannon, later designated Gun, Automatic, 37 mm, M4 (Aircraft). The cannon and ammunition were in the forward fuselage, above the engine driveshaft. The gun fired through the reduction gear box and propeller hub.

The XP-39 was originally powered by a liquid-cooled, turbo-supercharged and supercharged 1,710.597-cubic-inch-displacement (28.032 l) Allison Engineering Co. V-1710-E2 (V-1710-17), a single overhead cam (SOHC) 60° V-12 engine with a compression ratio of 6.65:1.

The V-1710-17 had a Maximum Continuous Power rating of 1,000 horsepower at 2,600 r.p.m. at 25,000 feet (7,620 m), and Takeoff/Military Power rating of 1,150 horsepower at 3,000 r.p.m. at 25,000 feet, burning 91 octane gasoline.

The engine was installed in an unusual configuration behind the cockpit, with a two-piece drive shaft passing under the cockpit and turning the three-bladed Curtiss Electric constant-speed propeller through a remotely-mounted 1.8:1 gear reduction gear box.

The V-1710-17 was 16 feet, 1.79 inches (4.922 m) long, including the drive shaft and remote gear box. It was 2 feet, 11.45 inches (0.900 m) high, 2 feet, 5.28 inches (0.744 m) wide and weighed 1,350 pounds (612 kg).



Bell XP-39 Airacobra 38-326 in the NACA wind tunnel at Langley Field. The man at the base of the supports shows scale. (NASA)

Army Air Corps strategy changed the role of the P-39 from a high-altitude interceptor to a low-altitude tactical strike fighter. The original turbocharged V-1710-17 was replaced with a V-1710-37 (V-1710-E5) engine. The turbo-supercharger had been removed, which reduced the airplane’s power at altitudes above 15,000 feet (4,572 m).

The V-1710-37 had a maximum power of 1,090 horsepower at 3,000 r.p.m. at 13,300 feet (4,054 m). With the NACA-recommended aerodynamic changes and the new engine, the prototype Airacobra was re-designated XP-39B.


Displayed in the main image: a Bell P-39 Airacobra fires all of its guns at night. (U.S. Air Force)