Bell XP-39B Airacobra | This Day in Aviation

The Bell XP-39 prototype in the original turbosupercharged configuration. The intercooler and waste gates created significant aerodynamic drag. (Bell Aircraft Corporation) — The Bell XP-39 prototype, 38-326, in the original turbosupercharged configuration. The intercooler and waste gates created significant aerodynamic drag. (U.S. Air Force)

6 April 1939: ¹ After being shipped by truck from the Bell Aircraft Company factory at Buffalo, New York, the XP-39 prototype, 38-326, (Bell Model 4) made its first flight at Wright Field, Ohio, with test pilot James Taylor in the cockpit. During the test flight, Taylor flew the XP-39 to 390 miles per hour (628 kilometers per hour) at 20,000 feet (6,096 meters). The service ceiling was 32,000 feet (9,754 meters).

The XP-39 was designed by Bell’s chief engineer, Robert J. Woods, to meet a U.S. Army Air Corps requirement, X-609, issued in March 1937, for a high altitude interceptor. A contract for the prototype was issued 7 October 1937. On 15 April 1939, Assistant Secretary of War Louis Johnson announced that the U.S. Army had purchased the experimental Bell XP-39 and the Seversky XP-41.

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.

Bell XP-39 Airacobra 38-326. Left profile. (United States Air Force G3162) — Bell XP-39 Airacobra 38-326. (U.S. Air Force)

The Bell XP-39 Aircobra in original configuration. (Allison Engine Historical Society) — The Bell XP-39 Aircobra in original configuration. (U. S. Air Force)

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).² The cannon and ammunition were in the forward fuselage, above the engine driveshaft. The gun fired through the reduction gear box and propeller hub.

Bell P-39 Airacobra center fuselage detail with maintenance panels open. (U.S. Air Force photo)

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 meters) long, including the drive shaft and remote gear box. It was 2 feet, 11.45 inches (0.900 meters) high, 2 feet, 5.28 inches (0.744 meters) wide and weighed 1,350 pounds (612 kilograms).

Allison V-1710 E19 (V-1710-85) with extension drive shaft and remote propeller drive gear unit. (Allison Division of General Motors)

On 6 June 1939 the XP-39 was flown to the National Advisory Committee for Aeronautics (NACA) Langley Memorial Aeronautical Laboratory at Hampton, Virginia, by Lieutenant Mark E. Bradley, Jr. The prototype was tested in the Full-Scale Wind Tunnel. Improvements in aerodynamics were recommended and Bell rebuilt the airplane as the XP-39B with an Allison V-1710-E5 (V-1710-37) engine.

Bell XP-39 Airacobra 38-326 in the NACA Langley Memorial Aeronautical Laboratory Full-Scale Wind Tunnel, Langley Field, Virginia, 1939. Right front quarter, high oblique view. (National Aeronautics and Space Administration NACA 18423) — Bell XP-39 Airacobra 38-326 in the NACA Langley Memorial Aeronautical Laboratory Full-Scale Wind Tunnel, Langley Field, Virginia. (NASA)

Bell XP-39 Airacobra 38-326 in the NACA Langley Memorial Aeronautical Laboratory Full-Scale Wind Tunnel, Langley Field, Hampton, Virginia, 9 August 1939. The fuselage has had all protrusions removed. Right quartering profile view. (National Aeronautics and Space Administration NACA 18281) — Bell XP-39 Airacobra 38-326 in the NACA Langley Memorial Aeronautical Laboratory Full-Scale Wind Tunnel, Langley Field, Hampton, Virginia, 9 August 1939. The fuselage has had all protrusions removed. (NASA)

Bell XP-39 Airacobra 38-326 in the NACA Langley Memorial Aeronautical Laboratory Full-Scale Wind Tunnel, Langley Field, Hampton, Virginia, 1939. Front low oblique view. National Aeronautics and Space Administration NACA 18421) — Bell XP-39 Airacobra 38-326 in the NACA Langley Memorial Aeronautical Laboratory Full-Scale Wind Tunnel, Langley Field, Hampton, Virginia. (NASA)

The turbosupercharger had been removed, which reduced the airplane’s power at altitudes above 15,000 feet (4,572 meters). The V-1710-37 had a maximum power of 1,090 horsepower at 3,000 r.p.m. at 13,300 feet (4,054 meters). This resulted in the P-39 being used primarily as a ground-attack weapon.

The XP-39B, with test pilot George Price in the cockpit, was damaged when when its landing gear did not fully extend, 6 January 1940. It was repaired and test flights resumed. On 6 August 1940, Captain Ernest K. Warburton stalled the prototype on landing. The impact resulted in significant structural damage, beyond economic repair. The airplane was later scrapped.

Bell Model 12 (XP-39) prototype 38-326, at Bell Aircraft Co., Buffalo, New York — Bell XP-39B Airacobra prototype, 38-326, at the Bell Aircraft Corporation airfield, Buffalo, New York, 1940. (Bell Aircraft Corporation)

On 27 April 1939, the U.S. Army announced that a contract to Bell Aircraft had been issued in the amount of $1,073,445 for delivery of thirteen YP-39s. 9,584 Bell P-39 Airacobras were built during World War II. More than half were sent to the Soviet Union.

Bell XP-39 prototype, serial number 38-326. (Bell Aircraft Corporation) — Bell XP-39B prototype, serial number 38-326. (Bell Aircraft Corporation)

¹ Reliable sources indicate the date of the first flight as both 6 April 1938 and 6 April 1939. The Bell Helicopter Company web site, “The History of Bell Helicopter: 1935–1949” states 1938. However, contemporary newspaper articles strongly suggest that the date was 1939. The first newspaper references to the XP-39 located by TDiA are dated 16 April 1939.

² The 37-mm Aircraft Gun Matériel M4 is a recoil-operated aircraft weapon designed by John M. Browning. It has an overall length of 7 feet, 5 inches (2.26 meters). The barrel, or “tube,” is 5 feet, 5 inches (1.65 meters) long with a caliber of 1.457 inches (37.0 millimeters) and weighs 55 pounds (25 kilograms). The barrel is part of the recoiling section of the gun and moves rearward 9-5/8 inches (245 millimeters). The weight of the gun with a loaded 30-round magazine is 306.4 pounds (138.98 kilograms). The M4 fires a high-explosive tracer round with a muzzle velocity of 2,000 feet per second (607 meters per second). Each M54 shell is 9.75 inches (248 millimeters) long and weighs 1.93 pounds, of which the projectile makes up 1.34 pounds (0.608 kilograms). The cannon has a cyclic rate of fire of 125–150 rounds per minute.

Bell XP-39 Airacobra 38-326 in the Full Scale Wind Tunnel at the Langley Memorial Aeronautical Laboratory, Langley Field, Virginia. (National Aeronautics and Space Administration NACA 18281) — 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)

9 August 1939: After General Henry H. Arnold had ordered that the prototype Bell Aircraft Corporation XP-39 Airacobra be evaluated in the National Advisory Committee for Aeronautics (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 kilometers per hour).

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 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)

As originally built, the XP-39 was 28 feet, 8 inches (8.738 meters) long with a wingspan of 35 feet, 10 inches (10.922 meters). The prototype had an empty weight of 3,995 pounds (1,812 kilograms) and gross weight of 5,550 pounds (2,517 kilograms). Changes recommended by NACA resulted in a recontoured canopy, lengthened the airplane to 29 feet, 9 inches (9.068 meters) and reduced the wing span to 34 feet, 0 inches (10.362 meters). Its empty weight increased to 4,530 pounds (2,055 kilograms) and gross weight to 5,834 pounds (2,646 kilograms).

Bell XP-39 in the NACA wind tunnel at Langley Field. (NASA) — 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)

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, turbosupercharged and supercharged 1,710.597-cubic-inch-displacement (28.032 liter) 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 meters), 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 meters) long, including the drive shaft and remote gear box. It was 2 feet, 11.45 inches (0.900 meters) high, 2 feet, 5.28 inches (0.744 meters) wide and weighed 1,350 pounds (612 kilograms).

Bell XP-39B prototype, s/n 38-326, at Bell Aircraft Co., Buffalo, New York

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 turbosupercharger had been removed, which reduced the airplane’s power at altitudes above 15,000 feet (4,572 meters). The V-1710-37 had a maximum power of 1,090 horsepower at 3,000 r.p.m. at 13,300 feet (4,054 meters). With the NACA-recommended aerodynamic changes and the new engine, the prototype Airacobra was redesignated XP-39B.

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6 April 1939

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