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, Dayton, Ohio, with test pilot James Taylor ² 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 War Department announced a $1,073,445 contract to purchase 12 YP-39s and one YP-39A on 27 April 1939.
The Bell XP-39 Airacobra was a single-place, single-engine prototype fighter with a low wing and retractable tricycle landing gear. The airplane was primarily built of aluminum, though control surfaces were fabric covered.
Bell XP-39 Airacobra 38-326. (U.S. Air Force)
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).
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).³ 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.
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. (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. (NASA)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 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-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. On 18 April 1939, the Dayton Daily News reported:
The radically-designed XP-39 is at the field now undergoing further testing. Its purchase was announced Saturday in Washington. It had been at the materiel division for about two months before its initial flight on Army Day, April 6.
² James Taylor may have been James Blackstone (“Jimmie”) Taylor, Jr. (23 September 1897–25 May 1942). Taylor (Lieutenant Commander, United States Naval Reserve) was a well-known test pilot of the time. He was famous for his high-speed power dive from 20,000 feet (6,096 meters) over Farmingdale, New York, in the Seversky NF-1, 25 June 1937.
³ 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.
NACA pilots Robert Apgar Champine, on left, and Herbert Henry Hoover with the Bell X-1-2, 46-063, 1 September 1949. (NASA E49-0005)
10 March 1948: National Advisory Committee for Aeronautics (NACA) chief test pilot Herbert Henry (“Herb”) Hoover became the first civilian pilot to exceed the Speed of Sound when he flew a Bell X-1 supersonic research rocketplane near Muroc Air Force Base (Edwards AFB after 1949) in the high desert of southern California.
Hoover was flying the second of the three X-1s, serial number 46-063. Dropped from a B-29 “mother ship” on a stability and loads test, Hoover climbed to 42,000 feet (12,802 meters) while using three chambers of the rocketplane’s Reaction Motors XLR11-RM-3 engine. At 0.93 Mach (613.614 miles per hour/987.516 kilometers per hour), he fired the fourth chamber and accelerated to Mach 1.065 (702.687 miles per hour/1,130.865 kilometers per hour).
Hoover glided to a landing on Rogers Dry Lake. The rocketplane’s nose wheel would not extend, so Hoover held the nose up as long as possible before it settled onto the hard sand surface. 46-063 suffered minor damage.
This was the seventy-second flight of the X-1 series.
Bell X-1-2, 46-063, with the Boeing B-29 drop ship, B-29-96-BW Superfortress 45-21800. Originally painted orange, 46-063 was repainted white in 1948. (National Aeronautics and Space Administration E49-0004)
The 4 March 1948 flight Hoover’s eleventh in an X-1. Hoover had been the first NACA pilot to fly an X-1, having made a glide flight 21 October 1947. He made a total of fourteen X-1 flights before moving on to other flight test programs.
For this flight Herbert H. Hoover was awarded the Octave Chanute Award by the Institute of Aeronautical Sciences for “contributions to the application of flight test procedures to basic research in aerodynamics, and the development of methods for scientific study of transonic flight.” The award was presented at the Hotel Ambassador, in Los Angeles, California, 16 July 1948, by John Knudsen (“Jack”) Northrop, founder of the Northrop Corporation. Hoover was the initial recipient Air Force Association’s David C. Schilling Award, then known as the Flight Trophy, also awarded in 1948. In 1949, he was awarded the Air Medal by the United States Air Force, “for meritorious achievement while participating in aerial flight on March 10, 1948.” The medal was presented by President Harry S. Truman.
NACA Chief Test Pilot Herbert Henry Hoover, with a North American P-51 Mustang, December 1948. (NASA)Herbert H. Hoover, 1929
Herbert Henry Hoover was born 18 May 1912 at Knoxville, Tennessee. He was the son of Benjamin Roscoe Hoover, railway conductor, and Zella Mae Edington Hoover. He attended Central High School in Knoxville, graduating in 1929.
In 1930, Hoover was employed as a civil engineer’s assistant. He then attended the University of Tennessee, graduating from the College of Engineering, 24 August 1934, with a bachelors degree in mechanical engineering.
Herbert Henry Hoover
Joining the United States Army Air Corps, Hoover was trained as a pilot at Randolph and Kelly Fields, San Antonio, Texas. Completing training, he was commissioned as a second lieutenant in the Air Corps Reserve and assigned to Mitchel Field, Long Island, New York.
Released from active duty in 1937 after three years of service, Hoover was employed by the Standard Oil Company as a pilot, flying in South America.
After returning to the United States, on 16 December 1940, Hoover became an experimental test pilot for the National Advisory Committee for Aeronautics at the NACA Langley Memorial Aeronautical Laboratory, Hampton, Virginia.
Ruth Anadda Rhyne
Hoover registered for Selective Service (conscription), 4 April 1942. On his draft registration card, he was described as 5 feet, 7 inches (170 centimeters) tall, 175 pounds (79 kilograms, with blond hair, gray eyes, and a light complexion.
On 29 August 1942, Hoover married Miss Ruth Anadda Rhyne at the Stanley Presbyterian Church, River Bend, North Carolina. The ceremony was presided over by Rev. R.H. Ratchford. They would have two children, Anadda Susan Hoover and Herbert Henry (“Hank”) Hoover, Jr.
Experienced at flying in bad weather, Hoover volunteered to fly the Lockheed XC-35 Supercharged Cabin Transport Airplane, 36-353, the first airplane to be built with a pressurized cabin, through thunderstorms for weather research.
Lockheed XC-35 Supercharged Cabin Transport Airplane 36-353, the first airplane built with a pressurized cabin.
In July 1943, while flying a Curtiss SB2C Helldiver on an instrument calibration flight, Hoover was badly injured when the airplane’s canopy came loose and struck him in the head. He was able to safely return to Langley.
A Curtiss SB2C-1 Helldiver at the NACA Langley Memorial Aeonautical Laboratory, 31 May 1944. (NASA EL-2000-00241)
A 1948 Newport News, Virginia, newspaper article described the incident:
In illustrating the infrequency of mishaps in test flights, Hoover recalls that he was the principal in the first accident involving a NACA test pilot. This incidentally was his only air accident, and occurred in July 1943, while he was calibrating 1,700 pounds[771 kilograms]of instruments to be used in flight instrument investigations of the Navy Helldiver.
The canopy over the cockpit of the Helldiver tore loose and as it fluttered away, an edge of the structure smashed through Hoover’s helmet and goggles. He found his sight blurred by blood streaming from his forehead. Although in pain and almost blinded, Hoover kept his seat in the now open airplane, turned back to Langley and put his ship into a maneuver that would attract attention. The men on the ground instantly understood that something was wrong and cleared the afield of other aircraft, and Hoover brought the Helldiver to a safe landing.
—Daily Press, Vol. LIII, No. 188, 15 July 1948, at Page 8, Columns 1 and 2
On another occasion, while firing a rocket-propelled model during a 0.7 Mach dive, the rocket exploded and seriously damaged Hoover’s North American Aviation P-51 Mustang. The Mustang’s coolant tank was punctured, but he was able to make a successful forced landing.
Hoover was appointed NACA’s chief test pilot. He was assigned to the NACA Muroc Flight Test Unit at Muroc Air Force Base, California, to begin NACA flight testing of the Bell X-1. He made his first flight in the X-1 one week after Captain Charles Elwood (“Chuck”) Yeager broke the “sound barrier” flying the number one Bell X-1, 46-062, 14 October 1947.
Herb Hoover was killed when the North American Aviation B-45A-1-NA Tornado, 47-021, “NACA 121,” suffered a structural failure in flight near Burrowsville, Virginia 14 August 1952. It is believed that he struck the aircraft, or parts of the aircraft, during ejection. His body was found with his parachute unopened, but with his hand on the rip cord’s “D”-ring. The airplane’s copilot, John A. Harper, survived with minor injuries.
NACA 121, a North American Aviation B-45A-1-NA Tornado, 47-021, photographed at the NACA Langley Memorial Aeronautical Laboratory, Hampton, Virginia, 6 November 1949. NACA test pilot Herb Hoover was killed when this airplane suffered a structural failure of its right wing, 14 August 1952. (NASA EL-2000-00269)
The St. Louis Post-Dispatch reported:
PIONEER JET FLYER KILLED IN PARACHUTING
Herbert H. Hoover and Colleague Testing B-45 Bomber When It Catches Fire
BURROWSVILLE, Va., Aug. 15 (AP)—One of the pioneers of faster than sound flight fell to his death yesterday from a crippled B-45 jet bomber.
He was test pilot Herbert H. Hoover, the first man to fly the Bell X-1, and experimental prototype of the present day supersonic aircraft.¹
The heavy four-jet aircraft caught fire over this south-eastern Virginia community and Hoover and a companion, J.A. Harper bailed out.
Harper landed safe, except for a bruised shoulder. Searchers found the crumpled body of Hoover, his hand clutching the ripcord of his unopened parachute.
Both men were employed by the National Advisory Committee for Aeronautics Laboratory at Langley Field, Va., for which they were testing aircraft.
Officials of the NACA said the crash was caused by the failure of the outboard panel of the right wing of the bomber. A spokesman, asked about a published report that the plane exploded, said, “there was no explosion.”
Officials who had talked with Harper said an overload was imposed during a maneuver to check research instruments and as a result the panel failed.
They said that there was afire following the wing panel’s collapse, but “it is not believed that the fire had any material effect on the accident.”
Hoover was the first pilot to exceed the speed of sound in an NACA aircraft and the second to break the sound barrier in any plane.
He had made more than a dozen flights in all. He received the Air Force Association award for 1948 for that year’s most notable achievement in flight contributing to the nation’s air defense.
Hoover served in the Army Air Corps from 1934 to 1937. He was a member of the first active long-range reconnaissance unit, the Eighteenth Reconnaissance Squadron, then stationed at Mitchel Field, N.Y.
—St. Louis Post Dispatch, Vol. 104, No. 323, 15 August 1952, Page 39, Column 4
In eighteen years of flying Herbert Henry Hoover had flown more than 100 aircraft types. He was the third NACA test pilot to be killed.² His remains were interred at the Peninsula Memorial Park, Newport News, Virginia.
Bell X-1 46-063 with its B-29 carrier aircraft. (Flight Test Historical Foundation)
The Bell XS-1, later re-designated X-1, was the first of a series of rocket-powered research airplanes which included the Douglas D-558-II Skyrocket, the Bell X-2, and the North American Aviation X-15, which were flown by the U.S. Air Force, U.S. Navy, NACA and its successor, NASA, at Edwards Air Force Base to explore supersonic and hypersonic flight and at altitudes to and beyond the limits of Earth’s atmosphere.
The X-1 has an ogive nose, similar to the shape of a .50-caliber machine gun bullet, and has straight wings and tail surfaces. It is 30 feet, 10.98 inches (9.423 meters) long with a wing span of 28.00 feet (8.534 meters) and overall height of 10 feet, 10.20 inches (3.307 meters).
46-062 was built with a thin 8% aspect ratio wing, while 46-063 had a 10% thick wing. The wings were tapered, having a root chord of 6 feet, 2.2 inches (1.885 meters) and tip chord of 3 feet, 1.1 inches (0.942 meters), resulting in a total area of 130 square feet (12.1 square meters). The wings have an angle of incidence of 2.5° with -1.0° twist and 0° dihedral. The leading edges are swept aft 5.05°.
The horizontal stabilizer has a span of 11.4 feet (3.475 meters) and an area of 26.0 square feet (2.42 square meters). 062’s stabilizer has an aspect ratio of 6%, and 063’s, 5%.
The fuselage cross section is circular. At its widest point, the diameter of the X-1 fuselage is 4 feet, 7 inches (1.397 meters).
46-062 had an empty weight is 6,784.9 pounds (3,077.6 kilograms), but loaded with propellant, oxidizer and its pilot with his equipment, the weight increased to 13,034 pounds (5,912 kilograms).
The X-1 was designed to withstand an ultimate structural load of 18g.
The X-1 was powered by a four-chamber Reaction Motors, Inc., 6000C4 (XLR11-RM-3 ) rocket engine which produced 6,000 pounds of thrust (26,689 Newtons). This engine burned a 75/25 mixture of ethyl alcohol and water with liquid oxygen. Fuel capacity is 293 gallons (1,109 liters) of water/alcohol and 311 gallons (1,177 liters) of liquid oxygen. The fuel system was pressurized by nitrogen at 1,500 pounds per square inch (103.4 Bar).
The X-1 was usually dropped from the B-29 flying at 30,000 feet (9,144 meters) and 345 miles per hour (555 kilometers per hour). It fell as much as 1,000 feet (305 meters) before beginning to climb under its own power.
The X-1’s performance was limited by its fuel capacity. Flying at 50,000 feet (15,240 meters), it could reach 916 miles per hour (1,474 kilometers per hour), but at 70,000 feet (21,336 meters) the maximum speed that could be reached was 898 miles per hour (1,445 kilometers per hour). During a maximum climb, fuel would be exhausted as the X-1 reached 74,800 feet (2,799 meters). The absolute ceiling is 87,750 feet (26,746 meters).
The X-1 had a minimum landing speed of 135 miles per hour (217 kilometers per hour) using 60% flaps.
There were 157 flights with the three X-1 rocket planes. The number one ship, 46-062, Glamorous Glennis, made 78 flights. On 26 March 1948, with Chuck Yeager again in the cockpit, it reached reached Mach 1.45 (957 miles per hour/1,540 kilometers per hour) at 71,900 feet (21,915 meters).
The second X-1, 46-063, was later modified to the X-1E. It is on display at the NASA Dryden Research Center at Edwards Air Force Base. Glamorous Glennis is on display at the Smithsonian Institution National Air and Space Museum, next to Charles A. Lindbergh’s Spirit of St. Louis.
The third X-1, 46-064, made just one glide flight before it was destroyed 9 November 1951 in an accidental explosion.
Bell X-1E 46-063 on Rogers Dry Lake. (NASA)
¹ This is incorrect. The first pilot to fly the Bell X-1 was Bell Aircraft Corporation Senior Experimental Test Pilot Jack Valentine Woolams. Please see This Day in Aviation at https://www.thisdayinaviation.com/19-january-1946/ Herb Hoover had been the first NACA pilot to fly an X-1.
² The first was Howard Clifton (“Tick”) Lilly, when the compressor section of a Douglas D-558-I Skystreak exploded 3 May 1948. Please see TDiA at https://www.thisdayinaviation.com/3-may-1948/
North American Aviation XP-51 Mustang 41-038 at Langley Field, Virginia, 29 December 1941. (NASA)
29 December 1941: The first North American Aviation XP-51 fighter prototype, Air Corps serial number 41-038, arrived at the National Advisory Committee for Aeronautics Langley Memorial Aeronautical Laboratory, Langley Field, Hampton, Virginia, for flight testing. This airplane was the fourth production Royal Air Force Mustang Mk.I, North American serial number 73-3101.
The test program resulted in an improved aileron design which significantly improved the Mustang’s maneuverability. The new aileron was used on all production models.
North American Aviation XP-51 41-038 at the NACA Langley Memorial Aeronautical Laboratory. (NASA)
41-038 was returned to Wright Field on 2 November 1942. The second XP-51, 41-039, arrived at Langley in March 1943 for continued testing.
The Mustang Mk.I (NAA Model NA-73) was a single-place, single-engine fighter of all metal construction. It was 32 feet, 3 inches (9.830 meters) long with a wingspan of 37 feet, ½-inch (11.290 meters) and overall height of 12 feet, 2½ inches (3.719 meters). The airplane’s empty weight was 6,280 pounds (25,848.6 kilograms) and loaded weight was 8,400 pounds (3,810.2 kilograms).
North American Aviation XP-51 41-038 at NACA Langley Memorial Aeronautical Laboratory, right profile. (NASA)
The Mustang Mk.I/XP-51 was powered by a liquid-cooled, supercharged 1,710.60-cubic-inch-displacement (28.032 liter) Allison Engineering Company V-1710-F3R (V-1710-39) single overhead cam 60° V-12 engine, with a compression ratio of 6.65:1 and a single-stage, single-speed supercharger. This was a right-hand tractor engine (the V-1710 was built in both right-hand and left-hand configurations) which drove a 10 foot, 6 inch (3.200 meter) diameter, three-bladed, Curtiss Electric constant-speed propeller through a 2.00:1 gear reduction.
The V-1710-39 had a Normal Power rating of 880 horsepower at 2,600 r.p.m. at Sea Level; Take Off Power rating of 1,150 horsepower at 3,000 r.p.m. at Sea Level, with 44.5 inches of manifold pressure (1.51 Bar), 5 minute limit; and a War Emergency Power rating of 1,490 horsepower at 3,000 r.p.m., with 56 inches of manifold pressure (1.90 Bar). The V-1710-F3R was 3 feet, ¾ inches (0.934 meters) high, 2 feet, 5-9/32 inches (0.744 meters) wide and 7 feet, 1-5/8 inches (2.175 meters) long. It had a dry weight of 1,310 pounds (594 kilograms).
North American Aviation XP-51 41-038 at NACA Langley Memorial Aeronautical Laboratory. (NASA)
The XP-51 tested at Wright Field had a maximum speed of 382.0 miles per hour (614.8 kilometers per hour) at 13,000 feet (3,962 meters) at wide open throttle, and cruise speed of 300 miles per hour (483 kilometers per hour).
Below 20,000 feet (6,096 meters), the Mustang was the fastest fighter in the world. The Mk.I was 30 m.p.h. (48 kilometers per hour) faster than its contemporary, the Curtiss P-40 Warhawk, though both used exactly the same engine. Below 15,000 feet (4,572 meters), the Mustang was also 30–35 m.p.h (48–56 km/h) faster than a Supermarine Spitfire, which was equipped with the more powerful Roll-Royce Merlin V-12.
The service ceiling was 30,800 feet (9,388 meters) and range was 750 miles (1,207 kilometers).
North American Aviation XP-51 41-038 at NACA Langley Memorial Aeronautical Laboratory, rear view. (NASA)
Armament consisted of two synchronized Browning AN-M2 .50-caliber machine guns mounted in the nose under the engine and firing through the propeller, and two more .50-caliber and four Browning .303 Mk.II machine guns in the wings.
North American Aviation XP-51 instrument panel. (U.S. Air Force)
Two Mustang Mk.Is, AG348 and AG354, were taken from the first RAF production order and sent to Wright Field for testing by the U.S. Army Air Corps. These airplanes, assigned Air Corps serial numbers 41-038 and 41-039, were designated XP-51. They would be developed into the legendary P-51 Mustang. In production from 1941 to 1945, a total of 16,766 Mustangs of all variants were built.
North American XP-51 41-038 was transferred to the Smithsonian Institution National Air and Space Museum. It was restored in 1976. It is now in the collection of the EAA AirVenture Museum, Oshkosh, Wisconsin. It has a current FAA registration number, N51NA.
North American Aviation XP-51 41-038 in the collection of the Experimental Aircraft Association AirVenture Museum, Oshkosh, Wisconsin. (EAA AirVenture Museum)
North American Aviation P-51B Mustang fighter in the Full-Scale Tunnel, NACA Langley Memorial Aeronautical Laboratory, Hampton, Virginia, 23 September 1943. (NASA)Drag test of North American Aviation P-51B-1-NA Mustang 43-12105 in the NACA Full-Scale Tunnel. (NASA)
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)
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 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 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.
A Bell P-39 Airacobra fires all of its guns at night. (U.S. Air Force)
