The 1940s marked a revolution in wing design for military aircraft, with aeronautical engineers realizing that swept wings, delta wings, and variable geometry wings unlocked the aerodynamic secrets to flight at speeds of Mach 1 and beyond. Of all the revolutionary wing planforms analyzed for high-performance combat aircraft by the victorious Allied Powers thanks to captured treasure troves of German aeronautical documents, the delta wing and variable geometry wing would end up emerging as the most appropriate wing planforms for airplanes capable of flight beyond Mach 2 due to their ability to reduce drag at low speeds and absorb shockwaves in level supersonic flight. However, the delta wing design of the US jet fighters that patrol the skies of the US defending America's sacred freedoms wouldn't be possible if it were not for a single ancestor built in the distant past: the Convair XF-92.
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| Early design study for the Convair XP-92, early 1946 |
In the final months of World War II, the Vultee Division of Consolidated Vultee (Convair) looked at the idea of a swept-wing aircraft powered by a ducted rocket engine, whereby fuel would be added to the heat produced by small rocket engines in the duct. In August 1945, the US Army Air Force announced a requirement for a supersonic interceptor capable of flying at 700 mph (1,100 km/h) speeds and reaching an altitude of 50,000 feet (15,000 meters) in four minutes. Convair responded with a design featuring wings swept back 45 degrees, a V-shaped tail empennage, and a mixed propulsion system comprising one 1,560 lb (6.9 kN) thrust Westinghouse 19XB/J30 turbojet and four 1,200 lb (5.3 kN) thrust rocket motors positioned at the exhaust nozzle of the turbojet. To meet performance requirements set out by the USAAF, operating range and flight endurance were sacrificed. By May 1946, Convair's proposal was accepted for further development by the Air Material Command of the USAAF, and the AMC designation MX-813 was allocated to the Convair design.
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| Top: Artist's conception of the delta-wing XP-92/XF-92 design, mid-1946. Bottom: Full-scale mockup of the Convair XP-92/XF-92 point-defense interceptor, April 1948. |
Not too long after the Convair proposal was accepted by the USAAF, however, aerodynamic problems were discovered during wind tunnel tests of the backswept wing design by Convair. For instance, the backswept, narrow-chord wings would experience problems with wing tip stalling at low angles of attack and issues were discovered with lateral control surfaces of the backswept wings. Recognizing the aerodynamic instability of the back swept narrow-chord wing, aerodynamicist Ralph Schick suggested to Convair technologist Adolph Burstein and test pilot Frank Davis that the design be reworked with a highly swept delta wing having a straight trailing edge. Burstein and Davis agreed with Schick's proposal, and a new design was envisaged in July with a pure 60 degree delta wing and a relatively short cylindrical fuselage, powered by a ramjet engine with six 2,000 lb (8.9 kN) thrust rocket motors buried in the ramjet's combustion chamber. This design, assigned the company designation Model 115 by Convair, would take off and climb to altitude with the rocket motors, and once at high altitudes the ramjet would ignite to propel the Model 115 to Mach 1.65, with a planned endurance of 5.4 minutes. In the event of an emergency, the pilot would jettison the fuselage containing the engines. Is it often said that the delta wing for the Model 115 was influenced by wartime research by German aircraft designer Alexander Lippisch, because the Convair Model 115 design had the same delta wing planform as the proposed Lippisch P.13a ramjet-powered interceptor. However, as noted by Hallion (1979), even though Ralph Schick met with Lippisch at Wright-Patterson Air Force in Dayton, Ohio, he rejected the thick delta wing proposed for the P.13a and the unpowered DM-1 technology demonstrator, preferring instead a delta wing with a thin airfoil.
In June 1946, two prototypes and one structural test airframe of the Model 115 were ordered, and the design was officially designated XP-92 by the USAAF. Later, in September, Convair had the USAAF amend the contract whereby the structural test airframe would be built as a flying mock-up, in other words a full-scale technology demonstrator, arguing that a technology demonstrator was essential to test the flight characteristics of the XP-92. The technology demonstrator received the serial number 46-682, whereas the XP-92 prototypes were assigned the serial numbers 46-683 and 46-684. The flying mock-up was allocated the number 7002, but this was not a company designation, but instead an internal accounting number by Convair on an engineering work order (Bradley 2013). To save development time and costs, components for the 7002 were taken from other aircraft, with the main gear taken from a North American FJ-1 Fury, the nosewheel coming from a Bell P-63 Kingcobra, the engine and hydraulics systems coming from a Lockheed P-80 Shooting Star, the ejection seat and cockpit canopy being sourced from the Convair XP-81, and the rudder pedals taken from a BT-13 trainer. Construction of the 7002 was underway when the Vultee facility in Downey was taken over by North American Aviation in the summer of 1947, prompting Convair to move the airframe to the Convair factory in San Diego. The aircraft was finished in December, and sent to the NACA Ames Aeronautical Laboratory for wind tunnel tests. The 7002 used a single 5,200 lb (23.1 kN) thrust Allison J33 turbojet, and was similar to the XP-92 design in the wing and tail configuration, but differed in having a bubble cockpit canopy. By April 1948, a full-scale mockup of the XP-92 was completed and ready for inspection by US Air Force officials. After the US Air Force decided to classify fighter planes as fighter rather than pursuit planes, the XP-92 became XF-92, while the technology demonstrator would be designated XF-92A. However, the XF-92 project was cancelled in June due to the complexities of the planned propulsion system, rising development costs, and the fact that the requirement for a point-defense interceptor had vanished.
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| Top: Desktop model of the Convair XF-92A at the Western Museum of Flight (photo taken by me in April 2021). Bottom: Convair XF-92A (serial number 46-682) in flight. |
Even before the XF-92 point-defense interceptor program was cancelled, in March 1948, the XF-92A arrived back in San Diego, California to have the Allison J33 turbojet installed, and it was transported by vessel to LA harbor for shipment to Muroc Air Force Force (later renamed Edwards AFB). Taxi tests began in May, and on September 18, the XF-92A made its first flight, piloted by Convair test pilot Ellis D. Shannon. After 47 test flights, the XF-92A was delivered to the Air Force on August 26, 1949, with Frank Everest and Chuck Yeager in charge of flight testing. On one test flight, the XF-92A managed to reach Mach 1.05 for a brief time, becoming the first American delta-wing plane to go supersonic. In 1951, the XF-92A was refitted with an afterburning J33-A-29 yielding 7,500 lb (3.3 kN) of thrust, flying with this engine for the first time on July 20, but this engine was hamstrung by maintenance problems and offered the XF-92A only marginal improvement in performance, so only 21 flights were made with the afterburning version of the J33. The XF-92A began flight tests on behalf of NACA on April 9, 1953, and 25 flights were made by Scott Crossfield until October 14, when the aircraft had a nose gear collapse during landing after its last flight. After being retired, the XF-92A languished as a static exhibit until 1962, when the US Air Force retrieved the aircraft and donated it to the National Museum of the US Air Force in Dayton, Ohio, where the aircraft resides today.
Although the XF-92 was not built for the role for which it was intended, it nevertheless provided a library of aerodynamic data on the delta wing planform that would later be applied to development of all American delta-wing aircraft, including the B-58, XB-70, F-102, F-106, F-4, F-15, F-16, F-22, and F-35. To this day, designers of fighter aircraft largely tend to focus on the delta wing as the best wing planform for combat jets capable of flying at speeds beyond Mach 1.
References:
Bradley, R., 2013. Convair Advanced Designs II: Secret Fighters, Attack Aircraft, and Unique Concepts 1929-1973. Manchester, UK: Crécy Publishing.
Buttler, T., 2007. American Secret Projects: Fighters and Interceptors 1945 to 1978. Hinckley, UK: Midland Publishing.
Hallion, R., 1979. Lippisch, Gluhareff, and Jones: The Emergence of the Delta Planform and Origins of the Sweptwing in the United States. Aerospace Historian 26 (1): 1-10.
Jenkins, D.R., and Landis, T.R., 2008. Experimental & Prototype U.S. Air Force Jet Fighters. North Branch, MN: Specialty Press.
Yenne, B., 2009. Convair Deltas: From SeaDart to Hustler. North Branch, MN: Specialty Press.
