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XF-92A Dart

As soon as German World War II research on delta wings became known, Convair became an early convert to the principle. Triangular wings have large surface area, and hence generate a larger amount of lift than conventional wings. One approach to low-drag wings was the use of delta shaped planforms with a highly swept leading edge and a low thickness ratio. Research on delta wing designs had been done in Germany by Alexander M. Lippisch, best known for developing the Messerschmitt Me 163 Komet rocket fighter. Lippisch first produced and demonstrated a practical delta-wing aircraft in 1931.

Lippisch came to the US after the war and, working with the Air Force and Convair, aided in the design of the tailless delta airplane XF-92A which, in turn, lead to the Convair F-102, F-106 and B-58 delta wing airplanes. Many delta wing designs have been developed in many other countries.

With a low aspect ratio (short span and broad chord), they have a good lift-to-drag ratio at high speeds. However, they also have a generally poor lift-to-drag ratio at low speeds, which is why delta winged aircraft must be landed nose high. Delta wings have great rigidity and can be built thin, but still retain great internal volume for a large fuel supply. And most importantly, the delta shape has low parasite drag in the transonic range, making it easier to achieve supersonic speeds.

America's first delta-wing airplane, the Convair XF-92A Dart, looked futuristic when it appeared at Edwards Air Force Base for its flight test programs in April 1948. Built strictly for delta-wing research, its sharply raked (60 degrees) wing and vertical stabilizer promised low drag at transonic speeds, good low-speed handling characteristics, and stability at high altitudes. The new jet proved to be much slower than anticipated, however, and it was extremely sensitive on the controls.

The Convair XF-92A aircraft was powered by a Allison J-33-A turbo jet engine with an afterburner, and was unique in having America's first delta wing. The delta wing's large area (425 sq. ft), thin airfoil cross section, low weight, and structural strength made this a great combination for a supersonic airplane. The Air Forces had intended this aircraft to be a testbed for a first all-weather interceptor.

The single-place XF-92A airplane had a delta wing swept at 60 degrees. It was 48.2 feet long, had a 31.3-foot wingspan, and was 17.5 feet high at the tip of the vertical stabilizer. It was controlled by a conventional rudder and full-span elevons that functioned as elevators and ailerons.

Major "Chuck" Yeager was the first Air Force pilot to fly the XF-92A and pioneered the techniques of flying the delta configuration; he rolled the plane on his first flight and landed at a lower, safer, speed. It fell to Yeager to take the plane supersonic for the first and only time in its career; a full 90 degree, 4 g split-S dive forced the stubby delta to Mach 1.1 for a brief time. On the second of his 19 test missions in the aircraft, he commented, "It was a tricky plane to fly, but ... I got it out to 1.05 Mach." This was .20 Mach faster than the aircraft's developer, Convair, had attained. During the same flight, he decided to see how slow he could land it. Pointing the nose up at a 45-degree angle of attack, he landed at a speed of only 67 mph--more than 100 mph slower than Convair's test pilot. When Gen. Boyd first flew it, Yeager advised him: "You just go rolling down the runway, sir, and when that airspeed indicator gets to 180, you just blow a little on that stick."

The original XF-92A ship had a severe pitch-up problem but was tested with different wing-fence combinations to gather data on their contribution to solving that problem. The pilot also reported that the aircraft was sluggish and underpowered. Besides validating the thin delta wing principle, the XF-92A played a major role in supporting the development of the Convair F-102A interceptor, the Air Force's first attempt at an all-weather, supersonic interceptor. In 1953, the XF-92A experienced a landing gear failure on rollout after landing at the NACA High-Speed Research Station and the aircraft was retired.

The YF-102 Delta Dagger was essentially a scaled-up version of the Dart, built to accommodate a Wright J67 turbojet with afterburner, a Hughes MA-1 radar system, and six Falcon air-to-air missiles carried in an internal bay. On paper, the combination seemed unbeatable but severe real-world problems soon intruded.

After the F-92 interceptor failed to materialize, the NACA High-Speed Flight Research Station assumed the flight testing in 1953. NACA pilot A. Scott Crossfield flew all 25 flights over the six month test period. The XF-92A had a bad pitch-up problem which was solved eventually by adding different wing-fence combinations. The research on the XF-92A lead to the development of F-102 fighter and other similar aircraft.



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