The Dawn of Delta

From the late 1930s until the end of World War II, German aircraft designers produced many of the ideas that have become a part of modern aviation. One of these designs, the delta wing, was the brain child of German Professor Alexander Lippisch, who was fascinated by what he called the Fliegende Dreiecke –“Flying Triangles.”

After working through various gliders in the 1930s, he progressed in 1943 to a full fighter-sized glider – the DM-1 – which looked like two large triangles joined together as one formed the main structure and wing and the other the vertical stabilizer and cockpit. This profile was relatively thick, making it possible to place the pilot in the joint of the wing and fuselage with a glazed windshield and glazing under the nose to help landing visibility. The DM-1 was towed aloft several times, cut loose to glide down and proved to have excellent handling characteristics. Lippisch hoped to develop the glider into a fighter, the Lippisch P.13a, using rockets and a ram jet engine fueled by coal dust (no kidding…) to solve the problems of lack of fuel at the end of war, but the conflict ended before the project came to fruition. 

Back in the United States, in August 1945 U.S. Army Air Force asked for proposals for a supersonic “point defense” interceptor that could reach 50,000 feet in four minutes. Consolidated Vultee (later Convair) won with their proposal for a mixed power ramjet and rocket powered aircraft with a 35-degree swept wing and “V” tail, but for development the company decided to split the power plant and the airframe into two different projects. The engine project faltered, but with the war over, American aviation companies were pouring across the German designs that came to the United States as part of Operation Paperclip – the extraction of German technology for exploitation. Convair came across the work of Lippisch and brought him to Convair to work on the point interceptor project, and Lippisch changed the wing platform to a delta very much like the DM-1.

The power was a jet engine with the pilot sitting in the shock cone engine extension in the intake. This arrangement was too big for the fuselage so the pilot was moved out into a conventional cockpit, though aerodynamically the design was similar in general layout.

The new design was presented to the AAF in 1946, but the service found it unacceptable as an interceptor and told Convair to develop it as a delta wing research aircraft instead. To save development time and money, many components – the landing gear, ejection seat and cockpit canopy – were taken from other aircraft, and the engine and hydraulics were taken from a Lockheed P-80 Shooting Star.

The XF-92A made the first flight of a delta wing aircraft Sept. 18, 1948, and the flight tests – many flown by Chuck Yeager – were generally satisfactory. The Air Force asked that an even more powerful engine with an afterburner be installed, but the engine offered little increase in speed and the XF-92A could only go supersonic in a dive, so the Air Force turned it over to the National Advisory Committee for Aeronautics, the predecessor of NASA that was the agency responsible for institutional aeronautical research. Nevertheless, the Air Force was impressed enough to order a delta wing all-weather fighter from Convair, the F-102.

The XF-92A continued its test program until its nose wheel collapsed in a taxing accident on Oct. 14, 1954. Since the first F-102 was scheduled to fly 10 days later and would provide information on the delta wing, NACA decided not to repair it. It was returned to the Air Force where it was used for many years as a touring exhibition. In 1962, it was turned over to the Air Force Museum where it resides today.

Lippisch’s tailless delta wing design became a favorite for high-speed use and went on to power a number of very successful and not-so-successful aircraft, such as the F-102 and F-106, B-58, the French Mirage III and IV, the British Avro Vulcan bomber and the Concorde supersonic transport. The delta wing proved to be extremely efficient at high altitude and since the platform carried across the entire aircraft, it was much stronger than a swept wing and had more internal volume for fuel and other storage. But there is no free lunch – in fighters, the delta wing’s high-induced drag caused it to “bleed off” energy very rapidly in turns, a disadvantage in aerial combat maneuvering, and it had high drag at low altitude which made it burn fuel rapidly.

Questions or comments, contact Dr. Michel at marshall.michel@ramstein.af.mil.