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| As the performance of jet engines grew, fighers with more traditional planforms found they could perform almost as well as the deltas, but do so while maneuvering much harder and at a wider range of altitudes. Today a remnant of the compound delta can be found on most ], in the form of ]s. These are effectively very small delta wings placed so they remain parallel to the airflow in cruising flight, but start to generate a vortex at high angles of attack. The vortex is then captured on the top of the wing to provide additional lift, thereby combining the delta's high-alpha "trick" with a conventional highly efficient wing planform. | As the performance of jet engines grew, fighers with more traditional planforms found they could perform almost as well as the deltas, but do so while maneuvering much harder and at a wider range of altitudes. Today a remnant of the compound delta can be found on most ], in the form of ]s. These are effectively very small delta wings placed so they remain parallel to the airflow in cruising flight, but start to generate a vortex at high angles of attack. The vortex is then captured on the top of the wing to provide additional lift, thereby combining the delta's high-alpha "trick" with a conventional highly efficient wing planform. | ||
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Revision as of 06:20, 28 September 2004
The delta-wing is a wing planform in the form of a large triangle. Its use in the so called "tailless delta", i.e. without the horizontal tailplane, was pioneered especially by Alexander Lippisch in Germany and Boris Ivanovich Cheranovsky in the USSR prior to WWII; none of their glider and powered aeroplane designs saw widespread service, though. After the war the tailless delta became the favoured design for high-speed use, and was used almost to exclusion of other planforms by Convair in the United States and Dassault in France. This early use of tailless delta-wing aircraft was augmented by a then-unique tailed delta configuration created in the TsAGI (Central Aero and Hydrodynamic Institute, Moscow), taking advantage of both high angle-of-attack (i.e. manoueuvre) capability and high speeds. It was used on the MiG-21 (Fishbed) and Sukhoi Su-9/11/15 fighters, built in several tens of thousands of copies. Nowadays, with the relaxed or no natural stability of aircraft and the necessary electronic control (Fly by wire - FBW) the horizontal control surfaces move more often forward to become the so-called canard in front of the wing to control the aeroplane as the normal elevator does, and modifying favourably the airflow over the wing, most notably during lower altitude flight. Contrary to the classic tail-mounted elevators the canards add to the total lift, enabling to execute extreme maneuvres or to markedly reduce drag.
The primary advantage of the delta wing design is that the wing's leading edge remains behind the shock wave generated by the nose of the aircraft when flying at supersonic speeds, which was a distinct improvement on traditional wing designs. Another advantage is that as the angle of attack increases the leading edge of the wing generates a huge vortex which remains attached to the upper surface of the wing, making the delta have very high stall points. The combination of these two features is a dream come true, as a normal wing built for high speed use is typically dangerous at low speeds, but in this regime the delta transitions to a mode of lift based on the vortex it generates. The disadvantage, especially marked in the older tailless delta designs, is, firstly, the loss of total available lift caused by turning up the wing trailing edge or the control surfaces, as required to achieve a sufficient stability, and, secondly, the high induced drag of this low-aspect-ratio type of wing.
Lippisch created the aerodynamic predecessor to the combatworthy, series-produced and routinely operated Messerschmitt Me 163 rocket interceptor, a concept which was developed further in the wartime Germany and later elsewhere. Lippisch, Frenchman Payen and the DFS studied a number of ramjet powered (sometimes coal-fueled!) delta-wing interceptor aircraft during the war, one progressing as far as a glider prototype. After the war Lippisch was taken to the US, where he ended up working at Convair. Here the other engineers became very interested in his interceptor designs, and started work on a larger version known as the F-92. This project was eventually cancelled as impractical, but a prototype flying testbed was almost complete by that point, and was later flown widely as the XF-92. The design generated intense interest around the world. Soon almost every aircraft design, notably interceptors, were designed around a delta-wing. Examples include the Fairey FD-1 Delta, the Dassault Mirage fighter and bomber family up to the Mirage 2000, the North American XB-70 Valkyrie M-3 bomber, the Lockheed Blackbird family, the Convair B-58 Hustler bomber, the Avro Vulcan bomber and the Mikoyan-Gurevich MiG-21 and Sukhoi Su-9, Su-11 and Su-15 fighters.
Pure deltas fell out of favour somewhat due to some undesirable characteristics, notably flow-separation at high angles of attack (swept-wings have similar problems), and high drag at low altitudes. This limited them primarily to the high-speed, high-altitude interceptor roles. A modification, the compound delta such as seen on the SAAB J-35 Draken fighter or the prototype F-16XL, or the graceful ogival (or Gothic) delta used on the Anglo-French Concorde Mach 2 airliner, connected another much more highly swept piece of the delta wing to the forward root section of the main one, to create the high-lift vortex in a more controlled fashion, reduce the drag and thereby allow for landing the delta at acceptably slow speed.
As the performance of jet engines grew, fighers with more traditional planforms found they could perform almost as well as the deltas, but do so while maneuvering much harder and at a wider range of altitudes. Today a remnant of the compound delta can be found on most fighter aircraft, in the form of leading edge extensions. These are effectively very small delta wings placed so they remain parallel to the airflow in cruising flight, but start to generate a vortex at high angles of attack. The vortex is then captured on the top of the wing to provide additional lift, thereby combining the delta's high-alpha "trick" with a conventional highly efficient wing planform.
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