Variable-sweep wing

A Variable-sweep wing is an aeroplane wing that may be swept back and then returned to its original position during flight. It allows the aircraft's planform to be modified in flight, and is therefore an example of variable geometry.

Typically, a swept wing is more suitable for high speeds, while an unswept wing is suitable for lower speeds (such as when taking off and landing). A Variable-sweep wing allows a pilot to select the correct wing configuration for the plane's intended speed. The Variable-sweep wing is most useful for those aircraft that are expected to function at both low and high speed, and for this reason it has been used primarily in military aircraft.

The added mass and complexity required to design a plane with a Variable-sweep wing somewhat offset the benefits. As a result, Variable-sweep wings have not seen widespread adoption.

History of development

Variable-sweep wing aircraft developed from earlier experimental aircraft that were built to study the effects of a simple swept wing. The first of these was the Messerschmitt Me P.1101 whose sweep angle could be changed on the ground. A number of test flights were carried out at various angles to determine the trade offs.

At the end of World War II the P.1101 was taken to the United States for further study at Bell Aircraft, where versions were built that could vary their wing angle in flight. One problem discovered while testing the Bell X-5 was that as the wing pivoted rearward, the lift vector also moved to the rear, pushing the nose down. A system to compensate for this basic effect had to be added for any such design to be viable.

Immediately post war Barnes Wallis had started work on variable geometry to maximise the economy of supersonic flight. Initial work was on the military "Wild Goose" project, then he went onto the "Vickers Swallow", intended to achieve a return flight from Europe to Australia in 10 hours. It had a blended wing tailless design and he successfully tested several models including a six foot scale model at speeds of up to Mach 2 in the 1950s but government backing was withdrawn. Wallis and his team presented their work to the Americans seeking a grant to continue their studies but none was forthcoming. [ [http://www.barneswallistrust.org/swingwing.htm The Barnes Wallis Memorial Trust ] ] In 1949, L E Baynes, an aeronautical engineer and designer of the Baynes Bat, patented a design for a supersonic Variable-sweep wing fighter but the design was not built.

A Variable-sweep wing was tried on the Grumman F10F Jaguar in 1952. The XF10F never entered service; it possessed extremely poor flying characteristics and rather vicious spin tendencies. The idea was again revived in the early 1960s as a way to reconcile ever-growing aircraft weights (and thus wing loading) with the need to provide reasonable takeoff and landing performance. The United States adopted this configuration for the TFX (Tactical Fighter Experimental) program, which emerged as the General Dynamics F-111, the first production Variable-sweep wing aircraft.

Similar requirements in the Soviet Union also led TsAGI, the Soviet aerodynamics bureau, to explore the possibilities of variable geometry. TsAGI evolved two distinct planforms, differing mainly in the distance (expressed as a percentage of total wingspan) between the wing pivots. A wider spacing not only reduced the negative aerodynamic effects of changing wing sweep, but also provided a larger fixed wing section which could be used for landing gear or stores pylons. This could, in fact, be adapted to more-or-less existing airframes, which the Soviets soon did, with the Sukhoi Su-17 (based on the earlier swept wing Sukhoi Su-7) and the Tupolev Tu-22M (based on the Tupolev Tu-22). The limitation of the wide spacing, however, was that it reduced the benefits of variable geometry as much as it reduced their technical difficulties. For the new, "clean-sheet" Soviet designs, TsAGI devised a more narrowly spaced arrangement similar to that of the F-111. This design was used (albeit at different scales) for the MiG-23 fighter and the Sukhoi Su-24 interdictor, which flew in prototype forms at the end of the 1960s, entering service in the early 1970s.

In the aftermath of the cancellation of the TSR-2, the British had started a project with the French for the Anglo-French Variable Geometry aircraft (AFVG). When French commitment was curtailed the British sought a second partner in the F104 Consortium of European nations. This in turn led to the European consortium that adopted variable geometry for the Multi-Role Combat Aircraft (MRCA) project that emerged as the Panavia Tornado. [Buttler, Tony "British Secret Projects: Jet Bombers Since 1949"] This was an interdictor and stand-off interceptor similar in function to the F-111, albeit on a smaller scale. Meanwhile, the U.S. Navy arranged to replace the canceled F-111B fleet interceptor with the Grumman F-14 Tomcat. Variable-sweep wing were seen as a way to reconcile the low landing speeds necessary for carrier fighters with the fighter's role as a high-speed interceptor. As a side effect, though maneuverability was not a design aim,Fact|date=March 2008 the F-14 was remarkably agile, despite its underpowered engines. It was far more maneuverable than the F-111, the Tornado, or any of the Soviet Variable-sweep wing aircraft due to lower effective wing loading (thanks to a airfoil fuselage that provided additional lift) and an automatic wing sweep. Rockwell, meanwhile, adopted variable geometry for the Advanced Manned Strategic Bomber (AMSA) program that produced the B-1 Lancer bomber, intended to provide an optimum combination of high-speed cruising efficiency and fast, supersonic penetration speeds at extremely low level. The last Variable-sweep wing military aircraft to date was the Soviet Tupolev Tu-160 "Blackjack", which first flew in 1980.

A Variable-sweep wing was also used by Boeing's entry in the FAA's study for a supersonic transport, the 2707. However during the design stage it became clear that the mechanism was so heavy that it would leave little available payload in the cabin for paying passengers.Dubious|date=April 2008 The design was later abandoned in favor of a more conventional delta wing.

While variable-sweep provides many advantages, particularly in takeoff distance, load-carrying ability, and the fast, low-level penetration role, Variable-sweep wing impose a considerable penalty in weight and complexity. The advent of relaxed stability flight control systems in the 1970s negated many of the disadvantages of a fixed platform, and no new Variable-sweep wing aircraft have been built since the Tu-160.

Variable-sweep aircraft

Experimental

* Bell X-5
* Dassault Mirage G
* Messerschmitt Me P.1101
* XF10F Jaguar

Production

* B-1 Lancer
* EF-111A Raven
* F-14 Tomcat
* General Dynamics F-111
* Mikoyan-Gurevich MiG-23
* Mikoyan-Gurevich MiG-27
* Panavia Tornado
* Panavia Tornado ADV
* Sukhoi Su-17
* Sukhoi Su-24
* Tupolev Tu-22M
* Tupolev Tu-160

ee also

* Adaptive Compliant Wing
* Oblique Wing
* Variable-incidence wing
* Variable camber wing

References