A variable-sweep wing, colloquially known as a "swing wing", is an airplane wing, or set of wings, that may be swept back and then returned to its original straight position during flight. It allows the aircraft's shape to be modified in flight, and is therefore an example of a variable-geometry aircraft.
A straight wing is most efficient for low-speed flight, but for an aircraft designed for transonic or supersonic flight it is essential that the wing is swept. Most such fast aircraft usually have wings (either swept wing or delta wing) with a fixed sweep angle. These are simple and efficient wing designs for high speed flight, but there are performance tradeoffs. One is that the stalling speed becomes rather high, necessitating long runways (unless complex high-lift wing devices are built in). Another is that the aircraft's fuel consumption during subsonic cruise is higher than that of an unswept wing. These tradeoffs are particularly acute for naval carrier-based aircraft. A variable-sweep wing allows the pilot to use the optimum sweep angle for the aircraft’s current speed, slow or fast. The more efficient sweep angles available offset the weight and volume penalties imposed by the wing's mechanical sweep mechanisms. Its greater complexity and cost make it practical mostly for military aircraft.
A number of successful and experimental designs were introduced from the 1940s into the 1970s; however, the recent advances in flight control technology and structural materials have allowed designers to closely tailor the aerodynamics and structure of aircraft, removing the need for variable sweep angle to achieve the required performance; instead, wings are given computer-controlled flaps on both leading and trailing edges that increase or decrease the camber or chord of the wing automatically to adjust to the flight regime. This is another form of variable geometry, although it is not commonly called such.
The 1931 Westland-Hill Pterodactyl IV was a tailless design whose lightly swept wings could vary their sweep through a small angle during flight. This allowed longitudinal trim in the absence of a separate horizontal stabiliser.
Later, experimental aircraft 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. World War II in Europe ended a few weeks before its scheduled first flight.
The P.1101 was taken to the United States for study at Bell Aircraft, but because of missing documentation and structural damage, Bell decided against completing it. Instead, a close copy was constructed which featured wings that could adjust sweep angle in flight. As the position of the lift relative to the cg (and hence longitudinal stability) changes with wing sweep, the Bell X-5 wing translated forward as sweep increased to prevent excessive instability (i.e. extreme forward cg relative to center of lift). A more practical solution for subsequent swing-wing designs was the outboard pivot, or rotation-only concept pioneered by Barnes Wallis around 1954. The detailed implementation of the concept was done by the NASA Langley Laboratory team of Alford, Polhamus and Barnes Wallis.
In 1949, British engineer Barnes Wallis started work on variable geometry to maximise the economy of supersonic flight. His first study, for the military, was the Wild Goose project. He then studied the 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 in 1957, government backing was withdrawn for many aeronautical research and development programs, including Wallis' work. Wallis and his team presented their work to the Americans seeking a grant to continue their studies but none was forthcoming. In March 1949, British engineer L. E. Baynes designed a supersonic variable-sweep wing fighter. He lodged patent applications in Britain, and in May 1956 was granted US Patent 2,744,698 for "High Speed Aircraft Wing and Tail Surfaces Having Variable Sweep-back". In February 1951 he applied for another patent (granted as US 2,741,444 in April 1956) for a supersonic variable-sweep wing and tail fighter ["High Speed Aircraft Having Wings With Variable Sweepback"]. The design was built and wind tunnel tests were completed, but due to budget constraints at the time, the government did not provide financial backing.
A variable-sweep wing was tried on the Grumman F10F Jaguar in 1952. The F10F 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. Most aircraft with variable-sweep wings must carry external stores under their wing gloves or fuselages, but F-111s were equipped with pivoting wing pylons (two under each wing) which automatically adjusted to the sweep angle. The F-111, along with the Panavia Tornado and Sukhoi Su-24, were the only swing wing aircraft so equipped.
Similar requirements in the Soviet Union also led TsAGI, the Soviet aerodynamics bureau, to study variable geometry. TsAGI evolved two distinct designs, 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). 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 interceptor, which flew in prototype forms at the end of the 1960s, entering service in the early 1970s. As of 2014 more than 100 Tupolev Tu-22M strategic bombers are in use.
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 F-104 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. This was an interdictor and stand-off interceptor similar in function to the F-111, albeit on a smaller scale. After AFVG, Dassault Aviation built a prototype fighter in 1968, Dassault Mirage G, two variants Mirage G4 and G8, and in cooperation with Ling-Temco-Vought, the LTV V-507 for VFX project. Meanwhile, the U.S. Navy introduced the Grumman F-14 Tomcat to replace the canceled F-111B fleet interceptor with a fighter more nimble than the F-4 Phantom. Unlike the F-111, its variable-sweep wings were programmed automatically by speed and could be moved even during turns. In air combat, the wings could be swept forward for tight "bat" turns and back for dash speeds. 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" strategic bomber, which first flew in 1980.
A variable-sweep wing was also selected as the winning design used by Boeing's entry in the FAA's study for a supersonic transport, the 2707. However it evolved through several configurations during the design stage, finally adding a canard, and it eventually became clear that the design would be so heavy that it would be lacking sufficient payload for the fuel needed. The design was later abandoned in favor of a more conventional tailed delta wing.
While variable-sweep provides many advantages, particularly in takeoff and landing performance, fuel economy in subsonic cruise (due to higher aspect ratio when wings are unswept), load-carrying ability, and low gust response in the fast, low-level penetration role, the configuration imposes 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. No new variable-sweep wing aircraft have been built since the Tu-160 (produced until 1992), though it has been noted that the F-14's replacement - the F/A-18E - has a reduced payload/range capability largely because of its small fixed wings.
In 2015, the Russian Ministry of Defence announced plans to restart Tu-160 production, citing the aging of the current aircraft and likely protracted development of its eventual replacement, the PAK DA project. The production is planned to recommence in 2020, making the new aircraft the first new variable sweep airframes produced after 28 years.
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