Variable-sweep wing
A variable-sweep wing, colloquially known as a "swing wing", is an airplane wing, or set of wings, that may be modified during flight, swept back and then returned to its previous straight position. Because it allows the aircraft's shape to be changed, it is a feature 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 be swept. Most aircraft that travel at those speeds usually have wings 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 is increased, necessitating long runways. 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 speed at the moment, whether 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 impractical for most commercial applications and result in its use being primarily for military aircraft.
A number of aircraft, both experimental and production, were introduced between the 1940s and the 1970s. The majority of production aircraft to be furnished with variable-sweep wings have been strike-oriented aircraft, such as the Mikoyan-Gurevich MiG-27, Tupolev Tu-22M, and Panavia Tornado. The configuration was also used for a few fighter/interceptor aircraft, including the Mikoyan-Gurevich MiG-23, Grumman F-14 Tomcat, and the Panavia Tornado ADV. From the 1980s onwards, the development of such aircraft were curtailed by advances in flight control technology and structural materials which 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 technique is another form of variable geometry.
Characteristics
Variable sweep
A straight, unswept wing experiences high drag as it approaches the speed of sound, due to the progressive buildup of sonic shockwaves. Sweeping the wing at an angle, whether backwards or forwards, delays their onset and reduces their overall drag. However it also reduces the overall span of a given wing, leading to poor cruise efficiency and high takeoff and landing speeds.A fixed wing must be a compromise between these two requirements. Varying the sweep in flight allows it to be optimised for each phase of flight, offering a smaller aircraft with higher performance. However it has disadvantages which must be allowed for. As the wing sweeps its centre of lift moves with it. Some mechanism, such as a sliding wing root or larger tail stabiliser, must be incorporated to trim out the changes and maintain level flight. The added weight of the sweep and trim mechanisms eat into the performance gains, while their complexity adds to cost and maintenance.
By moving the wing pivots outboard and only sweeping part of the wing, the trim changes are reduced, but so too is the variation in span and accompanying operational flexibility.
Wing controlled aerodyne
British engineer Barnes Wallis developed a radical aircraft configuration for high-speed flight, which he regarded as distinct from the conventional fixed-wing aeroplane and called it the wing controlled aerodyne. His previous work on the stability of airships had impressed on him the high control forces that could be exerted on the body of an aircraft, through very small deflections. He conceived of a simple ichthyoid fuselage with a variable wing. No other control surfaces were needed. Subtle movements of the wings were able to induce the small deflections which controlled the direction of flight, while trim was maintained by adjusting the angle of sweep to compensate for the varying position of the centre of lift at different speeds.For supersonic flight a delta-planform lifting body is more suitable than a simple ichthyoid. A conflict also arises between the wing sweep angle necessary for trim and the optimal angle for supersonic cruise. Wallis resolved this by moving mass, typically the engines, out to the wing tips and swivelling them as the wing swept in order to maintain the thrust line. In the asymmetric engine-out condition, the remaining engines could be swivelled to divert the thrust line closer to the centre of pressure and reduce the asymmetry to manageable levels.
Asymmetric sweep
It is not necessary to sweep the port and starboard wings in the same sense - one can be swept back and the other forward, as in the oblique wing.Varying the sweep asymmetrically by small amounts was also fundamental to the principle of the wing controlled aerodyne.
History
Origins
The earliest use of variable sweep was to trim the aeroplane for level flight. The Westland-Hill Pterodactyl IV of 1931 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. The concept would later be incorporated in Barnes Wallis's wing-controlled aerodyne.During the Second World War, researchers in Nazi Germany discovered the advantages of the swept wing for transonic flight, and also its disadvantages at lower speeds. The Messerschmitt Me P.1101 was an experimental jet fighter which was, in part, developed to investigate the benefits of varying wing sweep. Its sweep angle mechanism, which could only be adjusted on the ground between three separate positions of 30, 40, and 45 degrees, was intended for testing only, and was unsuitable for combat operations. However, by Victory in Europe Day, the sole prototype was only 80 per cent complete.
Development
Following the end of the conflict, the partially complete P.1101 was recovered and transported to the United States, where it was studied in depth by Bell Aircraft. However, due to a lack of documentation as well as some structural damage sustained, Bell decided against completing the aircraft itself. Instead, a close copy, known as the Bell X-5, was constructed with wings that enabled the sweep angle to be altered mid-flight. As the wing swept back, the root also slid forwards, maintaining the centre of lift in a constant position. A variable-sweep wing of this sliding type was flown on the prototype Grumman XF10F Jaguar in 1952. However, flight testing of the F10F proved to be unacceptable, albeit for other factors such as a lack of engine power and considerable controllability issues.During the late 1940s, British engineer L. E. Baynes started studying the variable sweep wing. He devised a method of varying the tail geometry as well in order to stabilise the centre of lift; no sliding mechanism was necessary, instead, the wing wake interacted with the variable tail to effect the necessary trim changes. During 1949 and 1951, Baynes filed patent applications associated with this work. While the design reached the physical modelling stage and was subject to a complete round of wind tunnel tests, the British Government failed to provide financial backing for the work, allegedly due to budget constraints at the time.
Independently from Baynes, British engineer Barnes Wallis was also developing a more radical variable-geometry concept, which he called the wing controlled aerodyne, to maximise the economy of high-speed flight. His first study was the Wild Goose project. Subsequently, Barnes devised the Swallow, a blended wing tailless aircraft, which was envisioned to be capable of making return flights between Europe and Australia within ten hours. Later on, the Swallow was increasingly viewed as a potential supersonic successor to the subsonic Vickers Valiant, one of the RAF's V bombers. During the 1950s, several modes of the Swallow were subjected to promising tests, including a six-foot scale model, at speeds of up to Mach 2. However, in 1957, British government decided to withdraw backing from many aeronautical programs, including Wallis' work.
Despite this lack of backing, the Swallow attracted international attention for some time. During late 1958, research efforts were temporarily revived through cooperation with the Mutual Weapons Development Programme of NATO, under which all of Wallis' variable geometry research was shared with the Americans. According to aviation author James R. Hansen, American aerospace engineer John Stack was enthusiastic on the concept, as were numerous engineers at NASA; however, the United States Department of Defense was opposed to committing any resources to the project. Wallis collaborated with NASA's Langley Laboratory on a design study for a variable-sweep fighter. Although it used the pivot mechanism he had developed, NASA also insisted on implementing a conventional horizontal stabiliser to ease the issues of trim and manoeuvrability. Although it was no longer the wing-controlled aerodyne that Wallis envisaged, it would prove a more practical solution than either his or Bell's. Swallow research led to several new configurations, including the adoption of a compact folding tail section and canards.
Barnes' work inspired a number of further studies, including a wing controlled aerodyne in response to OR.346 for a supersonic STOL fighter-bomber, then as BAC two further submissions: the Type 583 to meet Naval ER.206 and Type 584 to meet NATO NBMR.3, both also being V/STOL requirements. In 1960, Maurice Brennan joined Folland Aircraft as its chief engineer and director; he soon set about harnessing his experience of variable-geometry wings. Accordingly, such a wing was combined with the firm's Folland Gnat light fighter for two different concepts – one tailless and one using with a conventional tail – for a multipurpose fighter/strike/trainer, designated as the Fo. 147. It had a unique mechanism for wing sweep that combined tracks on the fuselage sides and the underside of the wings, which was actuated by hydraulically-driven ball screws positioned at the wing's inner ends. The wings could be swept from 20 degrees to 70 degrees; at the 70-degree position, longitudinal control was maintained by wing tip-mounted elevons, while this was provided by a retractable canard arrangement when swept at the 20-degree position, using full auto-stabilisation. By providing trimming functionality via the canard, the necessity of a large tailplane was eliminated. The Fo. 147 was claimed to have been capable of speeds in excess of Mach 2, being limited only by the heat buildup generated by high speed flight. Ultimately, the concept would not be developed to the prototype stage while the RAF showed little interest in the prospective variable geometry trainer.