Variable-sweep wing
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.
A straight, unswept wing experiences high drag as it approaches the speed of sound, due to the progressive buildup of sonic shockwaves.
Some mechanism, such as a sliding wing root or larger tail stabiliser, must be incorporated to trim out the changes and maintain level flight.
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.
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.
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.
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.
[5] 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.
[6] 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.
[13][14] 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.
[citation needed] 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.
[5] Subsequently, Barnes devised the Swallow,[5] a blended wing tailless aircraft, which was envisioned to be capable of making return flights between Europe and Australia within ten hours.
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.
[1] In 1960, Maurice Brennan joined Folland Aircraft as its chief engineer and director; he soon set about harnessing his experience of variable-geometry wings.
[19] 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.
In the United States, such a configuration for the TFX (Tactical Fighter Experimental) program, which resulted in the development of the General Dynamics F-111, a sizable twin-engined aircraft intended to perform multiple roles.
[23][24] The F-111 is the first production aircraft to feature a variable-geometry wing and it, along with other systems such as terrain following radar and turbofan engines outfitted with afterburners, were innovative technologies for the era.
This design was used, albeit at different scales, for the Mikoyan-Gurevich MiG-23 fighter and the Sukhoi Su-24 tactical bomber, both of which flew in prototype forms around the end of the 1960s and entering service during the early 1970s.
[37][38] To replace the TSR-2, the Air Ministry initially placed an option for the American General Dynamics F-111K;[39][40] while the F-111K was promoted as being cheaper,[41] this too was terminated during January 1968 on grounds of cost.
[45] According to aviation author Derek Wood, both Dassault and the French Air Force were unenthusiastic participants in the AFVG, the former wanting to pursue its own indigenous variable geometry aircraft, while the latter had determined that the type did not align with its future equipment plans.
[54] Furthermore, Dassault also worked in cooperation with the American manufacturing interest Ling-Temco-Vought to develop the LTV V-507, which was submitted for US Navy's VFX project.
The F-14 was a more nimble fighter than the F-4 Phantom II and, unlike the F-111, its variable-sweep wings automatically adjusted over its speed range, and could be moved even during turns.
Furthermore, the wings could be swept forward for tight "bat" turns in close quarters aerial combat, as well as rearwards for dash speeds.
[58] When the wings were set to their widest position the aircraft had considerably better lift and power than the B-52, allowing the B-1 to operate from a much wider variety of bases.
[64] Designated as the Tupolev Tu-160, it entered operational service with the 184th Guards Heavy Bomber Regiment located at Pryluky Air Base, Ukrainian SSR, during April 1987.
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.
