History and Design
Armstrong Whitworth Aircraft proposed a jet-powered six or four-engine flying wing airliner design, utilising a laminar flow wing, during the Second World War. This had to be a large aircraft in order to provide passenger head-room within the wing. The low-speed characteristics of the design were tested on a 53 ft 10 in (16.41 m) span wooden glider known as the A.W.52G; the glider was designed to be roughly half the size of the powered A.W.52, which in turn would be about half the size of the final airliner. The A.W52G first flew in March 1943 and flight testing, with tug releases from 20,000 ft (6,096 m) giving flights of around 30 min continued, mostly satisfactorily until 1945. In 1944, Armstrong Whitworth received a contract that would allow them to produce two A.W.52 prototypes for evaluation, nominally as mail carrying aircraft.
The A.W.52 was intended for high speeds and was an all-metal turbojet-powered aircraft with a retractable undercarriage, but aerodynamically had much in common with the glider. Both were moderately-swept flying wings with a centre section having a straight trailing edge. The wings carried small (not full chord) end-plate fin and rudders at their extremities. The rudders operated differentially, with a greater angle on the outer one. Roll and pitch were controlled with elevons that extended inward from the wingtips over most (in the case of the A.W.52 about three-quarters) of the outer, swept part of the trailing edge. The elevons moved together as elevators and differentially as ailerons. They were quite complicated surfaces - which included trim tabs - and hinged not from the wing but from "correctors," which were themselves wing-mounted. These correctors provided pitch trim. Air was sucked out of a slot just in front of the elevons to delay tip stall by pumps powered by undercarriage-mounted fans on the glider and directly from the engine in the A.W.52. The inner centre section wing carried Fowler flaps and the upper surface of the outer section carried spoilers.
Maintenance of laminar flow over the wings was critical to the the design and so they were built with great attention to surface flatness. Rather than the usual approach, where skinning is added to a structure defined by ribs, the A.W.52's wings were built in two halves (upper and lower) from the inside out, starting from pre-formed surfaces, adding stringers and ribs then joining the two halves together. The result was a surface smooth to better than 2/1000 of an inch (50 μm).
The crew sat in tandem in a nacelle so that the pilot was just forward of the wing leading edge, providing a better view than in the glider. The pressurised cockpit was slightly off-set to port. The engines were mounted in the wing centre section, close the the centre line and not disturbing the upper wing surface.
The first prototype flew on 13 November 1947 powered by two Rolls Royce Nene engines which each provided a thrust of 5,000 lbf (22.2 kN). This was followed by the second prototype on 1 September 1948 with the lower-powered (3,500 lbf/15.5 kN) Rolls Royce Derwent. Trials were disappointing: laminar flow could not be maintained, so maximum speeds, though respectable, were less than expected. As in any tailess aircraft, take-off and landing runs were longer than for a conventional aircraft (at similar wing loadings) because at high angles of attack, downward elevon forces were much greater than those of elevators with their large moment.
The first prototype crashed without loss of life in May 1949, making it the first occasion of an emergency ejection by a British pilot, but the second prototype remained flying with the Royal Aircraft Establishment until 1954.
On 30 May 1949, test pilot J.O. Lancaster flying the first prototype encountered a pitch oscillation while diving at 320 mph (515 km/h), believed to be a result of elevon flutter. Starting at two cycles per second, it rapidly increased to incapacitating levels. With structural failure seemingly imminent, Lancaster bailed out of the aircraft using its Martin-Baker Type 1 ejection seat, becoming the first British pilot to use the apparatus in a "live" emergency. It was fortunate that he was alone in the aircraft as the second crew member was not provided with an ejector seat.
Specifications (First prototype, Nene powered)
Data fromapart from airfoil data
Published - July 2009
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