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Aviation History
1962
1962 - 1390.PDF
200 FLIGHT International, 9 August 196: REQUIEM FOR THE ROTODYNE An Account of Unusual Problems Met and Solved DURING the House of Commons debate on the aircraft industry a Member said. "In 1956 it seemed to many people, including many with expert knowledge of aeronautics, that the Roto- dyne was something with a potential which could possibly bring striking new developments in air transport and air communication." This article is an attempt to set forth the technical problems which manifested themselves during development of this ambitious aircraft, and to indicate how far Westland Aircraft Ltd's Fairey Aviation Division were justified in regarding the major ones as solved. The Rotodyne is a compound helicopter, able to fly in either of two modes. For VTOL it is lifted by a rotor driven by fuel-burning pressure-jet units at the tips of the blades, with the two propellers in approximately zero pitch to provide directional control. In translational (forwards) flight it is propelled by the two propellers, lift being provided by the fixed wing and by the rotor turning in autorotation. Originators of this scheme for a large compound helicopter were Dr J. A. J. Bennett and Capt A. G. Forsyth, of Fairey Avia tion, whose original study dates back to 1947. By 1950 the project had two Dart engines and 20 seats, but during the ensuing three years it grew to have three Mambas and 28 seats, two Darts plus two de Havilland engines and 40 seats, and finally two Eland engines and 40 seats. In 1959-60 the Rotodyne grew still further, and the FA-1, or Type Z, production aircraft was envisaged as having two Rolls-Royce Tynes and two Rolls-Royce RB.176 air producers, and a fuselage seating up to 70 passengers. Termination of the project was announced by the Minister of Aviation on February 26, 1962. Of the military version he said, " It is necessary to forgo the operational advances offered by this aircraft, in view of the cost involved." Regarding the civil version, he said that BEA had informed him that, '"The commercial pros pects of the Rotodyne on their routes are not sufficiently assured to justify the heavy liabilities involved . . ."" In the absence of any firm order, Westland Aircraft did not feel justified in proceeding. Total expenditure had been approximately £1 lm prior to the change to Tyne engines; the Tyne-Rotodyne had a £4m ceiling, much of which will not now be spent. Doubtless the decision not to proceed with the Rotodyne is a correct one. Nevertheless the Rotodyne FA-1 was a known quan tity, for which Westland had guaranteed a performance. Both civil and military versions had been studied, and it is Westland's view that these aircraft were entirely feasible on all counts. Vibration The prototype Rotodyne Y (XE521) first flew on November 6, 1957. Prior to that date the most feared problem had been ground resonance; and the aircraft was originally built with a temporary fixed undercarriage. Extensive research eventually led to a retractable undercarriage with very soft characteristics, which enabled the existing structural damping of the rotor and fuselage to be more effective. It was also found that gusts were causing the blades to oscillate at their natural frequency, and a cure was found by introducing a damper in a strut behind the steel tube which formed the inner part of the blade. When the aircraft was built, there was in Britain a dearth of know ledge of the precise loading experienced by a rotor. Without knowing the basic rotor forces the magnitude of the various har monic contents could not be calculated, and this in turn made it impossible to design the airframe to avoid significant rotor-induced vibration. Once the aircraft got into the air it was appreciated that the oscillatory loads induced by the rotor were sufficiently large to make knowledge of body vibration characteristics important, if only to alleviate pilot discomfort. Two main conditions had to be investigated: high-speed cruising flight as an autogiro, and flare-out as a helicopter. Rotor r.p.m. were approximately 120 in the first case and 140 in the second, so that the loads induced by the rotor came through at different frequencies. The airframe can vibrate in a whole gamut of modes and frequencies—body hogging, pylon forward, wings up, and so on. The various body frequencies lie so close together that it is hardly surprising that one mode was found to occur very close to each of the two critical values of rotor r.p.m. Resulting from extensive calculation and dynamic model experi ment, airframe modifications were introduced to demonstrate that the structure could be altered to remove the troublesome frequencies from the running r.p.m. range. The mode in the vicinity of 120 r.p.m. responded to pylon fore and aft stiffnesses, and a reduction of these stiffnesses, coupled with a flexible tailplane mounted with dampers, completely cured the trouble. The mode in the vicinity of 140 r.p.m. was modified in a more dramatic way by the addi tion of a large strut from the top of the pylon to the tail, which served to show that a much smoother ride would result when the practicable airframe stiffnesses were suitably modified. Continued research into the coupling between horizontal motion of the Rotodyne Y rotor hub with blade flexure steadily allowed the undercarriage stiffness to be raised close to that accepted for con ventional aeroplanes. In the FA-1 provision would have been made The Rotodyne Y flying with empennage removed and a fixed strut from pylon to tail to modify airframe stiffness
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