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Aviation History
1988
1988 - 1291.PDF
structures are now almost commonplace in modern aircraft. Boeing's Model 360 and Bell's Acap are both 100 per cent composite helicopter airframes, and have been entirely successful. It is an indication of the success with which composites have been applied in Osprey that they have been used for one of the most critical components of the aircraft. This is the spindle upon which the entire engine/nacelle installation, weighing about 5,0001b, is hinged. Besides being the axis about which the aircraft makes its transition, the pylon support spindle is the structure to which both the engine and nacelle are attached, and is also the attachment point for the prop- rotor and tilt-axis gearboxes. An extremely rigid structure was clearly needed, which, being located at the end of the wing, also had to be as light as possible. Again composites provided the answer, by means of a filament-wound carbonfibre foundation with a glassfibre top layer. There was, in fact, no option but to build the V-22 from composites if it was to carry a useful payload. Vtol aircraft are always extremely weight sensitive, and the Osprey is no exception. A top-priority weight reduction programme is therefore in place at Bell/Boeing (holidays in Hawaii are just one of the incentives offered), aimed at achieving the guaran teed 31,8001b empty weight by the time the 16th production Osprey is built. Weight, says Col Blot, is "now under control. The airplane is about the weight now that it ought to be when it finishes the flight-test programme." Col Blot reckons that the Osprey will pick up about 1,5001b weight during flight test, "which means that it is about that much too heavy right now". The contractor weight reduction programme is aimed, he says, at eliminating about 2,0001b from the aircraft by delivery time. Flight's impres sion from talking to programme people was that this could probably be achieved. V-22 deputy director Chuck Ellis says that much of the aircraft was conservatively designed. "We'll do things smarter in the production aircraft," he says. Power for the Osprey comes from two 6,000 s.h.p. Allison T406-AD-400 turbines. The engine was selected in December 1985 after intense competition with General Electric and Pratt & Whit ney. The latter each offered brand new engines developed under the US Army's Modern Technology Demonstrator Engine (MTDE) programme, and there was some controversy over the Navy's choice of an engine derived from the much older T56 turboshaft. Allison is quick to point out, however, that the T406 is based on the latest ver sion of the T56 (which has just gone into production) and, although it is still not as fuel-efficient as the MTDEs might have been, the company says there is plenty of development potential remaining. The T406 is equipped with full-authority digi tal electronic control, and features what Allison claims to be the most efficient high-pressure compressor in its class. The T406 also offered several other advantages, including a lower-risk and FLIGHT INTERNATIONAL, 14 May 1988
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