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Aviation History
1986
1986 - 2216.PDF
V-22 Osprey: multi-service workhorse Anybody watching the 1985 Paris Air l\ Show would have seen a unique •*• *• aircraft being put through its paces. In its daily routine the Bell XV-15 tilt- rotor gave an impressive demonstration of its ability to combine helicopter versatility with turboprop speed and range. In May, the future of tilt-rotor aircraft in the US Armed Services was virtually guaranteed by the award of $1 • 7 billion to Bell and Boeing Vertol for full-scale devel opment of a larger, all-composite version of the XV-15, designated the V-22 Osprey. The contract calls for the team to fabri cate and test six prototype V-22s, with a first flight in June 1988, followed by six months and 4,000hr of flight-testing. The V-22 is the result of the Joint Services Advanced Vertical Lift Aircraft (JVX) requirement initiated in December 1981 by the then US Defence Secretary, Frank Carlucci. The JVX specification identified a wide variety of missions, carried out by all four Services, that could be flown by a single aircraft which would replace existing fixed- and rotary-winged vehicles. The US Marine Corps, for example, needed 552 medium assault transports to replace CH-46 and CH-53 helicopters, while the Navy and Air Force wanted a new combat rescue machine to replace HH-53s. The Army's requirement was for a utility/medevac transport, and Full-scale development of the V-22 Osprey tilt-rotor will lead to a common, highly versatile medium-lift transport meeting the needs of all four US Ser vices. Julian Moxon reports. for an electronic warfare platform to replace OV-ls. Four vertical-lift configurations were studied for JVX: a high-speed conven tional helicopter; advancing-blade and compound helicopters; lift/cruise fan; and tilt-rotor. The tilt-rotor was chosen because of its moderate development risk, and because it had more operational advantages, particularly in its self- deployment capability and its role flexi bility. Even so, the demand that the aircraft should fulfil the needs of all four Services meant that compromises were inevitable. The US Marines wanted the V-22 to be compatible with operation from its LHA amphibious assault ships. This constrained rotor diameter to 38ft, neces sary to maintain the specified 12ft 8in rotor tip clearance from the island with the V-22 sitting with its wheels 5ft from the deck edge. According to its designers, the ideal rotor diameter for the V-22 would have been 42ft. At the smaller diameter, disc loading becomes high, at around 161b/ft2, which is in the same league as the CH-53E. This, in turn, limits V-22 lifting ability, and hover efficiency. The Navy and Marines demanded that the V-22 could be folded away to fit into its carrier lifts, requiring a complex wing/ rotor folding mechanism and a weight penalty for the Army, which did not need folding rotors. The weight penalty was further increased by the Air Force's tough require ments for the deployment of special combat troops, which meant that the drive train had to be stronger and heavier than necessary for the other Forces. For the other Services, however, the extra strength can be traded for longer life if the full capabilities of the V-22 are not used. The inter-Service differences have now been settled, and virtually all of their mission needs will be met, the only excep tion being the Air Force's 700 n.m. radius of action requirement for hot-day troop deployment. This had to be limited to 550 n.m. as a result of the controversial deci sion to use derivative instead of new engines, which have a higher fuel burn. Stage 1 preliminary design of the V-22 began in December 1981, and included several programmes for validating critical technologies. The XV-15 programme had, in fact, swept away most of the doubts surrounding the tilt-rotor concept, the principal V-22 innovation being in the use of an all-composite fuselage and rotors. V-22 workshare is split more or less 50/50 between Bell and Boeing. Bell is responsible for the lift/propulsion system, comprising the wing, nacelle, trans mission, and rotor. Boeing Vertol takes care of everything below the wing— fuselage and tail, landing gear, sub systems—and aerodynamics, perform ance, and handling qualities. The Bell-Boeing team was awarded two Stage 2 preliminary design contracts, in April 1983 and May 1984, worth a total of almost $200 million. These were aimed at reducing V-22 development and pro duction risks as much as possible. This led to the largest windtunnel testing effort ever conducted for a rotary-winged aircraft, involving ten different scale models and more than 6,600hr of time in eight tunnels. Full-scale development (FSD) of the V-22 was originally slated to begin in June 1985, but was delayed by the engine selec tion process. There was also a change of procurement policy from US Navy Secre tary John Lehman, who announced that The Bell/Boeing V-22 Osprey tilt-rotor will enter Marine Corps service in 1991 154 FLIGHT INTERNATIONAL, 30 August 1986
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