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Aviation History
1986
1986 - 2219.PDF
weight—bringing the maximum gross weight to 59,0001b. Fast taxiing, short-hop ship-to-ship staging, and some nap-of- the-earth operations would also be carried out with the rotors slightly forward. With the nacelles fully horizontal the V-22 flies like a fixed-wing turboprop, and will be able to achieve speeds of up to 300kt, at 30,000ft. Flight control is achieved with flaperons, elevators, and twin rudders. In this mode, the V-22 is predicted to have a maximum range, with a 7,5001b payload and full mission fuel, of around 550 n.m. The aircraft has a ferry range of 2,100 n.m. (ferry tanks are located in the cabin) which, says Boeing, means that V-22s can be self-deployed anywhere in the world—albeit in a series US Air Force (CV-22A) The USAF needs 80 V-22s to replace its HH-53s, and is also a significant contributor to the design demands on the aircraft. Principally, the USAF wanted the C V-22 A to carry 12 troops over a 700 n.m. mission radius, be able to hover out of ground effect for 15rnin at any ambient temperature and, if necessary, bring all the troops home again without refuelling. The 700 n.m mission radius has been reduced to 550 n.m. as a result of the engine design (see text). This overall requirement sets the maximum gross weight of the V-22, while the mid- mission hover determines the power train size. The Air Force is still looking at the possibility of fitting an external tank to the V-22 to increase range. All-up weight would also increase to around 54,0001b, at which level a 200-300ft ground roll would be necessary, with the rotors tilted forwards 20°. US Army (MV-22A) The Army wants at least 231 V-22s in Marine Corps configuration for logistics, utility, and medevac oper ations carrying 24 troops or 12 litters. Much of the Army's MV-22A oper ations will involve carrying external loads. For this the V-22 is equipped with two external cargo hooks, acces sible from within, capable of lifting 10,0001b each, or 15,0001b together. 14- 12- 10- 1,000 ) a 8- "8 1 6- * 2- C Nominal performance Nominal weight empty O.W,E. = 30,7521b ^^^ T.O.G.W. atSL/103T ^^_ 49,720lb ^^. R.H.P. 4,570 "^ "1 T.O.G.W. at SL/103T ^^^ I 47,1071b ^^^1 R.H.P. 4,200 ^^ Requirement/guarantee: 5,7601b payload 1 at 50 n.m. radius \ ^ 1 Full fuel 1 7,6601b -J T.O.G.W.: take-off gross weight R.H.P.. rated horsepower O.W.E.: overall weight empty 20 40 60 1)0 Radius (n.m.) of short hops. Flight control of the V-22 will be via a fully digital flight-management and triple- redundant fly-by-wire system. A fly-by- light flight control system was considered desirable, but was rejected, because the necessary transducers were not guaran teed to be available in time to meet the V-22 production schedule. Flight control system design is the responsibility of General Electric, which also developed the F-18 system. Cockpit ergonomics, and particularly the method by which power is demanded, either through the collective pitch lever, or with a separate power demand lever, are still unsettled. Boeing Vertol and Bell are each conducting simulation studies to resolve this question. The selection of engines to power the V-22 proved the most controversial element of the programme to date, and delayed the full-scale development go- ahead by four months. Three engines were on offer. Two were brand new designs from Pratt & Whitney and General Elec tric, and the third was a derivative power- plant based on the ubiquitous Allison T56, versions of which power the Navy's Orions, the Army's Chinooks, and the Air Force's Hercules transports. Pratt & Whitney's PW3005 and General Electric's GE27 were both devel oped under the US Army's Modern Tech nology Demonstrator Engine (MTDE) programme, and were clear favourites in the V-22 powerplant competition owing to their superior fuel efficiency and higher power-to-weight ratios. There was some surprise, therefore, when the decision went in favour of Allison. The reason, according to Navy Secretary John Lehman, was because a derivative engine presented less of a risk in what was already a technically demanding project, Far left Bell's XV-15 demonstrated tilt-rotor capabilities over much of the V-22 flight envelope. Left The Marine assault mission is well within V-22 s capabilities, says Bell/Boeing and also that the T56 was already in wide spread use. The engine is also 1,000 s.h.p more powerful than the MTDEs, and has significant growth capability. Allison's proposal, called the T406- AD-400, is a hybrid two-shaft free turbine combining a scaled low-pressure turbine taken from the T701 turboshaft, originally developed for the US Army's cancelled Heavy Lift Helicopter, with an improved T56 high-pressure section. The T406 will feature full-authority digital engine control, single-crystal turbine blades, and a full engine health monitoring system. Up to 6,150 s.h.p. will be available at sea level temperatures of 42°C, but this could, says Allison, be increased to 7,400 s.h.p. by raising turbine entry temperatures. Fuel burn of the T406 will be 7-10 per cent worse than that of the MTDEs, the result being that the Air Force's original 700 n.m. fully loaded radius of action requirement cannot be met. After some acrimony the Air Force settled for 550 n.m., apparently yielding because even at the shorter range the V-22's radius of action is considerably better than other troop deployment aircraft in its inventory. The first ground-test T406s are sched uled for delivery to Bell in mid-1987, with preliminary flight rating tests beginning later that year. In March, Allison selected Pratt & Whitney as its second source of engines, and the two will compete annu ally for further contracts after delivery of the first production batch. The V-22's engine/transmission and rotors are hinged to the wingtips on large axial bearings and tilted in synchro nisation by hydraulically driven screwjack actuators. An interconnecting driveshaft running between the two rotor gearboxes caters for single engine failure. It would normally be stationary, but clutches in automatically if one engine loses power, to maintain balanced thrust. Full-scale development of the V-22 calls for six prototype aircraft. Boeing Vertol will build six fuselage sections and ship three to Bell in Texas, while Bell will produce six wings and ship three to Boeing Vertol in Philadelphia. The first two aircraft will be assembled at Bell's facili ties. The first flight of the V-22 is set for June 1988. Approval for limited pro duction should occur in December 1989, with authorisation for full production in December 1991, concurrent with initial delivery to the US Marine Corps. Ten V-22 production lots are planned, peak production of 132 aircraft a year occurring by lot four. Unit cost of the V-22 will be held down by the Navy's policy on sharing tooling costs with the manufacturers, and by matching Bell against Boeing later in the production programme. The Navy esti mates that this, combined with the fixed- price full-scale development and pilot production contracts, reduces overall programme costs by 17 per cent, yielding V-22 unit flyaway costs of $16 million. Q FLIGHT INTERNATIONAL, 30 August 1986 157
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