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Aviation History
1955
1955 - 1815.PDF
938 f ••- .-"•* FLIGHT HUNTING PERCIVAL P.74 Research Test Vehicle Approaches the Flight Stage: a New Helicopter Philosophy THERE are three ways of using a gas turbine to provide powerfor a helicopter. One is to couple it by shaft drive to the rotorin accepted piston-engine style; another is to draw off com- pressed air from the engine to supply rotor jets—with or withouttip burning; and another is to extend, in effect, the jet pipe to the extremities of the rotors, using the whole through-put of theengine to drive the rotor. Examples of the shaft-drive techniques include the Piasecki Transporter (two Allison T38-A-6), and theSud-Est 3130 Alouette 2 (one Turbomeca Artouste 2); the West- land Whirlwind is also being developed with turbines (oneBlackburn Turbomeca Twin Turmo), and will be shaft driven, as also will be certain Bristol twin-rotor helicopters (two NapierGazelles). Compressed-air devotees include the Fairey Rotodyne, the Fairey Ultra-light, and the Sud-Ouest 1120 Ariel, 1220 Djinn,and 1310 Farfadet family. Last week there emerged from the experimental department ofHunting Percival at Luton the world's first exponent of the third technique—the P.74.Although each gas-turbine system displays advantages of one kind or another over conventional piston-engine designs, use ofrotor jets is undoubtedly the most natural way of exploiting the gas-producing potentialities of a turbine engine. Let gas-pressureenergy do the work of a mechanical transmission and immediately the helicopter designer is rid of the mechanical couplings whichconstitute his major development worry and which, because of the fixed relationship of rotor speed to engine speed, limit the operatingefficiency of the helicopter. The jet helicopter is by no means new—its advantages wereforeseen the day the jet.was born—and today we see flying, with conspicuous success, such machines as the Fairey Jet Gyrodyneand the French Sud-Ouest company's family of helicopters. It is the Hunting Percival/Napier approach to the jet helicopter whichis altogether new. Hunting Percival foresaw in 1950 the aero- dynamic advantages of a jet helicopter, and that the best efficiency—weight lifted per amount of fuel burnt—could be achieved by a gas turbine whose sole job in life was to deliver gas horse-powerrather than thrust or shaft horse-power. The result is the P.74, and so close has been the partnership between engine and air-frame/rotor designers that it is in fact difficult to tell where the Napier Oryx ends and the P.74 begins. The Oryx has been previously described in detail (Flight forAugust 5th, 1955) and it is necessary only briefly to recapitulate the principle of this fine engine. It is a true gas-producer, inwhich the power left over from the turbine after it has compressed its own air is used to compress further air. This by-pass air mixeswith the turbine exhaust, increasing and cooling the flow for ducting to the helicopter rotor. The engine thus consists of twocompressors, each with its own air intake and driven by a common turbine. No further combustion is necessary (although reheatat the nozzles is, of course, practicable) and it is interesting—in view of the criticisms levelled at jet helicopters on the score ofnoise—that the relatively low-pressure flow is not likely to be too disturbing to the ears. (Experience with the P.74 test rotor,although not truly representative with its Derwent powerplant, has tended to show that nozzle noise is in fact barely distinguish-able from the aerodynamic swish of the rotors.) Ground runs of the P.74 complete with rotor are due to begin very soon—rotor-less engine runs were first made on December 2nd—and it will bepossible then to verify the belief that the P.74 and its com- mercial derivative the P.105 will be among the quietest helicoptersyet flown. Before we recount the problems that confronted the HuntingPercival and Napier teams, and describe the way in which they were mastered, it is worth setting out the advantages offered bythis new conception of the jet helicopter. Foremost, as already mentioned, clutches, couplings, gearboxes and other mechanismsare eliminated from the scene, together with their associated maintenance, vibration and cooling problems. Secondly, sincethe rotor is gas-coupled to the engine, its speed is not fixed in relation to it and the optimum rotor speed can be chosen for anycondition of flight; hence it is possible to achieve lower rotor r.p.m., and therefore greater aerodynamic efficiency and lift, forthe same engine power in hovering flight. Comparative curves of power required against forward speed show that theoreticallyabout 10 per cent less power is required to lift the same weight in the hovering regime (as forward speed increases the two curvestend to converge). The potential superiority in economy is clearly evident. No less advantageous is the fact that the torque betweenairframe and rotor is of a very low order, consisting only of rotor-bearing and gas-duct friction. Only a small rudder-rotoris necessary for lateral control, and a negligible amount of power is robbed from the main lifting rotor. It is possible that, as aresult of P.74 flight trials, a simple fin may be substituted for the rudder-rotor, and the rear fuselage has been made detachablewith this possibility in view. A further virtue of the system is that it makes possible, withoutrecourse to intricate mechanical geometry, the use of a tilting rotor hub. (One tilting-rotor helicopter with shaft drive is success-fully operating in the U.S. This is the Doman YH-31 of the U.S.A.F.) As is well known, any inclination of the tip-path planeto the plane of the rotor hub in the classical rotor results in the production of in-plane forces which in turn necessitate the intro-duction of drag hinges. Being free to tilt, the hub takes up its own position normal to the tip path plane, drag hinges go, thedegree of flapping is much reduced, and blade "tracking" tends to be less critical. There are yet other attractions; the entire gas output of theengines can be spilt overboard before transfer to the rotor, enabling a full-power check to be carried out by the ground crew withoutthe aircraft becoming airborne; engine and reduction-gear cooling —of. special concern to a fully laden hovering helicopter—is noproblem; the turbines, being outside the cabin as they are to be on the P.105, will be a welcome aural relief from internally installedpiston engines; and, of course, the use of kerosine fuel implies the reduced fire risk common to most turbine aircraft. Here, then, is a helicopter which proclaims formidable improve-ments in economy, engineering simplicity, quietness, convenience and safety. Such* ideals are not easily attained, and it is not sur-prising that development so far has occupied nearly five years of sustained effort by both airframe and engine teams. There ismore work to be done before this promising experiment can be fully translated into operational reality, but the fact remainsthat the preliminary interpretation of the philosophy, in the shape Lett, the Oryx engine installation (port side) showing the primary compressor air intake. Centre, the spill valve, with the butterfly in the open position. Right, the tilting rotorheod mounted on the tower in the test-pit at Luton, clearly showing the flexible steel duct.
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