f - ••.-"*• 938 FLIGHT HUNTING PERCIVAL P.74

Research Test Approaches the Flight Stage: a New Philosophy

HERE are three ways of using a gas turbine to provide powenginer e runs were first made on December 2nd—and it wil be for a helicopter . One is to couple it by shaft drive to the rotopossiblr e then ot veriyf the belief that the P.74 and ist com- Tin accepted piston- style; another is to draw of commercia- l derivative the P.105 will be among the quietest pressed air from the engine to supply rotor jets—with or without yet flown. tip burning; and another is to extend, in effect, the jet pipe to the Before we recount the problems that confronted the Hunting extremities of the rotors, using the whoel through-put of the Percival and Napier teams, and describe the way in which they engine to drive the rotor. Examples of the shaft-drive techniqueswere mastered, it is worth setting out the advantages offered by include the Piasecki Transporter (two Allison T38-A-6), and the this new conception of the jet helicopter. Foremos, tas already Sud-Est 3130 Alouette 2 (one Turbomeca Artouste 2); the Westm- entioned, clutches, couplings, gearboxes and other mechanisms land Whirlwind si also being developed wiht turbines (one are eliminated from the scene, together wiht theri associated Blackburn Turbomeca Twin Turmo), and will be shaft driven, maintenance, vibration and cooling problems. Secondly, since as also will be certain Bristol twin-rotor helicopters (two Napier the rotor si gas-coupled to the engine, its speed si not fixed in Gazelles). Compressed-air devotees include the Fairey Rotodynrelatioe, n to it and the optimum rotor speed can be chosen for any the Fairey Ultra-light, and the Sud-Ouest 1120 Ariel, 1220 Djinnconditio, n of flight; hence ti si possible to achieve lower rotor and 1310 Farfadet family. r.p.m., and therefore greater aerodynamic efficiency and lift, for Last week there emerged from the experimental department otfhe same engine power in hovering flight. Comparative curves Hunting Percival at Luton the world's first exponent of the third of power required against forward speed show that theoretically technique—the P.74. about 10 per cent less power is required to lift the same weight Although each gas-turbine system displays advantages of onein the hovering regime (as forward speed increases the two curves kind or another over conventional piston-engine designs, use of tend to converge). The potential superiority in economy is clearly rotor jets is undoubtedly the most natural way of exploiting the evident. gas-producing potentialities of a turbine engine. Let gas-pressure No less advantageous si the fact that the torque between energy do the work of a mechanical transmission and immediatelayirframe and rotor si of a very low order, consisting only of the helicopter designer si rid of the mechanical couplings which rotor-bearing and gas-duct friction. Ony l a smal rudder-rotor constitute his major development worry and which, because of thise necessary for lateral control, and a negligible amount of power fixed relationship of rotor speed to engine speed, limit the operatinis grobbed from the main lifting rotor. t I si possible that, as a efficiency of the helicopter. result of P.74 flight trials, a simple fin may be substituted for the The jet helicopter si by no means new—its advantages were rudder-rotor, and the rear fuselage has been made detachable foreseen the day the jet.was born—and today we see flying, withwith this possibility in view. conspicuous success, such machines as the Fairey Jet A further virtue of the system is that it makes possible, without and the French Sud-Ouest company's family of helicopters. tI is recourse to intricate mechanical geometry, the use of a tilting the Hunting Percival/Napier approach to the jet helicopter whichrotor hub. (One tilting-rotor helicopter with shaft drive is success- is altogether new. Huntign Percival foresaw ni 1950 the aero- fully operating ni the U.S. Thsi is the Doman YH-31 of the dynamic advantages of a jet helicopter, and that the best efficienUcy.S.A.F. ) As is well known, any inclination of the tip-path plane —weight lifted per amount of fuel burnt—could be achieved by a to the plane of the rotor hub in the classical rotor results in the gas turbine whose sole job in life was to deliver gas horse-powerproduction of in-plane forces which in turn necessitate the intro- rather than thrust or shaft horse-power. The result si the P.74, duction of drag hinges. Beign free to tilt, the hub takes up its and so close has been the partnership between engine and air- own position normal to the tip path plane, drag hinges go, the frame/rotor designers that it is in fact difficult ot tell where the degree of flapping is much reduced, and blade "tracking" tends Napier Oryx ends and the P.74 begins. to be less critical. The Oryx has been previously described in detail (Flight for There are yet other attractions; the entire gas output of the August 5th, 1955) and ti is necessary only briefly to recapitulate can be spilt overboard before transfer to th e rotor, enabling the principle of thsi fine engine. tI si a true gas-producer, ni a full-power check to be carried out by the ground crew without which the power left over from the turbine after it has compressedthe becoming airborne; engine and reduction-gear cooling its own air is used to compress further air. Thsi by-pass air mixe—s of. special concern to a fully laden hovering helicopter—is no with the turbine exhaus,t increasing and cooling the flow for problem; the turbines, being outside the cabin as they are to be on ducting to the . The engine thus consists of two the P.105, will be a welcome aural relief from internally installed compressors, each with its own air intake and driven by a commopiston n engines; and, of course, the use of kerosine fuel implies the turbine. oN further combustion si necessary (although reheat reduced fire risk common to most turbine aircraft. at the nozzles is, of course, practicable) and it is interesting—in Here, then, is a helicopter which proclaims formidable improve- view of the criticisms levelled at jet helicopters on the score of ments in economy, engineering simplicity, quietness, convenience noise—that the relatively low-pressure flow is not likely to be tooand safety. Suc*h ideal s are not easily attained, and it is not sur- disturbing ot the ears. (Experience with the P.74 test rotor, prising that development so far has occupied nearly five years of although not truyl representative with its Derwent powerplan,t sustained effotr by both airframe and engine teams. Theer si has tended to show that nozzle noise is in fact barely distinguishm- ore work ot be done before thsi promising experiment can able from the aerodynamic swish of the rotors.) Ground runs of be fully translated into operational reality, but the fact remains the P.74 complete with rotor are due to begin very soon—rotor-lethsast 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. I } * '

2 1953 December5 939

ortotfefhhhear cPeh.s7edh4d,o hlas i basitsT efhlcy inpgr otrbiallesm.s have betoehnfe eatnmni rmeyalstered, many unfamiliar nature. F ierteshnTtog,wien installati.on e onngines were deemed desirable, each o anofviai er ct ihtrsesaidf,e t feedin,g o catwoot nhm e"mtroonu sdeur-ctleg," roertaeenohurrgn. ingseiWnhne s totheneaidr il"operatiny, g line", turbine p aoprimarthnwed esru ymm oatfches a oufx tihlieary compressor power; r.p.m. turaanbebdrohi n tofe compressor m auxiliaratdnc htehedy; compresrso dtsamauesirlr itha-vte et rhpressurse e bdjitioTneielenhi sve. xeyhraust which flow rsatisfhteheasq tuitseloyiref mesnt o rwtoffhethorih csystem, performance isnmtot poisaezssnz esl flowitaivree,,a p rteanedsmsupre,rature—e th upper lifixewmechanicaomsiyfbh itdihche l p rthofoe pttoehe.rr tiesLikew,ise d rototuc ctthrien hge froadm each engine ismsaoyf otshste ecriticam, reql upa-irt inootfgehp pcarefuosignl rinecteornecssiltiation betwene (Above) itsrotorhad, The Hunting Perciral P.74, minus t ehannedrgminoed yinasmtailclastino desig.n Ideal,l yductign its engine2nd.Decemberforon first outingruns sHdpbhaseioru tuaeu stsnclhtdsetigib n;lePercl ivdaesnig a rotomakisacma dntoorem epn, gaicinnstedivil dueant,ity ineairframenotfodxi tehWse hppsiereeen dst.ehsenito P Oo.eacatf1 r0ayhnx5 , whose rotor/engine group, consisting erusoctnoinfhd sea ntaoo dbert, restub wing just beneaht veadowfaBanefoxraeirteatu iytavhlym sece-rp^.-sallone,ep m ideaofe nttlhe engine-to-ror toductgin syste,m involvgin lceansinadgstchayd ienx,gperiment with suitable duct shapes s pwantinottirelehdlarosts ig tncrelceesa,siypr ldesirabll e osnfquickestih yetshetem,t usign relativye l direct ductin,g p wbotahesero siylPnislb. e7l te4he , tnimgien. seHenec (Right) of Cross-section orotofwtgbufotfhyesrasayeo trloau rthgsee, edeliverign their the powergas drive system. lifeunoufgp ss ttihdeeel awg.aelsl Bedeouafn tdn tg livhwteihneheye ilsrinstallatiot designn dcdtiosunhfreciot tIicrotoicmniags,l rpdi.verseoiggnmisin braetoefnhoteq wthrueoeier dneoymenneacns mt eifficieync hobdantoaenthnedhs dttseh e,pirgorsnseibel rotor duct flow (Below) ofAdiagrammatic view fadcastnfic oe Iwethcteshaie sdnedumerous difficulties. the generatorgas Napier Oryx. Pmciaat.owt7ttrcun dratu4sea rasdeminelvart pduct,s a wna soptimum design within this limitation. Totla duct area decmbatheynhteadnegs tromshinie-eicfnel,eowsd outptu from l alargasmalotfyah nbeile tnwdueumenscb tregivgin bfewehanbthiclaiaugdthke r- slodescutsicsoe t,nswiht brelattelnbetaIv eudhtteeuhre-tncsedc tlionsse.s TO KOTO* l afthrewaanyvodasouet ursetd,ainless-ls tedeusc teahc o diamete4f inapprorx iwmeatree luysed (the duct tempera- truauisnioefr seegt4urhieao0nol ydlfoegi C). Teh STftlftUTlON VALVE

MAIN COMPRESSOR TURBI

(Below) rotorof ef Vficiencyariation with rotor tip-speed.

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rvi RNItlO C TTOR 1IO IN1L1I1tt FLWO

a: . ' . ^ i — —h-T ii i i « N0FI L%OW (FFICKNCV TO- 7 > ^°

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MR V -ELOCITY RATOI Vo 940 FLIGHT HUNTING PERCIVAL P.74 HELICOPTER

1 Rear spar, 23 Rear fuel tank vent pipe. 44 Oleo leg. 2 Asbestos insulation no alternaet 24 Fuel filler cap near tank (stbd. side). ADJUSTABLE " EYELID " 45 V.H.F. aerial. ribs. 25 Flexibly mounted bearings. JET NOZZLE 4* Yaw control. 3 Mass balance. 26 Quick release fasteners . 47 Fuel cocks. 4 "Delta-3" hinge to aileron control. 27 Tail rotor pitch control linknge. 48 Collective-pitch lock. 5 Drum. 28 External electric starter connection. 49 Engine throttles. 6 Jack operating valve. 29 Jacking bulkhead 50 Rotor brake. 7 Jack. 30 Main keel member. 51 Starting master switch. 8 Hydraulic accumulators. 31 Oleo leg. 52 Selector switch (port and 9 Oil tank for servo control unit. 32 Spring-loaded fire-extinguisher stbd engi.nes). 10 Flexible steel bellows gas duct. doors. 53 Butterfly controls. t i Collective-pitch spider. 33 Cooling air from main compressors 54 Parking wheel brake. 12 Six lifting lugs. to rotor head. 55 Main structural bulkhead. 13 Flapping hinge. 34 Engine fire detector. 56 Cooling gas inlet. 14 Gas bleed. 35 Fire extinguisher (port and stbd.). 57 Auxiliary drive cone. 15 Gas bleed valve. 36 Engine mounting bracket. 58 Bevel wheel and pinion. 14 Micro switches. 37 Non-throttling valve. 59 Main rotor axle pick-up. 17 Actuator. 38 Engine oil tank (port and stbd.). 18 Speed sensing unit. 39 Engine oil filler cap. 19 Vacuum generator unit. 40 Oil tank vent. 20 Auxiliary gearbox oil tank. 41 V.H.F. radio. 21 Auxiliary gearbox oil tan k filler cap. 42 Cyclic pitch linkage. 22 Cabin light. 43 Engine synchronization unit.

LAMINAR FLOW ROTOR BLADE

MONOCOQUE STAINLESS STEEL SKIN SPOT-WELDED TO RIBS

COLLECTIVE-PITCH SERVO CONTROL UNIT

ONE-PIECE CANOPY

LIGHT ALLOY AILERON IN SXI SECTIONS TO ALLOW FOR FLEXING

CYCLIC PITCH CONTROL COLUMN

COLLECTIVE PITCH AND THROTTLE TWIST GRIP

OPENING WINDOW FOR DOWNWARD VISION

FORWARD FUEL TANK (10 IMP. GAL.)

750 G^S H P N*P

enclosing them, although necessariyl of thick section—225. per should be kept low (P.74 maximum tip Mach number is 0.55). nI cent—was designed ot have zero-incidence laminar-flow charac-reality the ratio of rotor horse-power to gas horse-power is of the teristics up to about mia-chord. The blades are non-feathering,order of 0.5. The set of curves on page 939 shows the variation and variation of incidence is taken care of by ailerons, which are oinf this ratio with velocity for three conditions: frictionless flow; six 3ft sections to compensate for blade-flexing, each attached by90 per cent flow efficiency; and without centrifugal compression. three hinges of Ferobestos plastic (no lubrication si necessary) to The optimum conditions for best rotor performance have been a C-section "tape"; centrifugal force on the blade tends to keep the subject of prolonged investigation on the Hunting Percival test the aileron in the up position, and downward movement is accomto-wer at Luton. As can be seen in the photograph, this was sunk plished by a cable attached via a linkage to a classical type of rotober!ow ground level, for reasons of silence and safety. nI general head pitch-control. (Collectiev pitch is achieved in the usual waylayout the pit is reminiscent of a Roman arena, with its stepped by means of a vertical screwjack). Teh arrangement si not as sides and underground entrance passage. A Derwent si used to complicated as ti sounds, and has worked well on the test-rig. provide power, its exhaust being cooled to the requisite tempera- When the thermodynamic and manufacturing problems of flat- ture by water. Teh installation first ran last March, and the tened ducts have been solved a better aerofoil section wil result,programme of testing the P.74 system has now accounted for some eliminating the need for ailerons. Thsi si one respect ni which 40 hours of running. Thsi may not seem a great deal on first the P.105, with its feathering blades, will be an impfovement on consideration, yet the amount of data it has yielded si apparent the P.74. from a typical recent programme in which 40 sets of aerodynamic The blade itsefl si a pure monocoque of stainless stee,l with measurements were taken ni a test which, although occupying 22-gauge skin averaging 130 deg C at the root. (Blaed de-icing five days (including a week-end), yielded only 4^ hours of rotor will obviously not be a problem.) Flow at the end of each duct running. is carefully cascaded through 90 deg into a single discharge nozzleThe fact that the two engines of the P.74 deliver gas into a which, as will be explained, has a two-position "eyelid" to vary common duct and nozzle means that special care has to be taken nozzle area. to ensure that each engine si running on its "operating line." The tilting rotor hub is a fabricated steel structure mounted onIt was assumed in the early stages of design that the engines would Timken taper-roller bearings to permit hub-tilt, this unit ni turn be particularly sensitive to back-pressure, and a comprehensive being mounted on a spherical bearing. Belwo this is a flexible automatic control system has been developed by Napiers to guard gas-tight bellows duct of stainless steel, designed to withstand tahegainst the likelihood—for example—of the following sequence of complex motion of bending and shearing. Asmall accessory events taking place: one engine, its output momentariyl falling gearbox is driven off the rotor, from which the rudder rotor takes behind that of its partner, si caused to surge and stal by back- its drive, and the whole is neatly encased in the rotor hub fairingpressure. ; its partner, working against suddenly reduced back-pres- A conventional type of friction rotor brake si fitted. Similar s usirme,ultaneously overspeeds. Napire test work has shown flexible dpuacstssin,g between the blade flapping hinges, convey that ni actual practice a pair of Oryxes delivering into a common the gas flow into the rotor ducts. The flapping hinges have plasductic t and nozzle are not so sensitive to one another as had been bearings requiring no lubrication. at first supposed; but only running tests with the complete P.74 It si evident that the centrifugal action of the rotor further will show how compatible they are. Al lthis, incidentally, wil compresses the flow, increasing the velociyt of the jet, which be no problem with the P.105, in which each engine feeds its own would theoretically go on increasing until the limiting speed of duct and nozzle. the rotor was reached. Dutc losses obviously preclude this, but The P.74 engine control system si a most interesting piece of it is academically interesting to note that (since rotor horse-poweerngineering, and si the subject fo a diagram included ni our increases with rotor speed) at infinite rotor speed gas horse-powseurmmary (pages 933-934) of a paper read before the Helicopter delivered by the engines would be converted entirely into lifting Association by Mr. A. W. Morley of Napiers. The two-position horse-power. Thsi of course is an ideal that si far from attained nozzle takes care of the engine-out case, when mass flow is of in practice, where for best aerodynamci efficiency rotor speed course halved—although, because of lower duct losses, power is

TAIL ROTOR (WOODEN BLADES)

This "Flight" copyright drawing is the first to show the interior lay- ALL WINDOWS EMERGENCY EXITS out of the Hunting Percival P.74 research helicopter (Napier Oryx N0r.1) which will shortly be ready for its ground running trials at EAR FUEL TANK the company's works at Luton. The aircraft is the precursor of ETWEEN KEEL MEMBERS •0 IMP. GAL.) the P.105 ten-passenger general purpose helicopter, also referred to in the accompanying article. The small inset drawing on the opposite page shows one of the blade- nozzles. 942 FLIGHT

The P.74's cockpit close- ly follows classical heli- copter practice, Of special interest are the engine-synchronizer in- dicator at the top, and the levers at the foot of the pedestal which select transfer of engine gas output either to the rotor or to spill overboard.

(Right) The neat under- carriage, which consists of a single light-alloy leg with the shock-absorber inside the fuselage.

it embodies a number of new ideas. The sturdy four-wheel under- carriage, for example, is the neatest helicopter chasssi we have s eTehen .usual angular struttery carrying each wheel is replaced by a single swinging link, a substantial magnesium alloy casting, the shock-absorber of which si mounted withni the fuselage. Wheels are fully castering, and the rear pair are fitted with Palmer pneumatic brakes. Tyer pressures are 80 lb/sq in. All the cabin windows, four on each side, are escape hatches, alternately open- able from inside and outside. Construction of the fuselage conforms with accepted helicopter practice in that it may be likened structurally, and without dis- respect, to the shopping basket: lifting loads are carried into the fuselage container by sturdy bulkheads ("handles") and are taken out by the skin and into the keel by shear members. Unde-r carriage landing loads are taken by stiff frames reacted down into the keel web structure. The door is of a useful size (4Łft by 5ft) and here ti si appropriate ot mention that, although the P.74 is purely a test vehicle for proving the new propulsion system, its designers have taken the trouble to make its commercial poten- tialities apparent. There is ample room aboard for eight passengers in addition to the crew of two, and plenty of volume and floor space for freight. The cockpit has dual flying controls, which will enable pilots to be easily initiated into the new feel of the P.74; a glazed nose has not been provided—although it will be on the P.105—an d downward vision is afforded by large D.V. panels. Tw o fuel tanks, of 170 Imp. gal total capacity, are housed ni the keel. Design wokr on the P.105, which as previousyl mentioned represents a considerable refinement of the P.74, is in hand, and construction of the prototype rotor has started. This ten-passenger machine wil be the first commercial interpretation of the new propulsion system. t Imay be expected ot make its debut ni 1 b9y5 8w, hich time a great deal of pioneering experience wil have been gained from dei P.74. fO ist promised merits— economy, quietness, simplicity, flexibility—the latter stands out most prominently. Sinec the essence of a helicopter is its rotor/ engine combination, the availability of a compact and independent unit, to which almost any kind of frame may be fitted, promises a significant broadening of the scope of future helicopter operations. LEADING DATA General arrangements, approximately to scale, of the P.74 research P.74 Rweorch Helicopter (two Napier Oryx NOr.1 turbines of 750 g.h.p.)— helicopter, and (right) the P.105 ten-passenger commercial vehicle. Maximum weight, 7.750 Ib: fuel capacity, 170 Imp. gal; rotor diameter, SS ft. P.105 General Purpose Helicopter (two Napier Oryx NOr.4 turbines of 825 g.h.p.).—Maximum weigh,t 10,0001b: seating capacity, two pilots and ten passengers; fuel capacity, 230 Imp. gal; cruising speed, 100 m.p.h.; vertical rats HUNTING PERCIVAL P.74 . . . of climb at loaded weight of 9,600 Ib, 250ft/min; range with full payload of 2,250 Ib, 165 milu at 5,000ft; rotor diam«c«r, 63ft. less than halved. f I an engine fails in flight it is automatically isolated from the common duct to prevent flow from the good engine passing into it. Thsi is accomplished by the hydraulicallThey ultimate aim of the Hunting Percival helicopter design philo- operated "non-throttling" valve at the start of the duct. At the sophy is a compact and independent rotor-engine system which permits same time the rotor nozzle is moved into its part-closed positioncomplete freedom of choice of the airtrame. On the left is the ten- by means of a mechanical linkage from this valve, this mechanism passenger P.105, and on the right the "aerial crane." being designed to "fail open." Theoretically, to keep the engine on its operating line the area of the rotor tip nozzle should be varied with rotor r.pjn. To avoid this a blow-off valve is fitted in the common duct; this valve is sensitive to rotor speed, and is operated by a governor driven by the accessory gearbox. A similar blow-off system will be included in the P.105. To start the P.74, each engine "non-throttling" valve is closed, to spill the whole gas output overboard via a duct in the fuselage side. Abutterfyl valve, flush with the skin, allows for varied loading of the engine. Each engine may then be brought up to its correct speed and delivery pressure, at which point the non- thrortling valves are smoothly opened to transfer flow, one engine at a time, to the rotor. Structurally, the P.74 airframe is quite conventional, although