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Aviation History
1955
1955 - 1810.PDF
23 December 1955 933 NAPIER TURBINES FOR HELICOPTERS Dr. A. W. Morleys Lecture to the Helicopter Association THE original work of the Napier Company in the field ofhelicopter powerplants was the subject of a paper readby Dr. A. W. Morley, Ph.D., M.Sc, A.C.G.I., A.F.R.Ae.S., the company's forward-projects engineer, before the Helicopter Association of Great Britain on December 10th. The paper, a synopsis of which follows, was concerned with the application of the gas turbine to helicopters from the point of view of the engine designer. It is appropriate that in this issue, on pages 938-942, we describe the Hunting Percival P.74 helicopter (two Napier Oryx turbines), whose design owes a great deal to the work of the Napier company. Special attributes of the gas turbine engine which make itattractive for helicopter propulsion [said the lecturer] are its low weight per horse-power, freedom from vibration, simplicity ofinstallation, low frontal area and bulk, the freedom of choice in rotor speed, and its ability to make use of fuels carrying less firerisk than petrol. Types of Installation with Gas Turbine.—The gas turbinecan be used in the helicopter to power the rotor in two distinct ways: by direct mechanical drive, or by tip-jet reaction. In theformer, continued the lecturer, there is a mechanical drive, typical of piston-engined helicopters, to the rotor via reduction gear. Anexample is a development of the Bristol 173 twin rotor helicopter powered by two Napier Gazelles. In the second class we have thereaction drive of the rotor by means of tip-jets. Power output can take the form of compressed air (extra fuel perhaps beingburnt at the discharge nozzles), or by using a slightly different thermodynamic cycle, where the rotor discharge can include allthe gas in the engine efflux including the turbine exhaust, the rotor receiving its energy in the form of gas horse power, as in theHunting Percival P.74 and P.105 helicopters (Napier Oryx). A further example of the second class of turbine installationis a combination of rotor drive by tip-jets with normal forward airscrew drive through reduction gears. This type of craft cantake off and land like a helicopter and cruise almost as a fixed-wing aircraft vide the Fairey Rotodyne powered by the Napier Eland). The most important question at the inception of a helicopterdesign, said Dr. Morley, is the choice of the best number of engines. Some of the general problems include (a) the necessityfor comprehensive engine control; (b) emergency power require- ments; (c) dirt accumulation on the compressor blades; and (d) thenovel situation of the engine. The lecturer then discussed each of these problems in turn: (a)The narrowness of the turbine's stable operating range means that changes in operating conditions must be met smoothly andrapidly by automatic control. Flexibility depends upon full use being made of the compressor's better performance near the surgeline without actually running into surge. The quick changes that must be met when one engine cuts, and full power is requiredfrom the remaining engines, can only be effectively carried out by an automatic control. With a satisfactory control system the gasturbine has ample flexibility. Turning to emergency power requirements, (b), the lecturersaid that eventually the great majority of helicopters would be multi-engined with ample reserves of power. At present, forexample, if a fully-loaded two-engined helicopter climbing at zero forward speed suffers an engine failure it is likely to be left withinsufficient power unless the remaining engine can give 30 per cent above its maximum take-off power. Such an over-load would onlybe needed for a short period until a moderate forward speed was reached. An efficient engine control system would lose no time in carry-ing out the engine running changes and the overload would last but a few seconds, being needed again only on landing. It is, therefore, most desirable for the engine manufacturer toguarantee the required emergency power to avoid the additional weight and complexity of emergency propulsion equipment. It islikely that a helicopter engine will be uprated for the emergency requirement above the comparable take-off power of fixed-wingaircraft, since overload times will be short and required rarely, adding up to a much less total time than the maximum powerperiods needed for every take-off of a fixed-wing aircraft. The speaker pointed out that in a jet reaction rotor-drive emergencypower cases can be met by tip-burning. The problem of dirt accumulation on compressor blades (c)was considerable, since helicopters spent most of their time flying near centres of population in polluted air, resulting in depositsof solid matter on the compressor blades affecting their aero- dynamic properties. This can be removed by the injection of asuitable turbo-cleansing concentrate. On the subject of the novel situation of the engine (d) Dr.Morley said that the easiest way to install an engine is vertically, with the power drive at the top from what is normally consideredto be the rear end of the engine, special arrangements having to be made for the compressed air intake at the bottom. The Napier Gazelle.—The Gazelle, said the lecturer, has notyet been released for publication, and only brief mention of it can be made. It is a free turbine unit of 1,250 s.h.p. intended formechanical drive and designed from the start for helicopter use— in particular with regard to its control system and power rating. The Napier Oryx and the Hunting Percival helicopters.—Thelecturer praised the work of the Hunting Percival company who pioneered the form of helicopter which uses the Oryx. The designof the Oryx was crystallized in May 1952, and the engine first ran in December 1953, after extensive component tests. Many 2 POSITION TIP NOZZLE ;V 'ft .. ROTOR / • / R.G.8.O. R.D.C. 1 — • _^ F. M.U. VA F.P. fR.P.M. SIGNAL i I JA +' ' —.v-tft"""fr /^ V i.G,V.A! ; L. Diagram of the engine control system of the Hunting Percival P.74 helicopter (described on pp. 938-942). F.M.U.F.P. O.C.A.. P. V.I.G.V. C.O.V. C.O.V.S. (1) FUEL IN (2) OR .OR A B VA, VB R.D.C.S.B. T. R.G.B.O. Fuel metering unit. Fuel pump. O'speed governor. Twin Oryx enginev Variable inlet guide vanes. Data fed in (ambi- ent pressure and temperature: turbine inlet temp.). Change-over valve. Servo to operate C.O.V. Oil pressure ad- mitted to shuttle valve VA if (NB— N,\) is In excess. Oil pressure ad- mitted to shuttle valve VB if (NA— NM) is in excess. Oil pressure accu- mulators for C.O.V. servos, engines A and B. Shuttles controlling C.O.V. servos and overriding pilot. R.P.M diff. control. Starting butterfly and. atmospheric discharge. Trip release (causes partiaf closure of tip nozzles when either C.O.V. opens). Rotor gas blow-off, com. by rotor speed.
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