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Aviation History
1949
1949 - 1845.PDF
. FLIGHT, 10 November 1949 AIRSCREWS FOR TURBINES A Review of Current Progress: New Problems of Pitch-control and Blade Design By L. G. Fairhurst, M.I.Mech.E. AIRSCREWS for reciprocating engines have for anumber of years embodied such features as constant-• speeding, feathering and, more recently, reverse pitch for use in braking. The use of the airscrew in ''windmilling" pitch for dive-braking has been experi- mented with on the reciprocating engines of dive bombers, but to date has found no definite application on this type of power unit. Gas-turbine power units have created for the airscrew designer problems which had not arisen in reciprocating engines, and have demanded a completely new range of airscrews for power outputs of from 1,000 h.p. to 7,000 h.p. The fact that there are at present three distinct basic types of turboprop units has contributed by no mean amount to existing airscrew design problems. The three consist of the "direct connected," "compounded com- pressor" and "free turbine" types shown in Fig. 1. The characteristics of each type demand special mechanical features in the airscrew design. Such features, and the associated aerodynamic, vibration, and noise-reduc- tion problems, are dealt with subsequently under the various engine types. One feature common to all power units having a central air intake is the reduction in bulk of the airscrew hub in order to house it inside the smaller- 609 Counter-rotating airscrews on the Armstrong Sidde'ey Python. THIS review, prepared by the Chief Engineer of Rotol, Ltd., is a special ' contribution to the forthcoming new edition of " Gas Turbines and Jet Propulsion," by G. Geoffrey imith, and will appear in it with certain illustrations additional to those reproduced here. Mr. Fairhurst shows how, though the development of the turboprop has presented airscrew designers with a number of new problems, it has also relieved them of others, such as those concerned with blade strength in relation to the stresses imposed by the reciprocating engine. diameter spinner associated with this type of intake. Thisrequirement in itself involved considerable re-design by reason of the need to cramp the operating mechanism intoa smaller compass. Direct-connected turbine.-—This type of power unit demands, in addition to the normal fine pitch-angle for take-off, a still finer pitch for starting purposes. The re- quirement can better be appreciated by reference to Fig. 2, which shows that the piston engine, under static conditions, is capable of starting and accelerating the airscrew with the latter set at take-off pitch. The direct-connected tur- bine curve, falling below the airscrew power-absorption curve, shows that the turboprop cannot start and accelerate the airscrew at normal fine pitch angle as its power equals the airscrew power-absorption only at maximum r.p m. Thus, it is necessary to provide an additional pitch angle below normal fine—usually between o deg and +6 deg. Furthermore, it is essential to interconnect the airscrew- pitch and engine-fuel controls; otherwise, if too low an r.p.m. be selected on the airscrew for a given engine power after starting, the airscrew will overlead the engine and result in stalling and overheating. Fig. 3 shows the relationship between the engine and airscrew on a sudden acceleration of the engine from slow running to maximum r.p.m. As the airscrew control would be set to call for maximum r.p.m., no change from starting- pitch angle would occur until the engine slightly over- speeded beyond its maximum r.p.m., at which point the pitch angle would then have to coarsen rapidly in order to hold the permissible maximum r.p.m. figure. Under such conditions some over- and under-swing about maxi- mum r.p.m. would be unavoidable. Interconnection of airscrew-pitch and fuel controls is necessary in order to Fig. I. Operation of the three basic types of turboprop units. (A) Direct-connected: Turbine drives compressor and airscrew. (B) Compounded compressor: First turbine stage drives free radial compressor; second turbine stage drives axial compressor and airscrew. (C) Free turbine: First and second turbine stages drive free compressor; third turbine stage drives airscrews B II
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