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Aviation History
1947
1947 - 1832.PDF
470 FLIGHT OCTOBER 23RD, 1947 POWER-PLANTS for HELICOPTERS Weight, Speed, Mounting and Cooling are the Problems I T was indicated recently in Flight that the problem otproviding power for helicopters is not the same as thatfor fixed-wing aircraft, and although it is quite possible and, in fact, current practice to adapt existing units for the purpose, it is rapidly becoming apparent that special power units will have to be developed for rotating-wing aircraft. The most important requirement is. of course, that the engine should be*light in weight for its power, but in addition the helicopter power plant must include a clutch, overrun clutch, flywheel, speed-reducing transmission and cooling fan. There is also the rotor mechanism and, in some cases, a drive for the anti-torque tail rotor. The power requirements of the helicopter are also different, and full power output is called for more often. In addition, although full power may only be required for vertical ascent and descent and for hovering, the fact that the rotor speed is more or less constant for all manoeuvres means that the engine is required to run at near to its maximum r.p.m. for a whole range of power outputs. It has. not so far been decided whether any one particular engine layout is more suited than others for helicopters in general, and to date the decision to use radial, opposed or in-line engines, either with crankshaft horizontal or vertical, seems to have been decided by considerations of availability of the engine in the required power class, and space in the helicopter engine bay. The chief engineer of Continental Motors Corporation of America, Mr. Robert Insley, recently had something to say on his company's experiences with helicopter power plants, and his paper is summarized in the September S.A.E. journal. He points out that there are two ways of achieving lower specific, weight from existing engines; power can be increased without increasing weight, or structural weight can be reduced without reducing power output Regarding the first alterna- tive, increased manifold pressure through supercharging would increase power, but the extra complication was not usually considered worth while. The speed-reduction mechanism be- tween the engine and rotor offered an easier way, and an engine could be redesigned to run at a higher speed and the rotor to have a higher reduction gear ratio. Under the second heading Mr. Insley said that magnesium parts of equal or greater strength than aluminium parts could be produced with a weight-saving of up to one-third. He also quoted the ex- ample of a crankcase for a 250 h.p. engine upon which 20 lb could be saved with no sacrifice in strength and little increase in cost. An indication of what could be done by increasing engine spe<d was indicated by a table of comparative figures for the Continental E-185 flat-six aircraft engine and its counterpart the 0-470 helicopter engine. These two units are identical in fundamental structure, but the 0-470 is designed to operate at 3,200 r.p.m. as compared with 2,300 r.p.m. for the E-185. E-185 0-470Bore and stroke (in) 5x4 5x4 Displacement (cu in) 472 472Max. rated power (b.h.p.) .... 185 270 Speed at rated power (r.p.m.) . . 2,300 3,200 Bare weight (1b) 340 304Specific weight (lb/h.p.) 1.840 I.III Radial engines permitted simple cooling arrangements but,particularly in small helicopters and with crankshafts vertical, they were clumsy to install. On the whole, however, radialengines had been found to operate more successfully with The fan is mounted on the main shaft of the aircooled motors 0-4 Of-? Transverse fan mounting on the Continental 0-470. crankshafts vertical than in the conventional horizontal plane.Large helicopters with relatively unrestricted engine space would continue to rely on radial engines, at least until gasturbine power plants were ready for this type of service. Most current small helicopters were powered by opposed engines, aselection probably made on the basis of availability. Up-ending of opposed engines had caused serious lubricationdifficulties. Oil drained down around cylinder barrels and on to rapidly-moving crank throws and connecting rods, and theresultant hot foaming oil caused breathing and sealing prob- lems: Adoption of deep sumps and double oil seals in addi-tion t© shielded drainage passages had helped the position. The engine designer who was successful in adapting exist-ing small engrvs to helicopter use with only minor external modifications would bring about impressive savings in manu-facturing costs. The demand for helicopter engines was so small that manufacturing costs would be high unless theengines were built and used for other purposes also. Cooling was one of the main problems of helicopter powerplants. Rotary-wing aircraft only provided flight-induced cooling draught under conditions at which the engine coolingrequirements were lowest. Hovering, the most valuable manoeuvre of the helicopter, demanded full engine power butinvolved zero vehicle velocity, therefore the engine needed to provide its own cooling. Engine-driven fans had been themost popular means of assisting cooling, and the fans might be mounted either directly on the drive shaft of the engineor transversely. The shaft-mounting fan had the virtue of simplicity, but the transverse fan, because of its latitude indesign, operating speed, and dimensions, was likely to be more efficient. The location of the fan was often determined bythe space available within a particular helicopter, and in future installations, said Mr. Insley, the engine-driven fanmight yield its place to the exhaust ejector. Responsibility for the cooling of an engine rested on thehelicopter designer as well as the engine designer. Perhaps it would be a good idea to have the helicopter designer con-centrate on providing a healthy environment for the engine, with a properly ventilated compartment to avoid fuel vapourlock and with precautions against re-circulation of air.
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