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Aviation History
1944
1944 - 1176.PDF
614 FLIGHT JUNE 8TH, 1944 POWER PLANT- PAST AND FUTURE place with gas turbines and jet engines for air craft propulsion, and there can be little doubt that in the course ot time the gas turbine in its various forms will, with the neces- sary development, gradually supersede the piston engine as the source of power for the aircraft of the future. Although the possibilities of the gas turbine as a prime mover have been investigated by various authorities over a period of more than 30 years, it can justly be claimed that Great Britain has played a leading part in the application of the continuous combustion turbine for aircraft propulsion. (Jjp, Capt. Whittle must be given the credit for appre ciating the possibilities of adopting the gas turbine for use as a jet-propulsion unit, and this country further owes a debt of gratitude to him and his colleagues, as well as to the staff of the Royal Aircraft Establishment, for the vision and perseverance put into the whole problem of the gas turbine in its difficult initial stages of development. Use of the Gas Turbine The propulsive characteristics of the gas turbine and jet combination differ fundamentally from those of the piston engine with airscrew, and an illustration shows curves of propulsive efficiency against speed, at 20,000ft., for the conventional airscrew, jet, and rocket." It is seen that the airscrew achieves its maximum efficiency at about 300 miles per hour, while the cross-over point occurs at about 550 miles per hour. There is undoubtedly a great future for the jet-propelled aircraft in the high-speed, high-altitude field, both for military types and special-purpose civil aircraft, but in connection with the latter, if they are to be a success, much will depend upon the efforts of the aircraft designer towards producing really low-drag machines. Turning now to the use of the gas turbine in con junction with a conventional airscrew, it appears possible to visualise a considerably wider field of application than for the simple jet unit. Such power plant will probably be of the multi-stage, axial-flow type, em ploying between 70 and 80 per cent, of the power output to drive the airscrew, and the remaining 20 to 30 per cent, for jet propulsion. The illus tration shows an im pression of such a unit driving contra- rotating pusher air screws, and com- A large transport air craft employing four submerged gas turbine power units driving contra-rotating pusher airscrews. pletely submerged ini^y the wing of a. large* machine. This installation is shown with divided exhaust ducts discharging outside the airscrew disc, but there will undoubtedly be many alternative methods of installing such a power plant. There is still much to be accomplished, and adequate efficiency, flexibility, and stability of operation of multi stage, axial-flow compressors and turbines will only be achieved by patient development, and, in addition, theiv will be many problems on turbine materials, blade vibra tion, differential expansion, etc., to be solved. Every encouragement should be given to the evolution and manu facture of new types of promising design, but with the gas turbine, as is the case with the conventional aircraft engines, we must plan our development programme wisely, be pre pared to take one step at a time, and not be tempted to rush into production with partially developed designs. However, when the gas turbine has been developed and brought to fruition, we may expect to have prime movers with altogether new standards of reliability and per formance. Some outstanding advantages should be reduced drag and weight of power plant, the provision of a smooth CURVES ft ACASON* r « BAM.0 ( M.E ASSUM M P1IONS BHP -or i6 v- \^ ..., . . p\ v\ v> \ \o ' \°- Vr \o \° BHP , i* \ \\ \\ \V / / \ • JOO «00 FORWARD SPEED MP.H too SOO 750 IOOO I2SO MAX. RATED SEA LEVEL B.H.P. jooo Curves of variation in optimum efficiency with altitude and forward speed for six-blade contra-rotating airscrews. The trends in weight against power for various fighter airscrews, both metal and wood, seem to show a relatively '*s slight advantage for the latter type.
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