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Aviation History
1945
1945 - 2385.PDF
598 FLIGHT DECEMBER 6TH, 1945 Approximate dimensions t Length - - /25m. Height • 50in. From the compressor, air is ducted through the parallel pipes to the heat exchanger, thence forward to the "bottle " shaped combustion chambers. will, nevertheless, provide quite a good proportion of its optimum power when run within a fairly small range of its optimum speed, and it is taking advantage of this quality that the Bristol turbine design team have done such a very good job. Their solution lies partly in providing separate drives for compressor and airscrew, and partly in an extremely clever control device on which we are not permitted to elaborate. How- ever, the net result is that the speed of the independent airscrew drive can be varied over a considerable range without affecting the overall turbine efficiency. With large aircraft it is generally more useful to increase airscrew r.p.m. rather than airscrew h.p. for take-off, and the possibility of inde- pendent choice of r.p.m. is therefore a valuable feature, the usefulness of which will, no doubt, be enhanced to reduce airscrew tip noise by the reduction of r.p.m. when cruising. Curiously, the operating characteristics of the gas turbine / air- screw combination (and also of pure jets) are vir- tually the direct opposite of those of the piston engine, for whereas with the latter, fuel consumption increases with throttle opening and height, in the case of gas turbines fuel consumption goes down with in- crease of speed and height. Thus, one has the seeming anomaly that, if the engine is kept at full power, then consumption goes down with gain in height, while it increases when the throttle is closed during level flight. As far back as 1924-5 the Bristol Aeroplane Company were actually flying a Bristol exhaust turbo-supercharger on an adapted Jupiter engine; this was flown successfully at over 30,000ft. and proved very promising, but the state of engine development made it necessary to concentrate on gear-driven superchargers from 1926 onwards. No further work with exhaust-turbos was done untilX 937> when the line of development envisaged by the Com- pany was a turbo-blown version of the sleeve-valve engine —the idea being that boost and back pressure could be pro- gressively increased, the sleeve valve being very well suited to operation at high back pressures. Thus, ultimately, a state of affairs would be reached where the engine and exhaust-turbo powers were equal and it would so be possible t o interchange a i r s c r ew and compressor, thereby making the engine and compressor vir- tually a gas genera- tor unit and driving the airscrew by the exhaust turbine. By the outbreak of war Bristols had a general plan for entry into the gas turbine field, and the primary aim was the production of a com- pound unit of low fuel consumption which would therefore be pecu- liarly applicable to long- range aircraft. Long I.erm research work of this kind, however, had to be closed down and the project was therefow' Fitted with a Rotol 5-blade airscrew andcomplete with mounting structure the unit makes a neat installation.
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