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Aviation History
1955
1955 - 0545.PDF
FLIGHT, 29 April 1955 THE SUPERCHARGED TURBOPROP Dr. Hooker's S.A.E. Lecture on the Bristol B.E. 25, Successor to the Proteus ALTHOUGH it has not yet run as a unit, the BristolB.E. 25 has already been widely discussed, since itLpromises to offer a combination of performance and economy previously unmatched by any engine, and certainly not even approached by any engine of comparable size. The manufacturers, the engine division of the Bristol Aeroplane Co., Ltd., were, until last week, reluctant to discuss the engine in detail—justifiably so since, as we have remarked, the engine has not yet been run. Nevertheless, a comprehensive account of the engine's background and characteristics has now been given by the man best qualified to do so, Dr. S. G. Hooker, O.B.E., A.R.C.Sc, B.Sc, D.I.C., D.Phil., F.R.Ae.S., F.R.S.A., the engine division's chief engineer. His paper on the con- cept of the supercharged turboprop was read before the (American) Society of Automotive Engineers in New York on April 18th. The concept of a supercharged turboprop, said Dr. Hooker,had been introduced by the Bristol B.E.25, and this design of engine was used to illustrate his paper. Going back to the con-ception of the B.E.25 some two and a half years ago, the under- lying thoughts were: (a) To produce a turboprop with a specific fuel consumption equalto that of the best compound piston engine. This implied a pressure ratio greater than 10:1, and hence necessitated the "two-spool"arrangement of compressors. (b) To produce a turboprop having a cruising power of the order of3,500 h.p. at 30,000ft, with the lowest possible specific weight. Such an engine would give more than 8,000 h.p. at full throttle at sea-leveland it was desired to restrict this power to between 4,000 and 5,000 h.p. in order to reduce the weight of the reduction gear and airscrew and thusachieve an appreciable improvement in the specific weight under cruising conditions. (c) To produce a turboprop the take-off power of which was inde-pendent of the altitude and air temperature of every aerodrome in the world. Naturally aspirated turbine engines (i.e., those now in use) suffered badly at the higher and hotter air-fields, although the adverse effect of high temperature could be partially mitigated by water/ methanol injection.(d) To exploit to the full the known ability of the gas A turbine to produce (relative to piston engines) large power for a small bulk and weight.It would, said the lecturer, be practically impos- ' sible to produce a reciprocating aero engine for trans-port use with a conservative cruising power of 3,500 h.p. at 30,000ft. The desirable properties outlinedin (a), (b) and (c) above were all enjoyed by the supercharged piston engine, and this was the funda-mental reason why such engines had almost com- pletely displaced the naturally aspirated engines ofthe early days of flying. A diagrammatic section of the B.E.25 showed asingle high-pressure turbine stage driving the high- pressure compressor and a three-stage 1-p turbinedriving the 1-p compressor and airscrew reduction gear. Basic engine control was effected by two levers,one of which was a fuel throttle governing the power and speed of the high-pressure system and the otherwas connected to a constant-speed unit controlling the pitch of the airscrew, and hence the speed of thelow-pressure (or supercharging) unit. The apportion- ing of compression between the low- and high-pres-sure systems had to be carefully chosen so that, whatever the speed-ratio between the two, neitherran into a surging condition. The engine incorporated a compound epicyclic Photographs of the mock-up B.E.25. Made almost entirely of steel, the new engine will weigh scarcely more than a Proteus. Provisional B.E.25 data were published in out isms of January 28th last A, fuel manifold; B, engine de-icing valve; C, bulkhead inner section; D, air supply to aircraft for pressurizing and wing de-icing (two ducts at top, two at bottom); E, oil tank; F, air- screw brake link; G, airscrew-turbine overspeed governor; H, airscrew feathering unit; J, airscrew synchronizer generator; K, electric starter; L, 50 kVA alternator; M, "Ultra" control unit; N, starting pump; O, throttle and air/fuel-ratio control; P, fuel filter; Q, four dynamic suspension units; R, scavenge filter; S, oil pump; T, oil-pressure transmitter; U, fuel pump; V, lOin oil-cooler (port and starboard sides); W, barometric- pressure control, torque limiter and shut-off cock; X, mounting ring (powerplant supply). reduction gear in which the torque transmitted was measured onthe final floating annulus gear by oil pressure on a torque-reacting piston. Torque limitation was provided by balancing the torque-meter pressure against fuel pressure at the engine burners. It was a characteristic of turboprops that their specific fuelconsumption decreased with increase in turbine-inlet temperature. Experience had shown that the reliability and longevity of thecombustion system, turbine blades and other "hot" components fell rapidly as their operating temperature was increased. It wastherefore desirable to strike a compromise with temperatures on the low side, despite the disadvantages in performance, in order toachieve a long life with a high order of reliability. Little advant- age was to be obtained by reducing the inlet temperature muchbelow 1,000 deg K or 1,300 deg F, and this was therefore chosen as the cruising temperature for the B.E.25. Fig. 1 showed thespecific consumption obtained at 25,000ft and 300 m.p.h. in engines with various compression ratios. It should be noted that, with this peak temperature (turbine-inlet temperature), and with a cruising pressure ratio of 10:1, an six. of 0.37 lb/hr/e.h.p. was indicated. Higher compressionratios would be an advantage, but the specifics of the best recipro- cating engine could be beaten at the figure quoted, which was wellwithin experience of two-spool compressors. Turboprops had so far lagged behind the turbojet in aero-dynamic performance, owing to the great advantage of jet pro- pulsion for military purposes and to the relative simplicity of suchengines. Now that this first phase was over, turboprops were being designed which were thermodynamicaUy equal, and greatlysuperior in propulsive efficiency, to turbojets at their appropriate flight conditions. In fact, the turboprop responded better toimprovement in the thermodynamic cycle than did the turbojet, since any improvement in compressor or turbine performanceof the latter increased the velocity of the jet and thus decreased the overall propulsive efficiency. On the other hand, corres-ponding improvements in the turboprop cycle yielded an increase
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