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Aviation History
1941
1941 - 2222.PDF
192 FLIGHT SEPTEMBER 25TH, 1941. JET PROPULSION it was suggested, could be driven by means of a rotor mounted in the discharge tube ; in which case, presumably, the engine would De used only for starting. Either the complete discharge tube or a terminal portion may be mounted to swivel horizontally and vertically to effect or assist the control of the aircraft. Furthermore, means of temporarily closing the discharge end of the tube may be provided to cause the gas stream to issue from the forward end of the tube to produce a braking effect when landing. A Swiss Design Dr. Gustav Eichelberg, of Zurich, is concerned to differentiate the respective air pressures for propulsion and for charging the motive unit driving the compressor. For efficient operation the pressure at the discharge nozzles should be about 2 atm., and as the flying altitude is in- creased this pressure should be reduced in approximate proportion to the fall in atmospheric pressure. In systems in which the engine is charged from the main air supply, this involves a fall in engine output and, as a consequence, a reduction in the propulsive effort. To in- crease the pressure of the propulsive air is undesirable, since it results in an increased discharge velocity which seriously impairs the efficiency of the plant. On the otherhand, an absolute pressure of from 4 to 5 kilograms per square centimetre (57 to 71 lb. per square inch) may be required for charging in order to keep the . weight and dimensions of the engine at a minimum. In a two-stroke engine, generally accepted as the most suitable type, the exhausting pressure would be too high for efficient use as a propellant. To enable optimum pressures to be employed for each function, the engine exhaust drives a turbine, which in turn drives a separate blower to raise a portion of the main air supply to the required charging pressure. Although the gases leave the turbine at a lowered pressure they are not passed into the main air supply, as this would give rise to losses on mixing, but instead are discharged from special nozzles to provide supplementary propulsive effort. Ihe entire power output of the turbine is absorbed in driving the charging blower. Thus, when operating at a high altitude, the air on the delivery side of the compressor can be adjusted to the appropriate pressure, whilst the charging pressure can remain substantially constant. This is possible as the increased pressure drop (energy output) at the gas turbine offsets the higher pressure ratio (energy input) at the charging blower. As the charging pressure is relatively high the air becomes hfeated by compression.. It is therefore desirable to cool it after leaving the blower to avoid a loss of charge weight at the engine. To dissipate this heat to the atmo- sphere by means of surface coolers would complicate construction, possibly increase aerodynamic drag and also involve a loss of energy. The same considerations apply to the heat dis- sipation necessary for the cooling of the engine. In each case the liberated heat is turned to useful account by transferring it to the main air supply in a separate heat-exchanger of the contra-flow type. The temperature of the fluid-cooling medium is likely to be lower than that of the charging air, so it is expedient that the respec- tive exchanges be inserted in that order. On the left of the illustration is a diagram of a complete unit. Air enters through a diffuser intake A, which is located either to take advan- tage of the dynamic pressure set up by the. aircraft in flight or to suck in the boundary layer from a suitable point of the fuselage or wing. .The compressor B is either mechanically coupled or, as indi- cated, structurally combined with a two-stroke engine r. From the air leaving the compressor a supply is draw a off by the charging blower D driven by the exhaust gtls turbine E. Cooling fluid from the engine cylinder jackets is circulated through heat-exchanger F, and the charging air is passed through exchanger G on its way to the engine intake manifold. After traversing the heat-exchangers, the main propulsion air passes a regulating valve H and combustion chamber J, into which supplementary fuel can be injected to furnish additional power for peak loads, to the discharge nozzle K. The waste gases from turbine H are discharged from the nozzle L. To obtain operational flexibility and reliability a com plete plant would comprise a plurality of compressor aggregates, charging units and heat-exchangers, suitably interconnected to enable various combinations of com- ponents to be brought into co-operative function. An example of such a plant is given on the right of the illus- tration. To simplify the diagram the components are lettered to correspond with those of the single unit and heat-exchangers, and supplementary combustion chambers have been omitted. Three compressor aggregates and two charging units are employed. Air entering intake A is distributed to the compressors B, and from these is collected in conduit M and fed to a distributing manifold N for discharge from nozzles K. Part of the compressed air is bled from con- duit M for the charging blowers D and thence distributed to the three engines C. The exhaust gases are collected for the turbines E, from which the waste gases are led to a distributing manifold P for the auxiliary discharge nozzles D. Suitable regulating and controlling valves, as indicated, enable independent components to be brought into operation or shut down as desired. The number of units may be varied to suit requirements and, of course, any suitable type of engine, compressor or charging blower may be utilised. British Anticipation The ideas embodied in the Swiss scheme would seem to have been to a large extent anticipated by the British inventor, F. Whittle. In 1936 he outlined a "dual thermal cycle." All the working medium, air, was passed through a "lower cycle," and a portion of the medium was passed through a second "higher cycle." The lower cycle consisted of a compression of air from atmospheric to an intermediate pressure, an expansion due to with- drawal of a portion of the air for the higher cycle, and finally an expansion back to atmospheric pressure. In the higher cycle the air was compressed from the intermediate to a higher pressure, heated by the combustion of a suitable fuel and expanded back to the intermediate pressure. As in the Eichelberg scheme, the effluent from the motive unit of the higher cycle was not mixed with the main air stream but conducted to an auxiliary propulsion nozzle. (Continued on page 201.) , This form of the Whittle "dual thermal cycle" scheme employs a diesel engine and compressor to supply air and combustion products as the working medium to the turbine, which drives the main compressor. The effluent from the turbine is utilised for an auxiliary propulsion jet.
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