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Aviation History
1946
1946 - 1408.PDF
74 FLIGHT JULY I8TH, 1946 CORRESPONDENCE change of momentum of a larger mass of air travelling at alower velocity, i.e., the slipstream. Hence, though the aircraft is stationary, a thrust can be exerted on it by virtue of therelative velocity of the jet or slipstream. 3. Torque resistance to motion of a jet engine rotor.—Ina simple jet engine about 9.0 per cent of the power developed by the turbine is absorbed by the compressor in com-pressing the air. You will now realize why at quarter throttle (the figure quoted in your letter) you will not fly off to toprevolutions. 4. Zero speed—zero thrust.—The thrust of the airstream isdefinitely equal and opposite to the total drag of the aircraft for a constant speed. At take-off, however, the thrust forzero speed can vary from zero to a value equal to the sum of the air resistance and the frictional resistance between thewheels and the ground. No,doubt in your reasoning you were neglecting the latter term in this static friction case. C. N. HAWKINS, B.Sc., Wh.Sc, A.C.G.I., G.I. Mech.E. Power Supplied in Different Forms IN Flight, June 20th, "Brown Job" wants to know why theoutputs of jet and piston engines are measured differently. A piston engine supplies power as torque in a rotating shaft;Power = Torque x Angular Velocity. A jet engine supplies power as the energy of a moving massof air, the energy being obtained from fuel burnt in the air. From thermodynamic considerations it can be shown thatthe efficiency is much enhanced (and the fuel consumption correspondingly reduced) by compressing the air before com-bustion. It is true that there must be a torque in the shaft to overcome the air forces on the compressor blades, as withan airscrew, the above equation applying in both cases, but the power required is "tapped off" from the energy of thegas stream as it passes through the turbine, only the residual energy being available for use in the jet. The residual energyclearly bears no relation to the power in the shaft, and. there- lore, there is no shaft power output to measure.Jet thrust, however, can be measured directly as the force on the engine mounting; and here the properties of a jet andan airscrew are analogous. If a jet engine and a piston engine plus airscrew were each put in a box with a hole at eitherend, the overall effect of each would be fundamentally the same, according to the equation: ,, ,, c „ (Jet Speed or Box Speed or"| lhrust^ManperSec. x Jsnpst£am Speed^ Aircraft Speed The reason why piston engines are not rated in thrust is thatthere is no thrust until an airscrew is fitted, and then the thrust depends on the efficiency of the particular airscrew used. Themaker is, therefore, required only to say what his engine alone will do, the rest being left for the aircraft designer to workout. With regard to "Brown Job's" final reasoning, he has for-gotten that the drag of a stationary aircraft " revving-up" is provided by wheel-brakes and chocks.As a note I would like to add that the actual power in the efflux from a jet engine is the sum of the kinetic energy ex-hausted per second plus the heat energy exhausted per second by virtue of the temperature of the gases above that of thesurrounding air. This is not the power available, as all of the heat energy is lost and also some of the kinetic energy asKing as the jet speed relative to the aircraft is greater than the aircraft speed. E. A. BRIDLE. Piston and Jet Engine Compared >ROWN Job" (Flight, June 20th) appears to have caused) some of his trouble by considering only aircraft instal- lations, which are concerned primarily with the thrust availablefor flight. On the jet turbine* engine insta^ atioa his argument on thrust horse-power is coirect and it is eq lly applicaLleto a piston engine/propeller installation. The difference is that a piston engine produces power in the form of shaft(brake) horse-power which can be developed and measvre 1 on a test bed, whereas a jet turbine produces only thrust. On the piston engine installation the airscrew is necessaryto convert the torque (shaft) horse-power into thrust and thrust horse-power, the efficiency of the airscrew being theratio of thrust horse-power to torque horse-power. As " Brown Job" states, the thrust horse-power is thrust times forwardvelocity The torque horse-power is shaft torque times angular , (Torque x 2* xr.p.m.). , , , velocity. 2 — Just as the thrust horee- 33,000 J power is zero at zero forward speed, so the airscrew efficiency BI< is zero at this condition although the piston engine may bedeveloping maximum (torque) horse-power. Reference to the internally absorbed power is apt to confusethe argument on thrust and useful power. In this respect, however, both types of engine are similar. Both have frictionat running surfaces to overcome, which requires power; both have accessories to be driven and most piston engines havea supercharger which can be compared with the compressor of the turbine engine. The power used in this way, however,does not form part of the useful power and does not directly affect the thrust horse-power in which "Brown Job" is inter-ested. G. N. USING GERMAN TECHNICIANS Why Not Attach Them to the Ministry of Supply ?R ECENT paragraphs in the Press concerning the employmentof German prisoners of war in the War Office, and the resulting explanation from an official that, owing to their know-ledge of the particular work, they would be more useful than civil servants, prompts one to suggest that perhaps this schemecould be applied with advantage to the Aircraft Production Branch of the Ministry of Supply.The Ministry at present consists of a nucleus of officials with little or no practical experience, surrounded by a host of civilservants who have no qualifications for the job they do, other than a limited repertoire of stock phrases for indirectly saying" no," a positive genius for obstruction, and who, after some time in the department, are referred to as technical experts.With long standing the higher officials achieve the status of oracles, and can then use academic jargon and new phrases andterms coined at their convenience or need, to overawe Royal Air Force officers whose technical qualification quite often con-sists of an R.A.F. engineering course taken some long time ago, and who are appointed for relatively short periods to"control" official activities. As it has been stated on the highest authority that officiallythe R.A.F. depends entirely on the advice of the Ministry, it is surely time that those who have consistently shown theirlack of vision, and have so resolutely resisted new ideas in the past, should be removed, retired, or promoted out of the wayin the usual manner. Otherwise, without the urgent necessity of war to force thetrial of new ideas—and, indeed, the adoption of developments which have already been proven in practice—we shall find our-selves facing the future wi'.'i equipment of a similar class to that which frittered away value personnel, man-hours andmaterial in the early stages of this last war. Great men have publicly given assurances that all stepswould be taken to ensure against the recurrence of a similar state of affairs, and if, after the weeding-out process is com-plete—and our own talented scientists and technicians do not desire to remain under Ministry control while others still feelthat initiative will be stifled—then one answer would be to employ, within the Ministry rather than at other centres, theGerman scientists brought to this country. These men have most effectively demonstrated their foresightand ingenuity, although hampered by difficult conditions and paucity of materials. J. R. ANDERSON (ex-Fit. Lt.). MAN'S INGRATITUDE Even Disabled Airmen are IgnoredT HE letter by " Man-in-thestreet" (Flight, May 23rd) wasconfirmed by an experience of my own last week. Walking back from the centre of the town to the Rehabilita-tion Unit, one evening, after missing the last bus, I "thumbed" six cars before giving up the attempt. All hadempty seats; none stopped, although it was raining slightly I and I walk with a fairly pronounced limp when tired. y? The unit is very well known in the town, and it is commonknowledge that it consists of disabled aircrew. Nevertheless, since the end of the war, incidents such as this have become therule rather than the exception. T. V. ARDEN, Fit. Lt. FORTHCOMING EVENTS July 21st.—Leicester and Auster Fl/ing Clubs Rally, Rearsby Airfield. Aug. 22nd.—Model Engineer Exhibition opens at the New Horticultural Hall, Vincent So., Westminster, London, S.W.I. (II a.m.) Sept. 9th.—Ministry of Civil Aviation demonstrations of technical radio equipment. More details pending. Sept. 12th and 13th.—S.B.A.C. Flying Display and Exhibition, Handley Page Airfield, Radlett, Herts. Nov. 15th.—Paris Aero Show, Grand Palais, Champs Elysees. Nov. 15th.—Engineering exhibition, Kelvin Hall, Glasgow.
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