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Aviation History
1944
1944 - 0412.PDF
212 FLIGHT FEBRUARY 24TH, T^,. CORRESPONDENCE although the latter is lighter in weight and gives better aero dynamic shape. I think the assessment of points for this and the vulnerability factor needs very careful investigation. F. j. HEYMANN. [An article on this subject appears on pp. 202-203.—ED.] JET PROPULSION OF AIRCRAFT Correspondent in Hot Water MAY I point out that the suggestion of your correspondent, H. Munday, of using water as a fuel, contains a basic fallacy. He suggests electrolysing the water into its compon ent parts, hydrogen and oxygen, and then burning the hydro gen in the oxygen, and diverting some of the energy produced to decompose more water, and so on. Now when water is decomposed, electrical energy is used, and if the hydrogen and oxygen so formed are recombined by burning, exactly the same amount of energy will appear as heat as was used to electrolyse the water. If, then, you divert enough energy-to decompose more water at the same rate as before, you will have to use all the energy and convert it into electricity at 100 per cent, efficiency. This is not a practical possibility, but even if it were, there would be no energy left over to propel the aircraft. If, as your correspondent suggests, you use some of the heat to turn water into steam, there will be even less energy available to electrolyse the water, and the system will be even more impossible of working. In any case, since the product of burning hydrogen in oxygen is water, it will be seen that, if the system works, it will be producing hot water at the same rate as we are supplying it with cold water. Since there is no external source of energy, this is impossible, and, therefore, the system will not work. D. HANSTEAD. Plea for Further Enlightenment MAY I now make a definite appeal for an article by Mr. G. Smith or other unimpeachable authority on jet propulsion. There seems to be so much loose thinking about this subject, due, no doubt, to attempts at over-simplification, or else to mischievous leg-pulls. I cannot follow Mr. Haviland's statements in your issue of February 10th, unless they are intended to provoke argu ment by deliberate misstatement—a method I have used effect ively when in search of enlightenment. May I deal with his points seriatim? Fluid flow can only take place from a region of higher to a region of lower pressure: jets, or turbine nozzles are no exception. Mr. Burgess's authoritative and informing letter shows that, in vacua, the analogy of the gun is correct during the period of acceleration. The speed of the gun, or rocket, has no bearing upon the value of the thrust in these conditions, and the jet efficiency remains constant irrespective of the speed. I cannot imagine why Mr. Haviland introduces " g" into an expression already containing mass. Perhaps it is this that causes him to deduce that a device which is doing no work can have an efficiency of 100 per cent., or even more. Would that this were true! Perhaps a definition of jet efficiency ought to be established so that all of your correspondents can meet on common ground. To me—and I venture to believe, to most engineers—it will include first, thrust, time, and pounds of fuel, then relative velocities, density, and viscosity of the medium in which travel takes place. It is simpler if the velocities are reckoned relative to the jet, and not considered in relation to the ground. Theoretically, jet propulsion can be made more efficient than an airscrew by the use of all the waste heat to increase the slip speed, as indicated by Mr. Shoham, even at airscrew speeds, and in addition it has the advantages which Mr. Havi land so rightly claims, particularly that of increased power/ weight ratio, although at the cost of higher specific fuel con sumption. To support my request that velocities be considered relative to the jet, take the hypothetical case of the machine gun on frictionless wheels in a vacuum, firing bullets with a muzzle velocity of 2,000 ft./sec. At the start an observer on the ground would see the bullet travelling at 2,000 ft./sec. away from the gun. When the M.G. has attained a velocity of 2,000 ft./sec. the observer sees the bullet drop out of the muzzle, and when the velocity is 4,000 ft./sec. the bullet is seen to be following the gun at -2,000 ft./sec.—how confusing for the observer! Far better if he travels with the gun and only has to deal with a constant slip speed of 2,000 ft./sec., a constant thrust for a constant expenditure of explosive fuel' and therefore, a constant jet efficiency, at any rate so long as he doesn't reach velocities high enough to bring Einstein into the argument. "PROJET." Movement from Unbalanced Pressure SURELY the aircraft could (all things being equal) be propelled by the impulsive forces created by the sudden release of gases into a total vacuum, similar to a rocket, A toy balloon filled with air 01 gas, if placed in a vacuum and punctured, will travel in a direction opposite the puncture, this movement being due entirely to the unbalanced pressure of gas, or air, on the side unpunctured. Obviously in a vacuum outside forces are non-existent. L. STUART WATTS, A.M.I.Struct.E., A.M.I.C.D. [Mr. Stuart Watts' reasoning is sound. In connection with the general controversy as to whether the jet "pushes" or redcts on the atmosphere, it is desirable to make it clear that the jet reacts on the aircraft and not on the atmosphere.—ED.1 The Same Old Mistake I M AY I comment upon the letter of your correspondent E. ri. Miles in your issue of February 3rd advocating the "orange-pip principle" for jet propulsion. First of all, may I ask where the air intake to the compressor and turbine is situated, or is this proposed aircraft self- contained, being of the rocket propulsion type ? Secondly and more important, Mr. Miles says: " . . . have several jets coming out of the shoulders, pressing air against and all round the fuselage, when the aircraft will shoot forward twice as quickly on only half the power." Surely Mr. Miles is making the same age-old mistake made by Mr. E. Colston Shepherd in his broadcast on January 7th, giving the im pression that propulsive effort is obtained by the action of the exhaust gases "pushing upon the atmosphere." Apart from this it would seem that the principle would not apply, for the '' squeezing'' force is moving with the aircraft and is not stationary as the fingers are when shooting an orange-pip. " ASTRO- AERON AUT." THE CIRCULATION THEORY A Useful Mathematical Abstraction I THINK the best answer to " Engineer's " question in Flight of January 27th re the existence of circulation is that it is a mathematical abstraction, in the sense that it is one of the components into which the actual motion may arbitrari divided. An obvious, though admittedly not a good, an^ is that of a motor car wheel as viewed from the road; no^>-'~t on the wheel "circulates," in the sense of completing a circuit and arriving back at its starting point, yet the rotary com ponent is the most striking feature of the motion. Somewhat similarly, to speak of circulation round an aero foil does not imply that any particle of air makes a tour round it and arrives back at its starting point, though, for the benefit of eager critics, it may be mentioned that individual molecules probably do quite often achieve this feat. That, however, has nothing to do with circulation theory. The point is that the theory starts with a figment of imagination, namely, an in- viscid, incompressible fluid capable of following any contours, regardless of the extremes of velocity and pressure that may result. In such a case the calculated flow past an aerofoil inclined to the stream divides just under the leading edge and unites on the upper surface just in front of the trailing edge; there is no lift and no drag, but there is a couple tending to increase incidence. In reality, of course, the air makes no attempt to get round the trailing edge on to the upper surface, but continues straight on, the junction of upper and lower streams being at the trail ing edge. This condition can be produced in the theoretical flow by superimposing a circulation of suitable strength. Since this mathematical trick also predicts a lift nearly equaMto the observed lift, it is considered to be well worth while. $S&t physically the lift is still due to the deflection of air down wards, just as it always was. Skin friction is only responsible to the extent that it is the main reason why the air fails to round the trailing edge. W. E. HICK.
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