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Aviation History
1944
1944 - 0453.PDF
MARCH 2ND, 1944 FLIGHT 235 CORRESPONDENCE The Editor does not hold himself responsible for the views expressed by correspondents. The names and addresses of the writers, not necessarily for publication, must in all cases accompany letters. JET PROPULSION Efficiency of Axial-flow Compressors REFERRING to the issue of Flight dated February 17th, 1944, and to your article on " Gas Turbine Power Plants," after examining the sketch by your artist, Max Miller, the type of axial flow compressor depicted will absorb, as you say, approximately 75 per cent, of the available power, which is ridiculous when there are other types of axial blowers that absorb only 1/ 200th of the power available. The mixing chambers are unusual and would be highly inefficient, but the flame grid bars would infringe patents. A power unit of the type you suggest at 3,000 r.p.m. instead of anything up to 10,000 r.p.m. with a fuel consumption of approximately 0.0078 lb. of fuel of a B.T.U. value Of 18,000 per injection will compete favourably with any reciprocating or C.I. engine. It is to be noted that foresight has been given to your sketch by attaching the conservative airscrew, and the time is not (far distant when the turbine power unit will be available for w variety of propulsion purposes on an economical basis. * G. W. STANLEY. A Simple Explanation HOW long will it take, I wonder, for the majority of inter ested people to understand, finally and without question, the operative principle of jet propulsion ? May I shove my oar in ? Then with your indulgence: The propulsive effort obtained from a jet power unit (whether fitted to an aircraft, autocycle or lawn mower) is derived purely from the reaction of the force exerted in ejecting the jet. Just that and nothing more. Without dashing into the specious realm of mathematics, probably the clearest analogy is provided by the operation of a gun or rifle. Consider a rifle freely suspended in space with some remote control method of firing it. When the charge is fired, a given pressure is generated in the breech a portion of which is expended in ejecting the bullet along the barrel; the remaining portion of the force generated is expended in moving the whole rifle backwards in an opposite direction to that taken by the bullet (that is, the recoil). If in place of a rifle one substitutes a jet aircraft, the bullet becomes the jet ejected, and the. recoil becomes the forward motion of the machine. The main difference is that the jet aircraft has a constant combustion and therefore continued motion, whereas the rifle has spasmodic combustion (only when it is lired) and therefore a like recoil motion. "AFFLATUS." Yet Another Explanatory Analogy .CT-RANGE how long it takes to dispel the fallacy that the 4k^ propulsive force is derived from the rearward-driven jet "pressing against the surrounding air." It is well known that such is not the case. Propulsion is derived from the reaction caused by acceler ating a mass of gases rearward, the reaction being in the oppo site direction—forward. There is no mystery about the reaction. Imagine a man seated on the ground, knees drawn up and feet planted against a 20olb. rock. His back is against, say, a wall. By straighten ing his legs he exerts a force on the rock and pushes it away from him, thus accelerating a mass (of rock). In order to exert this force his back is pressed against the wall. This pressure is the reaction, and is the simple natural result of his action in pushing the rock away. If his back were against another 20olb. rock instead of the wall, it is probable that both rocks would move as a result of his exertion. So in the jet-plane. The power plant may be likened to the man, the rearward-driven mass of gases to the 20olb. rock, and the aircraft itself to whatever the man is bracing his back against. The mass of gases is pushed rearward, and unless the aircraft is anchored it will move forward. It is also a fact ( rour correspondent R. L. Gladwell may be interested to know) that J.P. aircraft do not "fly at great heights with little reduction in speed." Their speed increases with altitude, for although the reaction which drives them can be maintained, the drag decreases owing to "thinner" air. The "intake suction" mentioned by N. V. Brittain is no more a part of the driving force than it is on an orthodox engine and aircraft. In fact, at high speeds considerable "ram effect" is experienced, which improves the output of the blower (compressor). It must also be obvious that a J.P. aircraft could not operate in a vacuum. Where would it obtain the air to burn, or to give lift to its wings? It should be equally obvious (and is an equally well-established fact) that a rocket will operate in a vacuum. It requires no oxygen from the air to support the combustion of its fuel, which is self-sufficient and anyway burns in an enclosed airless chamber. Inasmuch as a rocket is lifted without wings, simply because it is aimed upward, it would naturally travel faster'in a vacuum (if such condition were possible), where drag would be non-existent. Again it is the reaction of expelling or accelerating a mass of gases which sends the rocket skallyhootin' through the sky. In short, to move a mass (of anything) you must exert force against something. If that something happens to be a rocket which is not nailed down, the rocket gives way. As Archimides said: "I could lift the world with a big enough lever—if only I had some place to stand! " Or words to that effect. R. H. HENDERSON. Relative Overall Efficiency MAY I congratulate you upon the complete justification of your 1941 campaign for jet propulsion. It is surprising after all you have written about direct-reaction drive that so many of your correspondents still miss the point, namely, that a J.P. aircraft pulls itself up by its own braces and not by kicking" back against the surrounding atmosphere. Even the de Havilland advertisement No. 19 does not state the case quite clearly enough when it talks about throwing stuff back instead of about the reaction against the structure of the throwing back of that stuff. The picture would perhaps be clearer if the formula for efficiency were based on Jet Speed instead of Slip Speed, thus— R=tr and E = =r V r +R But in any case, comparison between J.P. and airscrew drive is useless without reduction to the final expression of overall efficiency. Orthodox aircraft engines show anything from 25 per cent, to 30 per cent, efficiency. Diesels go up to 35 per cent. In both of these considerable mechanical energy is lost in conversion and transmission, auxiliaries, exhaust, etc. With a compressor/turbine unit, where the only mechanical energy taken out is for an auxiliary, namely, the compressor drive, i.e., where the main source of energy required and available is in the exhaust gases, and where, also, the mechanical energy taken out is already in the desired state of rotary motion at the source where it is tapped and passed from rotor to co-axial compressor, the thermodynamic efficiency should be at least equal to that of a diesel. Axial-flow compressors have been constructed with 90-92 per cent, efficiency criterion and an annular or spiral combustion chamber cannot impose more than 2-5 per cent, loss by shock and turbulence. Turbines giving 90-95 per cent, output are not unknown provided the gases are fully expanded at the cutlet. The paradox is that full expansion means high outlet speed, which in turn means lower efficiency of propulsion, and if it is not sufficient to find a compromise between these two factors, I would suggest borrowing ideas from several of your corre spondents, "Projet," " Ne Fronte Crcde," and J. K. Haviland. Increase the mass of air handled by sucking off the boundary layer and turbulence on wings and fuselage by means of ejector ,~ effect around the jet. "LAYMAN, UNQUALIFIED." THE DUCTED RADIATOR A Form of Jet Propulsion I S answer to your correspondent, R. D. Morrell, in January 27th issue of Fliglit, the theory of the ducted radiator can be defined as follows:— The ducted radiator as incorporated in present-day cooling systems uses cooled air moving at a much lower velocity through the radiator than the forward velocity of the machine The cross-section of the duct increases in area to the radiator position, and decreases to the outlet. The velocity of the air
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