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Aviation History
1945
1945 - 2082.PDF
OCTOBER I8TH, 1945 FLIGHT 433 / • CORRESPONDENCE the itch will return to the palms of the hands, and if releaseof some kind is not organised we shall have aircraft stolen from R.A.F. stations and bodies falling with collapsedumbrellas from high buildings. It is no ordinary craving, this. It settles in the very marrowof the bones. The spectre of the atom bomb is as nothing to the still more gaunt spectres which will soon be seen abroad—those of the men and women torn from their aircraft. Already the disease can be seen in the Air Force itself, where thousandsof aircrew are eating their hearts out on the ground. If nothing can be done to satisfy the need, then I earnestlyadvise the Ministry of Health to arrange for doctors, ambu- lances and hospitals to stand by for the crack-ups. S.W.3. "JEREMIAH." CAMPAIGN AWARDS Aircrew Europe Star a Time AwardT HE Fleet Air Arm gentleman that feels maltreated about the "Aircrew Europe Star" should take comfort in the fact that it is not really a campaign star and is of very little consequence. It is purely a time award. It is given ad lib. _, to any member of aircrew who has done one operation prior to ffi D-DAY (even if L.M.F. after it), whereas a tour or more of ** operations since that date counts for nothing more than the "France and Germany Star," which ground personnel also receive. ANOTHER MISFIT. DEFLEX-REACTION PROPULSION No Improvement Possible YOU invite criticism of Mr. Umpleby's article in Flight,September 13th issue, so here goes: — (1) It is well known that the greater the (properly directed) exhaust velocity, relative to the aircraft, the greater the effec- tive thrust. And all experience of the flow of a gas in a pipe, e.g., high-speed petrol engines, goes to show that the less you bend it about, vary its cross-sectional area and so on, the faster it will flow. The example of the wave in the river mentioned by Mr. Umpleby is a case in point. A variation of cross-section induces a wave which requires a lot of energy to produce it, which energy can only be supplied at the expense of the speed of the current. (2) I am not suie that 1 understand "in the case of purely axial flow the momentum of the gas stream in relation to the acceleration of the fluid is reduced by an amount equivalent to the velocity of flight," but it is well known that the exhaust velocity relative to the machine may be Jess than the flight velocity relative to the starting point. And this is not sur- prising. A fireproof engineer sitting in the furnace could not know, except by a possible slight "ram effect," whether the Symachine was moving fast or slowly. ' • (3) I do not agree that "as the velocity of flight increases the effective propulsion force decreases." The velocity of flight does not go on increasing indefinitely owing to head resistance and drag, and because all the air picked up by the blower has to be accelerated from rest to the velocity of the aircraft. The propelling force probably increases with in- creased velocity owing to ram effect. • (4) Equation (1) does not seem to me correct. Newton recorded the fact' that the pushing force (in poundals) is equal to the mass (in lbs.) multiplied by the acceleration (i.e., the ft. per sec. of velocity added every second to the velocity of the mass). A pound weight is g poundals and is slightly vari- able, but engineers measure stresses, thrusts, etc., in lbs. Therefore, starting from T poundals = W.lb. mass x (V - V,) acceleration we get T -lb. = V - V,) W or T^V-V.) W.g. To return to general considerations, essentially a J.P. unit consists of a blower blowing air into an oil-fed furnace (a volume multiplier), a turbine giving the free-est possible pas- sage to the furnace gases consistent with its developing suffi- cient speed and torque to drive the blower to which it is coupled, and an expansion chamber leading as smoothly as possible to a jet-nozzle which is as large as possible consistent with the furnace pressure being maintained at the pressure which the blower can provide. The function of the furnace is to maintain, by multiplying the gas volume, the pressure supplied by the blower, in spite * of the open hole at the tail end of the chamber. So that one end of the force we are using for propulsion is taken by mechanism fixed to the aircraft and so transmitted to the air- craft and the other end is used, as nearly as possible exclu- sively, to accelerate the gas in the chamber. Any obstruc- tions, rough surfaces, baffle plates, turbines or ducts, cause a negative thrust which leaves so much less for the essential purpose of accelerating the gases at one end and the aircralt at the other. This seems to leave no room for improvement by deflection. A boat will often sail faster with a beam wind than with a following wind, which looks at first sight like an argument for the deflection principle, but the reason is that as the boat gains speed the relative wind falls off, whereas the jet propul- sion unit carries its own constant pressure with it. Rockets have been constructed squirting in the direction of flight and using a sort of little umbrella completely to reverse the blast, but that was for stability reasons—nobody claimed increased thrust. A Pelton wheel is an outstanding example of com- plete reversal, but that again is for constructional and mechanical reasons—a glorified lawn sprinkler would, in theory, be more efficient when running at speed. In short, it appears to me that the jet propulsion principle is unique in that the machine using it cannot run away from the force which is driving it, because the force originates in the machine, and, therefore, there is no hope of improving thrust or speed by any adaptation of the inclined plane prin- ciple. L SHELFORD BIDWELL. [A letter from Mr. Umpleby points out that the equation (4) in the original article should have been Axial Thrust in Flight WR or T = — (V-V-) - v. —ED.] g Mr. Umpleby Replies to His Critics FIRST I wish to thank everyone who has so kindly com-mented on my article in tile, issue of Flight of September13th, even though, up to the moment, we are disagreed. I trust they will accept a general reply concerning their various comments in place of individual treatment. In the case of axial flow the stuff flows through the jet unit axially from inlet to outlet. The charge at the inlet must have a certain ingoing velocity, and the change of momentum within the unit is the difference between inlet and exit velocities. If in flight the aircraft velocity reaches the efflux velocity there is no change in momentum within the unit, relative to the air- craft, no work is done in the unit and no thrust is generated. Intermediate velocities develop intermediate thrusts which vary in accordance with the respective velocities, as shown in Fig. 3, curve c. A body travels in a straight line unless it is acted upon by a force which changes its direction (Newton's First Law). This force has also its equal and opposite reaction (Newton's Third Law), and gas molecules are small bodies whose deflection imposes reaction upon the structure. If a constant mass of stuff is being deflected at a constant rate a constant reaction force is generated, and it is indepen- dent of the speed of flight. If the stuff passed through the unit at the efflux velocity,.and the aircraft attained efflux velocity, a constant reaction "force would still be generated. This is the important difference between axial jet propulsion and reflex reaction propulsion, particularly round about the 600 m.p.h. mark. The ram effect would increase the density of the air at the compressor inlets. If the deflex propulsion failed to act in this way the turbine, which is operated by deflected stuff, could not operate effec- tively at high flight velocities. Axial change in momentum and deflection can operate simultaneously in the same unit and both contribute thrust (Newton's Second Law). The next question that arises is : can deflection be so arranged as to provide useful forwardly acting thrust ? This is answered in Fig. 5, which is admittedly one particular development; it is not the only possible development, nor perhaps the most efficient for every requirement. Other circumstances may require other treatments. The gas friction, eddy losses and turbulence can be reduced by scientific design, and "the proof of the pudding is in the eating thereof." The rocket referred to is, I suppose, the Oberth, a number of which were built and tested in Germany. They worked well, but nothing much seems to be known about them. The writer has given these rockets some thought and investigation, and may give some results of the investigations later, if they should be of sufficient interest. F. UMPLEBY.
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