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Aviation History
1944
1944 - 0620.PDF
320 FLIGHT MARCH 23RD, 1944 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 FOR AIRCRAFT Suggested Modifications CONGRATULATE Mr. Geoffrey Smith on his clear R" (2) I exposition of the turbine-operated jet (Flight, Feb. 17th), and the combination with it of the cqntra-prop. which seems to promise interesting possibilities for the future when ton-miles per gallon becomes more important than enormous h.p. per lb. Mr. Max Millar's interesting drawing on page 171 of the com bination is most helpful in visualising possible developments, but the design is open to objections, which I will try to formulate. Having settled how the jet does its propelling, we are now entering the domain of thermodynamics, where the laws are as immutable to the engineer as those of Newton. First of all we have Carnot's well-known expression for the thermal efficiency of an ideal heat-engine, viz. ; T — T ~TT~=V (I) Next to be considered is the connection between temperature and pressure in adiabatic compression or expansion, viz. : ?» uv Combining (1) and (2), we have 71" * ~ (iTj*-» (3) This slunv.i the importance of the compression ratio, but there is a limit on Tj imposed by the materials of the turbine-blades, say, TL. If, in an ideal adiabatic compressor and turbine, K is such that TL is reached,, then the set will not give power because it is impossible to add fuel heat. It is futile in this case to cool the compressed air after compression, as shown by Mr. Millar, because the fuel would only replace the heat thrown away, and the final result would be the same as before. A lower R would reduce the possible efficiency, and the aim should be to have the highest possible R. True isothermal compression would solve the difficulty and could nearly be attained by cooling the diaphragms and guide blades between each stage of compression ; nevertheless, the possible thermal efficiency of the turbine is limited by TL. This temperature is reached by using less than the total oxygen content of the air, which is wasteful. By using all the oxygen, with fuel oil, a temperature in the neighbourhood of 5500° K is reached, but that is too high even for most refractories. I suggest that Mr. Geoffrey Smith's design be amended in the following manner :— (a) That the air intake be enlarged so that the coutra-prop forms the first stage of the compressor during take-off and cruising, but is finally stopped and feathered at abnormal speeds. (b) That the compression be in two isothermal stages, the first stage to an R which is suitable to the turbine, the exhaust from which forms one jet, and the second stage to a higher R, the air from which is taken to combustion cjiambers of refractory material, and the products of combustion expanded in de Laval jets. The jets, R, and fuel consumption should be so balanced, if possible, that the eject is at the lowest attainable Ts, and that I\ should be practically that of the atmosphere. (c) That the jet outlet should be an expanding one so that the slip stream velocity should be as low as possible compatible with the mass of air and desired thrust. I still object to high velocity jets ! It would bo interesting to have particulars of the blower nn-litionrd by Mr. G. W. Stanley (Flight. March 2nd), without which his figure of g per cent, of the power generated seems incredible. ["PROJET."] THE FLYING BOAT A Jet-driven Plastic Flying Wing M R. C. A. N. POLLITT has raised a very crucial point in his article in Flight of Feb. 24th, i.e., "First of all . . . persistent claims of the flying boat having a readv-made land ing surlace may prove misleading . . " The truth is that the development of aircraft is making them more and moie absolutely dependent on prepared bases, whether they be solid or liquid. In war, of course, the bases must also be held and supplied by the surface forces, but in peace it is only necessary to prepare the surface and provide terminal facilities. Now, half this job can be said to be re.ady-made in the case of flying-boat bases on lakes and rivers, but let us altogether discount this.advantage for forced landings, since only one quarter of the globe provides water. Let us say " Given bases, how does the aircraft best evolve— landplane or flying boat? " I will describe extremely briefly such evolution along three channels (a) materials and stressing, (b) propulsion and take-off, (c) performance and control. In the first place, there is a sound practical reason for plastic materials ousting metals, other things being equal, so that the stressing of a huge aircraft, efficiently and lightly designed, may be done by photo-elastic analysis of a model, also made of a plastic material with similar elastic peculiarities: Conse quently the objection of the corrosion of metals by sea water can be replaced by the objection to holes for retracting wheels. Secondly, the advent of jet propulsion, while removing the flying boat's headache oi airscrew clearance, introduces the problem of where to blow out the jet. Instead of striving to keep such a hot and erroding blast away from the ground during take-off, it would be quite desirable to blow out as near as possible to a water surface. Finally, the now generally accepted "flying wing" idea1 can be used practically without modification as a flying boat, as explained by Dr. Roxbee Cox in the Royal Aeronautical Society Journal of July, 1938. The C.L.W. Curlew, described in Flight of Sept. 17th, rqjG, had quite remarkable aileron control near the stall, and this was ascribed to a peculiarity in the wing shape, i.e., elliptical in plan and straight tapered in elevation (Prov. Pot. No. 24623/36) from N.Ae.A. 242r to 2418 aerofoil sections, using neither wash-out nor wash-in. The result was that the relatively thinnest section occurred near the inboard end oi the, aileron (approx. N.Ae.2412), and a study of the lift curves will show that the outboard portion of such a wing might stall last. Similarly an increase in root chord in plan only, i.e., a drastic reversal of the elliptical curves of leading and trailing edges for about one-third span, might have an effect similar to increased aspect ratio—always keeping within the limits of peculiarity of the chosen aerofoil sections. If now our "flying wing" is of this rather unusual shape, it will be seen, if anyone cares to sketch it out, that we have a distinct pull formation which Dr. Roxbee Cox might prefer to his own suggestion in the paper referred to above. R. C. ABEL. THE WORLD'S BEST AIRCRAFT Why% Masefield's Sudden Change of Front ? T HE other day, while looking through some old papers, I came across a copy of the Sunday Times of August 16th, 1942. In this appeared an article by Mr. Masefield in which he displayed little enthusiasm for American bombers. In fact, he wrote: " Much as we need bombers over the Atlantic, first- rate heavy boml>ers are needed over Germany in the greatest numbers possible. American production of heavy bombers is concentrated on the inadequate Fortress and Liberator." He continues: "If one may venture an honest—and constructive-^ criticism from this country, it is that America's greatest duction need would seem to be to turn over her vast potential to new and improved types of heavy bombers without delay. She owes it to her men and to the Allies' cause." "SIMPLETON." Accent on Anglo-American Superiority 1 WAS very pleased indeed to read in Flight of February 24th the article by F. Lloyd and R. L. Lickley on " The World's Best Aircraft," in answer to Mr. P. Masefield's article on the same subject. I do not profess to know anything about aircraft which are •still on the secret list, but, taking fighters and bombers only, I should name four aircraft as being the most outstanding aircraft in this war, so far. They are the Spitfire, Mosquito, Fortress and Lancaster. First, the Spitfire, because it started this war as. a first-line fighter, and Mark IX is definitely a first-line fighter now, and 1 am sure at the end of this war future Marks of the " Spit " will be ranking alongside other first-line fighters. The Mosquito, because of its unusual construction, its fire power, speed and versatility, lam sure it has no equal, j The Fortress, because of its tiemendous fire power and ability to defend itself, makes it the ideal day bomber. The Lancaster, became of its anility to carry a henvv load
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