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Aviation History
1953
1953 - 1358.PDF
512 FLIGHT CORRESPONDENCE The Editor of "Flight" does not hold himself respqnsible for the views expressed by correspondents in these columns; the names and addresses of the writers, not necessarily for publication, must in all cases accompany letters. Puffs and Bangs I T was with considerable shock that I read Mr. Warren's review (September 11th) of the controversy on supersonic bangs. There is a point of view which requires that in any legal action the jury and not the litigants produce the verdict. Would it be impertinent of me to suggest that this principle be adopted here? The uncompromising attitude that Mr. Warren is correct and all others "wildly" false cannot go unchallenged. The explana tion I put forward last year, that the passage through Mach 1 caused the audible bangs, was suggested by the fact that only at this Mach number does the disturbance remain localized, in con trast with purely supersonic conditions when the shock-wave propagates sideways from its source. This seemed to explain why the pilot needs to aim the bang with accuracy and, incidentally, can explain the localized puffs of vapour and of vapour trails which have been observed. Anyone qualified to judge Mr. Warren's toy-balloon explana tion will readily observe a flaw in his argument. (See Flight, October 17th, 1952.) The flaw is not immediately obvious in the treatment he gives, but an exact two-dimensional treatment— as opposed to his approximate one—or a three-dimensional dis cussion shows that it is difficult to explain reinforcements of disturbance at a point due to straight-line flight except at Mach 1. I therefore challenge Mr. Warren to produce a simple two- dimensional explanation of this reinforcement, using the principle of simple superposition of disturbance for the case of an aircraft travelling along a straight path at any Mach number greater than one. Kingston-upon-Thames. B. A. HUNN. F T C. H. E. Warren's useful summing-up (p. 382, September 11th) of the state of knowledge on "bangs" he mentions one unsolved mystery—that of "puffs" in the sky during the high speed dive. At Farnborough I heard the commentator call attention to puffs at a time when an aeroplane was diving towards us at about M=1.. It had been making a vapour trail during its climb and level approach, but during its dive the trail became intermittent, and one detached length of trail was described by the commentator as a puff, because he was looking at it almost end-on. I suggest that the puffs are short lengths of vapour trail (contrail) looked at end-on, and that these are not necessarily associated with super sonic flight at all. The local pressures round an aeroplane differ from that else where at the same height by amounts depending on the position of die point in question relative to the aeroplane and its speed and attitude. If the local pressure falls, the air is cooled; and, if the dewpoint is reached, the water vapour in the air condenses to form cloud, or, if the air is cold enough, ice crystals. During high-speed pull-outs the pressure in the wing-tip vortices becomes very low and wing-tip trails are often seen even at low altitudes. The water droplets quickly evaporate as the tip vortex-strength is dissipated. If the air is humid enough the low pressures over the wings and round the fuselage produced by a fast level run are enough to cause a temporary cloud to form, and at recent displays fast aeroplanes have been accompanied by a flickering cloud which continuously forms at the front of the aeroplane and evaporates an instant later at the back. When these effects occur at high altitude they tend to be more permanent. Instead of droplets of water the vapour now turns directly into ice crystals. These act as nuclei to which other molecules of water are attracted, and the trail may remain for a considerable time—sometimes growing to become a long sheet of cloud. At other times the pressure is not low enough, but the extra water vapour from the aeroplane exhaust gases is enough to saturate the air locally. The type of water-vapour effect depends on the amount of vapour already present in the air. This is far from uniform in the atmosphere and varies especially with height. A diving aeroplane passing through layers having a varying water-vapour content will thus often leave a broken trail—perhaps only a few short lengths. When the dive is towards the observer he will see the sudden appearance of puffs of cloud, which may persist or rapidly evaporate according to the local humidity. Thanks to these vapour trails at Farnborough at this year's S.B.A.C. show we knew when to listen for the bangs. The bangs from Bedford's Hunter on the Tuesday and Wednes day numbered three from each supersonic dive—six when he exceeded M = l twice. The first two (the bow and tail waves from the wing) appeared loudest to me and were about one-fifth of a second apart, while the quieter rear wave, shed at M=l, followed about one second later. These three accord well with the theories summed up in your issues of June 5th, 1953, and by Mr. Warren on September 11th last. The 1953 novelty was the six bangs from the Hunter double dive. What is being thought up for 1954? I hope some suitable reward will be given to the first pilot to burst the marquee. Cranfield. A. H. YATES. Reaction Propulsion Research T HE method developed by O.N.E.R.A. "for studying the flow of liquids in rocket motors" ("Parisian Round-Up," p. 83, Flight, July 17th, 1953) appears not unlike that used, over five years ago, at New York University, for studying the combustion processes of pulsejet engines. Models, similar to the famous "Dynajet" (produced by the Aeromarine Company, Dayton Municipal Airport, Vandalia, Ohio, as power units for model aircraft, cars, and hydroplanes), with normal metal intake, fuel inlet and valves, had a Pyrex com bustion chamber and tail-pipe. From when, a few seconds after starting, the engine was operating "smoothly," until the Pyrex tube, under the combined effects of heat, pressure and resonance generated by the intermittent explosions, finally fractured (the average "life" was betwen 20 and 30 sec), several hundred com plete combustion cycles were recorded by a camera taking photo graphs at 2,000 frames per second. These films, in addition to providing other data, clearly showed how air is sucked in through the tail-pipe after each explosion, which fact often surprises "the layman." Cambridge. PEDANTICA. World Speed Records ET us bury deep in the ground once and for all two false -*—' claims for world speed records frequently claimed for Britain by writers and advertisers. The first claim is for 226.75 m.p.h. by Henri Biard in a Supermarine S.4 (Napier) in 1925. According to the F.A.I. list the record was set at 278.50 in 1924 by Adjutant Bonnet of France in a Bernard. Then a claim is made for an S.5 (Napier) flown by D'Arcy Grieg in 1928 at 319.57 m.p.h. The latter has more substance than the former, for it beat the 318.64 m.p.h. by di Bernardi in a Macchi for Italy; but as it did not beat the previous figure by the necessary 5 m.p.h. it was not confirmed. Let us also bury that awful F.A.I, word "homologated" and in future say "confirmed." London, S.W.3. GEOFFREY DORMAN. [In our Supermarine historical article last week—which, incidentally, Mr. Dorman had not seen when he wrote the above letter—we quoted the S.5's 319.57 m.p.h. as a British record. We were, however, in error in repeating the Supermarine claim to a world record with the S.4 in 1925; research suggests that our correspondent's correction is valid.—ED.] FORTHCOMING EVENTS Oct. 9. Airlines Ball, Royal Albert Hall, London. Oct. 13. R.Ae.S. Section Lecture: "Use of Materials in the Plastics Range," by H. L. Cox, M.A., F.R.Ae.S. Oct 20. R.Ae.S. Graduates'and Students'Section: "An Introduction to Gas Turbines," by D. H. Mallinson. Oct. 23. Aerauto Association: Annual Dinner and Dance, Westminster Arms, Page Street, London, S.W.1. Oct. 23. R.Ae.S. (Birmingham): "Helicopters." Oct. 28-30. Institute of Welding: Annual Dinner and London Autumn Meeting. Oct. 29. R.Ae.S. Main Lecture: "A Quantitative Study of Instrument Approach," by J. F. W. Mercer, B.Sc, A.C.G.I. Oct. 30. R.Ae.S. Graduates' and Students' Section: Informal Reception and Dance. Nov. 3. R.Ae.S. Section Lecture: "Development of 4,000 lb Hydraulic Systems," by G. Orloff. Nov, 5. R.Ae.S. Graduates' and Students' Section; "Exploration of Space," by Arthur C. Clarke, B.Sc, F.R.A.S. Nov. 6. Helicopter Association: Discussion: "The Independent Airline Operator and the Helicopter." Nov. 6-11. "Dates" Air Rally, Biskra, Algeria. Nov. 10. R.Ae S. Section Lecture: "Gusts and their Measurement," by J. Taylor, M.A.. A.F.R.Ae.S. Nov. 12. R.Ae.S. Main Lecture: "Stability and Control in Aircraft Design," by J. C. Wimpenny. Nov. 19. R.Ae.S. Section Lecture: "Aircraft Stability and Control," by A. W. Babister, M.A., A.F.R.Ae.S. Nov. 26. R.Ae.S. Graduates' and Students' Section: "The Case for the Light Fighter," by P. A. Norman. Dec. 1. R.Ae.S. Section Lecture: "Integral Construction," by H. G. Legg. Dec. 4. Helicopter Association: "Ground Resonance," by R. Howarth
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