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Aviation History
1953
1953 - 1226.PDF
382 FLIGHT, H September 1953 SONIC BANGS A Year's Theories Reviewed SINCE the last S.B.A.C. Flying Display there has been much correspondence and several articles in Flight on the subject of sonic bangs. The "Brief Note on Bangs" (Flight, September 12th, 1952, p. 345) was the fore runner of a flood of explanations, and it may be as well now to review the year's contributions, and to take stock of the situation. Although it would appear that many explanations of the sonic bang have been put forward, the writer believes that the main contributors are, in fact, agreed on the essential cause of the phenomenon. Accordingly, it is time that the impression that there are many different theories was corrected. There are, of course, many ways of putting the explanation, simply because different contributors find different ways more convincing. In Flight alone we have had an explanation in terms of shock-waves, an explanation involving the analogy with surface waves on water, and an explanation involving a row of hypothetical balloons spread out along the flight-path of the aircraft. In all these cases the same fundamental explanation of the phenomenon is implicit. The explanation in terms of shock-waves (the second explanation by A. H. Yates, Flight, October 3rd, 1952, p. 442) gives the best physical answer as to what the bang is, without being terribly convincing as to the process whereby the shock-waves reach the observer from the aircraft. The explanation that draws on the analogy with surface waves on water (G. M. Lilley and others, Flight, June 5th, 1953, p. 712) probably enables the layman to see most clearly the manner in which the shock-waves from an accelerating, and then decelerating, aircraft spread out, and the general direction in which they travel. The explanation invoking a row of balloons (C. H. E. Warren, Flight, October 17th, 1952, p. 494) is convenient if one wishes to make a mathematical assess ment of the occurrence and intensity of the bangs generated by the (known) motion of a given aircraft. Each of the above explanations, therefore, caters for a different type of reader. Whilst the above explanations agree on the fundamental cause of the sonic bang, mere are various minor respects in which they are not quite accurate. For example, the presence of capillary waves due to surface-tension effects implies that the surface wave analogy cannot be expected to exhibit precisely the same features as would be found with compression waves in air. Likewise the predictions of mathematical linear theory must be accepted with some reserve, to allow for the effect of the assumptions that are made in developing the theory. The three explanations do, how ever, agree on the essential cause of the phenomenon. The Correct Explanation Sonic bangs are due to the shock-waves that are created when an aircraft flies at and above the speed of sound. This was pointed out in the original "Brief Note" on September 12th, 1952, and was further elaborated by Yates in his second explana tion. Briefly, when an aircraft accelerates at high subsonic speeds shock-waves begin to appear at some Mach number beyond the so-called critical Mach number, and on accelerating through the speed of sound to supersonic speeds shock-waves are left behind by the aircraft, as was demonstrated by Lilley in his hydraulic tank. When the process is reversed, and the aircraft decelerates back to subsonic speeds, shock-waves move ahead of the aircraft. The shock-wave pattern about an aircraft, particularly when it is accelerating or decelerating, is exceedingly complex, but a few simple facts concerning it can be stated as follows: Shock-waves are liable to arise whenever the relative motion is supersonic, for in such cases the air ahead receives no warning of the approach of the body, and does not have time to adjust itself to give a smooth rise in pressure as the body passes. Instead, the pressure rises very, very rapidly, almost discontinuously, through what is known as a shock-wave. And noting that it is relative motion that matters, we see that such waves can occur before the speed of the aircraft itself is supersonic, owing to the fact that the velocity of the air over the surfaces of the aircraft is greater than the forward speed of the aircraft. Once shock-waves have formed, they move normal to their fronts at speeds only slightly greater than that of sound. For examole, a shock-wave corresponding to a typical sonic bang (1 lb/ft2, or 128 decibels) travels at only 1.0002 times the "speed of sound." Even when it left the aircraft, when the pressure ratio across it might be 1.1 or thereabouts, the velocity of the shock- wave normal to its front would be only 1.05 times the speed of sound. One may ask why all the shock-waves are not left behind when the aircraft accelerates to supersonic speed: the reason is mat at By C. H. E. WARREN, M.A., A.F.R.AeS. THIS week's flying at the Farnborough Display has again brought interest in sonic bangs to the forefront. In this article Mr. C. H. E. Warren, who is a principal scientific officer at the R.A.E., examines the validity of some explanations put forv/ard during the past year. supersonic speeds the shock-waves are oblique, and they move in roughly the direction in which the component of the aircraft velocity is sonic. At supersonic speeds we have, in fact, shock- waves moving out laterally from the aircraft, and being con tinually created in the vicinity of the aircraft by its motion. Whether or not an observer will hear a bang will depend upon whether he is in the path of one of the shock-waves sent out by the aircraft. It follows fairly obviously from the foregoing remarks that an observer will hear a bang from every occasion on which the aircraft is approaching him at sonic speeds, for the portions of oblique shock-wave created on such occasions will move towards him. The number of bangs that he will hear will depend not only upon the number of occasions—during the dive, say—on which the aircraft is in the favourable (or unfavourable!) orienta tion just mentioned, but also upon the number of shock-waves sent out on each occasion, from the wing, tail, fuselage, etc. The precise number of bangs that he would hear is difficult to deter mine, owing to two facts : — (i) the human ear cannot distinguish two successive sounds of short duration, unless the time interval between them is greater than about 0.01 sec. And in attempting to determine whether two shock waves will arrive at a greater interval than this, one must make an exceedingly involved calculation allowing for the fact that the shock waves do not move at precisely the speed of sound, or, indeed, at the same speed as each other. (ii) it is possible for one shock-wave to overtake another, since they do not move at exactly the same speed, and for the two to coalesce. For these two reasons the author believes that it is almost impossible, in general, to predict precisely how many bangs may be expected from any given manoeuvre. All that one can say definitely is that the number of bangs that one will hear on any given occasion will depend upon where one is, and arguments between differently situated observers are pointless. Single, double, triple, and even quadruple bangs have been reported. The sonic bang is not a new phenomenon. Indeed, it has been known to ballisticians for years, for every bullet and shell causes a sonic bang in just the same way as an aircraft does as it flies through the air, the sonic bang being something quite distinct from the explosion that comes from the rifle or gun. Because of the smaller size, however, the sonic bang from a bullet or shell is much more of a crack than a boom, and, moreover, with a bullet or shell, the sonic bang is nearly always drowned by the accompanying explosion. During the V-2 attacks on London, there were reports of people hearing a "bang in the sky" as well as the "explosion on the ground." The former was, of course, a sonic bang created when the V-2 was travelling towards the respective observers at sonic speed. False Explanations Although the cause of sonic bangs from aircraft should have been well understood—and, indeed, they had been forecast as early as 1948 in Sutton's excellent Pelican book The Science of Flight (p. 147)—the S.B.A.C. Flying Display of 1952 brought the phenomenon into the everyday discussion of large numbers of people, and any number of wild explanations were put forward. It might be as well, therefore, to record some of these false explanations, in order to clear the air for those who are puzzled by the many and varied explanations that are still prevalent. One false explanation that received considerable support depended upon the fact that an aircraft is a source of sound- energy in that it houses one or more very noisy jet engines, the sonic bang being an accumulation of this sound-energy. The contributions invoking this explanation were started by A. H. Yates (his first explanation) and by D. R. H. Dickinson (Flight, October 3rd, 1952, p. 458). Although the effect that this explana tion puts forward does exist, the effect is far too small to explain die relatively large intensity of the sonic bang. In other words, if an aircraft could be propelled by silent means, we should still experience sonic bangs whenever it attained supsrsonic speeds. A further false explanation that has received considerable sup port is that the sonic bang is associated with "breaking through the sound barrier," or, more scientifically, with flight at Mach 1 (Concluded on page 390)
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