Aircraft Engineering THE MONTHLY ORGAN OF THE AERONAUTICAL ENGINEERING PROFESSION VOL XXVI No 299 JANUARY 1954 other words they are chiefly looking at the matter from the view point of those living in the immediate neighbourhood of aerodromes HE nuisance caused by the noise produced by aircraft is a or factories; while we feel that the wider question of those matter which has been arousing antagonism now for some who are merely annoyed by the passage of aeroplanes overhead, Tforty years and there have been repeated demands that 'some on a busy route or below an area where they are 'stacked' to await thing should be done about it'. their turn to land, is not one that can be ignored. This is not easy, for the reason, apart from others, that it has never In any case the survey of the experimental work that has been been decided what are the precise causes of the variety of noises going on in various places to discover the root causes of the emission that give offence—let alone a cure found for them. It was, for in of noise by jets and its possible reduction that appears in this issue stance, pointed out twenty or thirty years ago that even if all the by M R ALAN POWELL is of the greatest interest and importance. banging and clattering of the innumerable moving parts of a piston Incidentally, M R POWELL is working at the University of Southamp engine were eliminated, or at any rate reduced, there would still be ton which by long-established tradition has interested itself in this the noise caused by the passage of the propeller blades through the subject, since WING-COMMANDER T. R. CAVE-BROWNE-CAVE, Pro air; which, in fact, there were those who maintained was the principal fessor of Engineering there, instituted investigations on the subject cause of annoyance. some twenty-five or thirty years ago. Idiosyncrasies Rapid Progress Apart from anything else, we are not aware that anyone has It is evident from the list of references given by the author that satisfactorily explained why it is that certain types of aeroplane at jet noise is quite a new subject of research, the bulk of the papers certain speeds—or when heard from the ground from certain angles— mentioned having been published within the last couple of years. It produce a quite distinctive noise compared with other types of seems to us that in view of the shortness of this time quite remarkable apparently similar characteristics fitted with the same engines. The progress has been made and the phenomenon is already well under outstanding example of this, so familiar during the last war, was stood. The noise does not originate, as perhaps might have been the peculiar whistle emitted by the Spitfire. Though to the normal supposed, sooner than behind the outlet of the jet pipe and is due to ear this was exceptionally individual there was no doubt that ex the turbulence arising from the contact of the efflux of gases with the perienced observers could identify many of the principal types of surrounding air. There are two types of noise—high frequency and both British and German aeroplanes from sound alone even when low frequency—whose point of origin is different. In the case of the they were invisible'. Two notable examples of the present day we former it is comparatively close to the orifice while the lower fre would say from our own experience are the Comet which when quency noise arises further downstream. Similarly, the maximum passing in flight at certain speeds, distances or angles—we are un intensity of the high frequency noise is at fairly large, and of low able to determine which—emits a note which is to our ears strangely frequency at smaller, angles to the jet axis. One cause of confusion reminiscent of the Spitfire; while the Viscount—a turbo-prop be it that is mentioned is that as the observer goes farther away from the noted—also at times produces for the listener on the ground a sound jet the apparent direction of maximum intensity will 'swing around' which, again, can only be described as a 'whistle'. To take another and may give rise to the illusion that the sound is emanating from a example, does anyone at Bristol know why the Britannia is so point other than its known origin. A semi-empirical rule has been notably silent compared with its contemporaries—or, for that evolved that the noise largely depends on the jet velocity to the matter, can Manchester tell us why the Vulcan seemed quieter than eighth power and the square of the je t diameter. This gives a rough other 'jets' when passing overhead at Farnborough? estimate of the noise level to be expected. Viewpoints Guarded Optimism We hope we are not being too difficult in discussing this aspect A hopeful feature is that there are indications that some quite of the matter when so many minds are working on the problem of the cause of the emission of noise as a more basic matter. Another simple specifics may have considerable effect in reducing the noise. aspect of our approach which may be noted is that we have been Apart from the obvious remedy of reducing the jet velocity, if this referring to the noise of aeroplanes passing overhead more or less be possible, such slight modifications as the forming of castellations on their lawful occasions, whereas those professionally interested— at the orifice of subsonic jets, or flaring the exit or adding a gauze cylinder as an extension of the nozzle in the case of choked jets so to speak—deal with the problem as mainly one of quietening engines when being tested, run up on the ground, or at take-off. In have been proved to reduce the noise.
Aircraft Engineering and Aerospace Technology – Emerald Publishing
Published: Jan 1, 1954