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Aviation History
1964
1964 - 0454.PDF
FUGHT International, 20 February 1964 EXPENSIVE NOISE . . . 277 steam produced must be exhausted into the atmosphere, the noise is just like that of a launch from Cape Canaveral. Fortunately, however, most of the noise is radiated upwards. Reducing Jet Noise Because the noise energy radiated varies so critically with jet velocity, the obvious way to reduce noise is to reduce velocity. This means that the engine airflow must be in- creased, and with it the size and weight of the engine. However, the efficiency of a turbine engine (though not a rocket, which carries its own oxidant as well as fuel) is increased in this way, to that there may be no overall penalty. These factors have been considered in many recent designs and have led to engines of increasing by-pass ratio such as the Rolls-Royce Spey. Alterations which involve less of a weight penalty, but do cause a loss of efficiency, are to flute the edges of the exhaust nozzle so as to reduce the shear and mixing at the outlet; and the use of an array of small nozzles which tend to shield the noise from one another. The exact flight course chosen by the pilot also has an effect, but before dealing with this I had better say something about how noise is measured. Noise Intensity The noise intensity at a point is the rate of flow of energy per unit area there. Obviously the local intensity gets smaller as one goes further away from a source, because the area through which the radiation is passing increases with distance. A simple source, emitting a sound of one Watt, would produce an intensity of 10-4 Watt/cm8 at one-foot distance; this would tend to be painful to the ears. At 100 yards the intensity would be 10-9 Watt/ cm2, which is a cocktail party in its early stages. At 20 miles it would be 10^14 Watt/cm2, which is about the limit of hearing for most people. These figures show that the ear can cope with an astonishing range of intensities, so we use a logarithmic scale for its measurement. Zero is the quietest sound one could conceivably hear, and each step of ten decibels means an increase of intensity of ten times: thus 20db is 100 times the minimum, 30db is 1,000 times, and so on. This scale gives a few typical points:— db0 threshold of hearing at 1,000 c/s 20 studio for sound pictures 40 residence (no children) . . . 60 conversation ; • ' 80 heavy traffic- •- 100 underground train : • 120 close to pneumatic drill i140 gas turbine engine at 100ft (damage to ears) 160 rocket engine at 100ft (panting of stomach) The Wilson Committee on noise has laid down a noise level limit of HOdb in the neighbourhood of airports, and said that this figure must be decreased as the frequency of jet flights increases. Actually the HOdb limit does not quite compare with the table, as allowance has also to be made for subjective effects—if the noise contains high-frequency components, which are more annoying, the absolute level must be reduced. Flight", Procedures have now been worked out for pilots which enable the take-off noise level to be considerably reduced. As shown in Fig 17, the aircraft is accelerated at full throttle and the pilot takes off and starts to climb as soon, and as fast, as he can. With modern high-powered engines the aircraft has gained a thousand feet in altitude before it is over the first of the houses surrounding the airport. The pilot then throttles back, climbing as slowly as possible; this considerably reduces the noise received at the houses. Using these procedures, and with engines incorporating the quietening devices already mentioned, we believe that the take- off noise level will be no worse, and may well be better, with the new aircraft now coming along. There is, however, plenty of evidence to show that landing noise is becoming a more and more serious problem. At landing, the air- craft is aimed at the near end of the runway and descends at a much shallower angle than that at which it climbs away, in order to keep the rate of descent low. The powerful engines, even when throttled back, make a lot of noise, particularly at the objectionable higher frequencies. And I also think that the fact that the aircraft is still aPproaching when the noise reaches its crescendo is psychologically more alarming than the take-off case. TAKE-OFF ^THROTTLE BACK 1 LANDING JS) -AIRFIELD- -RESIDENTIAL- AREA Fig 17 Take-off and landing noise from airliners. Throttling back the initial climb reduces the noise effect in residential areas near the airport Compressor Noise By muffling first the front and then the back of the engine in turn, it can be shown that under landing con- ditions the approach noise comes from the compressor: the re- treating noise is louder than the jet itself could produce and must come from the turbine. In both cases the noise comprises a general "white" background, on which are superimposed a few discrete tones. The pitches of these tones correspond to the frequencies at which compressor and turbine blades pass fixed points. If you listen carefully to a tape record of a Boeing 707 landing you can pick out one particularly shrill whining tone from the general background. Detailed studies have shown that the noise is dipole in origin— that is, it must be due to fluctuating forces imposed on the air by solid boundaries. The discrete tones are almost certainly associated with the passage of each row of blades through the wakes of blades in a preceding row. Because of these wakes, the force on a blade fluctuates—just as a ship sailing in the lee of a series of vessels passing on the opposite tack would experience a steady fluctuation of force on its sails, and be rocked accordingly. Since the blades pass through the wakes at uniform time intervals, they act as synco- pated sources of sound, and their individual contributions combine to form spiral waves, which screw forward out of the mouth of the engine. There are various ways with which one can deal with these tones. One would be to remove the wakes by sucking the boundary layer of slow-moving air off the surface of the blades. Another would be to space the blades irregularly, as is occasionally done with fans in motor cars. We also have others under test at the moment. The general noise background is more difficult to account for and to deal with. We ourselves believe that it is due to the inevitable irregularities in the pattern of flow over the blades—something like the vortex-shedding from telephone wires that was discussed earlier, but much less regular. One idea that we are trying is to run the front stage of the compressor much more slowly than the others, hoping it will then make little noise itself but will still attenuate the noise from the stages behind it. Certainly the evidence suggests that at least half the noise comes from the first stage, noise from the others getting reflected, re-reflected and largely absorbed in- ternally. Any other ideas would be warmly welcomed. I am afraid that in some respects the subjects we have considered in this Discourse are something of a miscellany. At first sight they appear unrelated to each other, and each looks rather unfinished. One unifying factor, however, is that they are all problems that are still with us. If the subjects look unfinished, it is because the riddles are still unsolved, and serious difficulties are only kept at bay by development palliatives. The second unifying factor is that they all relate to unsteady flow. We seem to be reaching a stage in the development of jet engines where it is no longer worthwhile to spend huge sums in order to produce small improvements in steady- flow efficiencies which are already approaching 100 per cent. It is the unsteady problems—like those involved in starting, combustion oscillations, vibration and noise—which give us the really tough nuts to crack: such problems provide the blends of aerodynamics, combustion theory, materials technology and acoustics which con- tinue to make the jet engine such a fascinating study.
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