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Aviation History
1955
1955 - 0556.PDF
556-557 FLIGHT, 29 April 1955 SILENCING JET HELICOPTERS A Notable Paper by Professor E. J. Richards AS briefly recorded in Flight last week, a paper entitledNoise Problems in Helicopter Design was presented by"•Professor E. J. Richards, M.A., Professor of Aero- nautics, Southampton University, at the final 1954-55 meeting of the Helicopter Association, held on April 15th. Dr. G. S. Hislop was in the chair. The lecturer began by describing the work which had alreadybeen carried out on orthodox piston-engined helicopters. So far as internal noise was concerned, B.E.A. had stipulated theirrequirements for passenger comfort (Fig. 1) based on fixed-wing experience. A study of internal noise level measurements (Fig. 2)showed that in present helicopters, without engine-exhaust silencers, a reduction of some 20 decibels was needed. This couldbe achieved partly by soundproofing and partly by reducing external noise. To establish a criterion for the maximum acceptable externalnoise was not easy without operating experience in city centres. Dr. R. H. Bolt of the Massachusetts Institute of Technology hadput forward a series of curves (Figs. 3a and 3b) indicating the response of American communities to objectionable noises; thesecould be used as a guide. In this respect, some tests carried out at the Houses of Parliament with a Bristol 171 helicopter (Fig. 4)were interesting. Measurements of the overhaul sound level were made which represented approximately the octave band soundpressure levels of 150 c.p.s. It could be seen from the curves that die noise level in the Chamber was acceptable while that inthe committee room was not. For the purpose of standardiza- tion, it had been decided that, unless otherwise stated, noiselevels should be measured at a distance of 200ft from the source. A further series of tests, to determine the source of the greatestnoise, had been carried out by Dr. N. Fleming, of the National Physical Laboratory, on the Bristol 173 helicopter. To separateexhaust noise from rotor noise, tests had been made with a single rotor on the spinning tower. There had followed further testswith the helicopter suspended on a gantry, the blades being replaced by paddles. Later, continuous recording tests on theactual machine in flight had been made (Fig. 5). For com- parison with these figures for the 173 helicopter, the maximum recorded noise levels measured on the Bristol 171 were includedon another graph (Fig. 6) which also showed die effect of height and distance on the reduction of noise.An analysis had shown that the noise could be appreciably reduced by silencing the engine exhausts. At the distance of200ft, and for the power used in the 173 helicopter (i.e., two engines each developing 400 h.p.), spectra of noise pressure levelsas shown in Fig. 7 might be expected if silencers were fitted. It could be seen that even these required a marked reduction. Direct evidence of the efficacy of exhaust silencing was derivedfrom comparative tests carried out on the Westland WS-55 Whirlwind (Fig. 8), with and without the Vokes silencer fitted.From these tests it could be seen that a reduction of some 12 db had been achieved and it might be concluded that such silencerswould be fitted on future orthodox helicopters. Rotor blade and gearbox noise was of a much higher order than was generallyappreciated (Fig. 9). The curve for basic rotor noise (Fig. 7) showed that a reduction of some 20 db was required to bringdown the level below the point of serious interruption, but this would be most difficult to achieve. While it might, perhaps, befeasible to effect some improvement by the use of boundary-layer suction in the blade design, it was unlikely that designers wouldbe in a position to adopt such a principle for many years. In jet helicopters, the gearbox noise would, presumably, beeliminated, but there was little point in attempting to reduce the noise of the jets to a level below that of the threshold of bladenoise. The immediate problem, then, was to find a means of reducing the tip-jet noise to the level of the basic rotor noisecurve, a reduction of approximately 15-20 db. The source of the noise was turbulence at the jet outlet and combustion. Aero-dynamic noise from tip jets emanated in three ways: — (i) as a simple acoustic source on a pulse jet of frequency equalto that of die pulse and of a mean noise level given by Db = 10 logl0 & ^ (V,™ - Vmin)2 fV x 109 above 10-16 watts/cm2 where p0 is the ambient air density, c is die velocity of sound andV that of the jet, of exit area s. The frequency is f, and r is the distance from the source. - - COMFORT CRITERION IK) 100 to R.AE AIRCRAFT RECOMMENDATION 2 B.E.A. RECOMMENDATION 375 73 I JO 300 600 1200 7S 150 500 600 1200 2400 FREQUENCY 8AN0S(c.p ») Fig. 1 (left) Helicopter internal-noise recommendations. Fig. 2 (right) Measured noise levels inside helicopters. Bell 47 S-5S Alt. ft 750 4S0 Ens. r.p.m. 3,100 2.250 Boost in 19 27 I.A.S. kt 50 72 2M0 4400 MOO Fig. 3a Fig. 3a Relation between community re- sponse and composite noise rating. Fig. 3b Composite noise rating. Fig. 4 Helicopter noise in Houses of Par- liament; aircraft proceeding in up-river direction at 500ft. Fig. 3b PILOT'S POSITIOH BELL 47 57-5 75 75 ISO 150 100 300 600 FREQUENCY 1 600 I£OO BANDS* £00 2/100 2*00 4*00 Fig. 4 VIGOROUS LEGAL ACTION THREATS OF LEGAL ACTION STRONG COMPLAINTS MILD COMPLAINTS MILD ANNOYANCE NO ANNOYANCE AWFRACr rutrrTrn 1 RESPONSE 44yfty A? RANCE OF EXPECTED RESPONSES IN NORMAL COMMUNITIES __i 75 ™ 300150 600 rBrnlicurv 2400 1200 4W04JB00 2,400 9600 >«e 1O 2O 3O 4OWESTMINSTER LAMBETH
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