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Aviation History
1955
1955 - 0558.PDF
558-559 FLIGHT, 29 April 1955 ROTATING 'JET WITHOUT SILENCER Fig. 13a Noise level on Fairey pressure jet. 2 3 4 5 6 7 8 OCTAVE 6AND ; SILENCING JET HELICOPTERS Fig. 13b Noise levels from static and rotating pres- sure jets at comparable powers and at a distance of about 6ft from nearest point in jet periphery. •.-, FREQUENCY BAND examination of the effect of varying fuel/air ratios in the com-bustion chamber and the effect of the whole range of silencing devices (Figs. 14 and 15). There was little purpose in referringto the former tests in detail, since the results showed that for a constant combustion-chamber pressure the overall noise levelsremained substantially constant over a wide range of fuel/air ratios. These results had to be treated with some reserve, how-ever, as any variations in fuel /air ratio made a large difference to the temperature, and thus to the density of the mixture. The results obtained with the various silencing devices attachedto the jet nozzles (Fig. 16) showed that the greatest reduction in overall noise had been achieved with the cross-tail silencer—though only with a thrust loss of 12 per cent, which was unaccept- able for practical purposes. Another device which showedpromise for the future was the corrugated nozzle (Fig. 17) which had been brought to a highly developed state by Dr. F. B.Greatrex of RoDs-Royce. . : The lecturer concluded by describing a new form of pressure-or exhaust-jet nozzle (Fig. 18), investigated at Southampton University, and from which the gas was discharged through anannular outlet. Some tests had been made at the University with such a device, using cold air haying little initial turbulence andno combustion noise. It was anticipated that the thrust loss with the annular jet would be small and the noise reduction achievedin the preliminary tests was most encouraging (Fig. 19). A logical development from this would be a two-dimensional jet(Fig. 20). In this last-mentioned type, with a thin corrugated outlet, thenoise pressure levels should be well down to the threshold of the basic rotor noise. Considerable performance advantages mightalso be obtained if this type of jet were deflected down through an angle of, say, 40 to 50 degrees. It might even be possible, withthe resultant increase of lift on the retreating blade, to do away with the necessity for cyclic- and collective-pitch change. Muchwork remained to be done, but the low-velocity exhaust jet should be regarded with favour as far as noise was concerned, and tip-jetproblems in general were not insurmountable. c oTzr Fig. 14 Silencing devicesfor combustion chamber -.-_.,.•''"'.(as tested in third rangeof tests on the Fairey Company's static rig):— PUIM UM|T-1, Jet unit without silencer; 2, tooth silencer, ViinX'/i'nteeth; 3, flute silencer No. 1 ; 4, gauze silencer; 5, ductedgome silencer; 6, fishtail silencer; 7, crosstail silencer;8, fish-cone silencer; 9, star- cone silencer; 10, flutesilencer No. 2. OVERALL NOISE 100 110 120 TOOTHED NOZZLE- SHORT FLUTED N0Z2LE TZT WUZE AW DUCT- N* 1 CROSSTAIL NOZZLE N» 2 CMSSTAIL NOZZLE Fig. 76 Overall sound levels with various silencers. Distance 200tt (o=45 deg to axis; • = 22 deg to axis). Thrust ~ • loss (per cent):— STAR CONE — Plain nozzle, 0; fluted nozzle,2.8; toothed nozzle, 2.5; plain nozzle, 0; gauze and duct,, 3; crosstoil No. 1, 12; LOW FLUTED" star cone, 5; plain gauze, 3. NOZZLEo Ftg. 15 iarly Fairey devices tested. Fig. 17 (right) Rolls-Royce corrugated nozzle. 44**
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