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Aviation History
1955
1955 - 0126.PDF
126 FLIGHT SILENCING A WS-55 The Vokes silencer is shown clearly in this take-off view. The ground observer (right) is operating a Dawe sound-meter. A N effective method of muffling the sound of/^L piston-engined helicopters was demon- •*- •*• strated last Monday at the South Bank sitein London. The demonstration took the form of several flights by a Westland S-55 Whirlwindfitted with an external silencer designed by Vokes, Ltd., and developed by Westland under M.o.S. contract. The silencer was produced "hurriedly" in order to determinethe value of this means of quietening helicopters; design and installation took only ten days to complete. Nevertheless, obser-vers on the South Bank agreed that the noise level was greatly reduced. With the S-55 cruising overhead at only 500ft, thepurr of its Pratt and Whitney Wasp (600 h.p.) seemed scarcely audible above the surrounding traffic sounds—an impression con-firmed by measurements taken on the spot with a Dawe sound- level meter, which recorded the noise-level of a steam train passingover nearby Hungerford Bridge as 84 db, compared with 82-83 db from the helicopter. When the WS-55 touched down some 50 yd away, the metershowed 104 db, which is comparable with the level of interior noise experienced in an underground train. In offices (including The audience at the South Bank demonstration included Sir Edward Boyle and Mr. John Profumo, Joint Parliamentary Secretaries to the M.o.S. and the M.T.C.A. respect/ve/y. Shown here are (left to right) Sir Edward Boyle, Mr. G. H. Vokes, Mr. Profumo and Mr. C. G. Vokes. SWEDEN'S TEN-YEAR PLAN THE defences of Sweden are to be modernized under a ten-yearplan, according to an announcement by the Supreme Com- mand, and guided missiles will play a key role. They will be usedby all three Services and some have already been tested by the Army, which has lately reduced its number of field battalions by12 per cent. Missiles, too, will form the main armament of the destroyers Smaaland and Holland, now under construction atGothenburg, and will, of course, be carried by the Saab Lansen attack aircraft. In the anti-aircraft role—operated by the AirForce—missiles will also figure prominently, and one type now under development should have a range of up to 150 miles. PLANNING IN HINDUSTAN THE Hindustan Aircraft Company at Bangalore, which is ownedby the Indian Government, is planning to build aircraft not only for all the requirements of India, but for those of neighbour- ing countries, including Ceylon. This has been announced in Colombo by the sales executive of the company, Mr. H. S. "Flight" photographs those of Flight) immediately adjacent to the line of flight andwell above the street the noise was practically inaudible. Detailed measurements of the degree of sound-reduction effectedhave been made by the National Physical Laboratory, and though full figures are not yet available, it has been stated that the averagereduction is of the order of 10 db. Resembling a large exhaust system of car or motor-cycle type,the pipe and silencer lead aft from the normal stub terminating just beyond the port-side rear window. Exhaust gases flow straightthrough the system, without baffles and a minimum dia- meter of 5£in; thus, there is virtually no back-pressure (the figureis about lfin of water) to cause loss of power. Mr. G. H. Vokes, assistant chief designer to the firm whichproduced the silencer, explained that the expanded portion of the pipe incorporates two Helmholtz resonators, or acoustic chambers,tuned to a frequency of 169 c.p.s. (which corresponds to the mean firing frequency of the engine at the most noisy conditions); twosmaller chambers to suppress the sound at medium frequencies; and lagging of the outlet pipe to deal with high frequencies. Thesystem is not applicable to turbines. The weight of the Vokes silencer is quoted as approximately60 lb, and of the provisional installation as a whole about 85 lb. It is understood that a silencing system for an S-55 tried out inAmerica early last year also weighed approximately 85 lb. A careful examination is now to be made by all concerned—manu-facturers, operators and Ministries of Supply and Civil Aviation —to discover the best ways of applying the knowledge now beinggained and of designing a really practical silencing system. For example, it has already been suggested that twin silencers, fittedin conjunction with the floats and involving only short delivery- pipes, might offer advantages. If the system is to be adopted on the two WS-55s recentlypurchased by B.E.A. for their proposed shuttle service between the South Bank and London Airport it will be necessary to reducethe weight to the absolute minimum. The weight of amphibious gear already to be carried by these helicopters will limit theirseating capacity to five passengers (instead of a possible eight), and the Corporation will be understandably anxious to avoid furtherloss of payload. At the same time, one of the purposes of the early services is to establish public confidence in the helicopter,and a measure of noise reduction should help to dispel the antagonism already evoked in some quarters by the comparativelymodest number of flights made from the South Bank last summer. Kaviratne. Designs are in existence for a basic jet trainer—theHJT-16—which is said to resemble the Morane Saulnier Fleuret, and for an advanced jet fighter known as the HJT-17. A "turbo-propelled" (presumably turboprop) transport, designated HP-18 and intended as a Dakota replacement, is also foreseen.Piston-engined trainers of the HT-2 type, entirely designed and manufactured by the Hindustan Aircraft Factory, took partin the recent display at Ratmalana Airport, Colombo. AIR LEAGUE'S NEW CHAIRMAN IT was announced this week that Air Chief Marshal Sir GeorgePirie, K.C.B., K.B.E., M.C., D.F.C., LL.D., has been elected chairman of the council of the Air League of the British Empire.He succeeds Air Chief Marshal Sir Guy Garrod, G.B.E., K.C.B., M.C., D.F.C., LL.D., F.R.Ae.S., who remains a member of thecouncil. Sir George Pirie retired from the Service in 1951. His last appointment was as Head of the Air Force Staff, British JointServices Mission, Washington. He had also been C-in-C. Far East Air Forces and Air Member for Supply and Organization.
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