The Chairman said the subject was of paramount importance to all helicopter designers, manufacturers and operators The Association had not previously had a lecture which dealt specifically with the problem and the importance of the subject meant that such a lecture was long overdue They knew that, in spite of the claims made from time to time by sundry manufacturers, helicopters did vibrate, some to a very great extent Indeed, one eminent fixed wing designer thought that by virtue of this vibration helicopters were rather more grandiose fatigue testing machines than the conventional test equipment for the purpose

The Chairman said the Association were transferring their thoughts for the evening from the pure technicalities of the vehicle to the equally important problem of what facilities it required for operation, particularly for the inter-city transport role which had caught the imagination in the last two years, not only in this country but in the United States and elsewhere abroad

They had been very fortunate in that Mr Hough, the City Engineer and Surveyor of the City of Liverpool, would address them on the subject Liverpool was one of the cities in this country which had given a great deal of thought to the matter

During the past few years, it has become recognised that one of the major objections to the introduction of helicopters fitted with pressure jets at the blade tips is the high level of noise set up by the jets The source of this noise lies m the jet stream itself When air travelling at high speed mixes with the surrounding air, turbulence is formed in the region of high velocity shear and noise is emitted into the stationary or near stationary medium surrounding the jet The large velocity gradient or shear actually amplifies the sound (Ref 1) which increases as a very high power of the jet velocity, normally about the eighth power Needless to say, if turbulence exists initially in the jet stream, either from poor internal shaping of the engine or from incomplete burning of the fuel, the noise will be aggravated and a different noise law achieved With well designed jet umts, however, the eighth power law is obeyed surprisingly well

During the course of fundamental research into noise suppression on jet engines, the need for a jet system which embodies quick mixing has become apparent, one way of doing this is by the use of a long thin peripheral slot in place of the usual circular nozzle Indeed noise measurements on such a jet unit indicate a reduction such as to make the use of pressure jets on helicopters once more feasible from the noise point of view