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Aviation History
1951
1951 - 0214.PDF
•138 FLIGHT, 1 February 1951 VIBRATIONS in HELICOPTERS A Symposium at the Royal Aeronautical Society FOUR PAPERS were read at a symposium on " The practical Approach to Vibrations in Helicopters" Arranged by the Helicopter Association of Great Britain and held in the library of the Royal Aeronautical Society last Saturday, January 21th. The chairman, Mr. N. E. Rowe, C.B.E., D.I.C., B.Sc, A.C.G.I., F.R.Ae.S., opened the meeting, which comprised four short papers, twelve six-minute contributions, and an open discussion period. The papers summarized herewith are : Operational Aspects of Helicopter Vibration, by 4. McClements, A.R.T.C., A.M.I.Mech.E.; A Survey on the Practical Approach to Characteristic Vibration Sources in Helicopters, by J. S. Shapiro, Dipl. Ing., A.F.R.Ae.S. ; Measurement of Vibrations, by Dr. G. E. Bennett, B.Sc, Ph.D., F.Inst. P.; Airworthiness and the Presence of Vibrations, by R. H. Warde, A.M.I.Mech.E. • OPERATIONAL ASPECTS INTRODUCING his lecture, Mr. McClements (Ministry ofSupply) said: "The helicopter as we know it is a goodexample of a potential vibration-producing machine. It has in it the means for providing vibratory frequencies similar tothose which we meet in fixed-wing aircraft and, in addition, it can throw up periodic disturbances having much lower frequen-cies. With this in mind it will be easy to appreciate that the vibrations produced in a helicopter can be a major source ofworry to the operator unless they are carefully kept under control. " To the helicopter operator, vibratory disturbances mean pos-sible structural failure from fatigue, and probable difficulty in the protection of equipment from the lower-frequency disturb-ances. In addition, vibratory sensations have to be contended with in respect of both passengers and crew. Many of thesesensations come within the aural frequency band; they are all within the frequency range which can be felt, and the lower fre-quency displacements originating from the main rotor may be seen and followed visually. These sensations, if excessive, willresult in physical fatigue and discomfort; further, they are not conducive to safety. These points, and many others, are notmerely theoretical postulations, but, as most of us know from experience, they are practical considerations which must be dealtwith." Mr. McClements then went on to discuss some of the aspectsof helicopter vibration which appeared to him to be operationally important; he did so under the broad headings of " Noise " and" Mechanical Vibration." He said that, in practice, a " noisy " helicopter would almost certainly cause annoyance to passengersthrough difficulty in carrying on conversation, and perhaps by headaches and subsequent ringing in the ears. The effects oncrew—namely, physical fatigue, irritability and perhaps tem- porary loss of hearing—might be more important because safetywas dependent on crew well-being and economy in crew utiliza- tion. Another aspect of helicopter noise which might be veryimportant in the future was the external-noise level. It was visualized that helicopters would fly into the centres of built-upareas and thus, to the man in the street, they might well be a much greater source of annoyance than the aircraft which con-fined itself to airfields. Stating that the human ear has an approximately logarithmicresponse to sound, the lecturer stressed the importance of know- ing the relative values of the noises which combined to formthe resulting noise which we hear, because it implied suppression of all sources of approximately equal level if any material im-provement was to be made in the apparent loudness associated with that level. He had been unable to discover for the helicopterdetailed information (of the type available for fixed-wing aircraft) regarding the various sources of noise and intensity levels; asurvey of noise sources in current types would be a fruitful line of investigation. In the absence of detailed information hesuggested headings under which the noise might be considered. They were as follows: — (a) Engine Exhaust Noise.—This, more often than not, wasrelatively high. Silencing was usually associated with loss in power, and power was critical in the case of the helicopter.Operators would welcome the development of a silencing system efficient in noise-suppression without appreciable power-loss. (b) Engine Mechanism Noise.—This was also relatively highand, because the source was likely to be housed in the fuselage, suppression might be more difficult than in fixed-wing transports.It was a problem of suspension and soundproofing, and one which was in the hands of the designer. Avoidance of associatedclatter and rattle would call for maintenance by the operator. (c) Transmission Noise.—There was no equivalent of thetransmission noise in the fixed-wing aircraft, so its relative im- portance was not known, but transmission systems under powercould cause a considerable amount of high-frequency noise and, further, it had been noticed that different transmissions of thesame type could have appreciably different noise levels, depend- ing upon their condition of assembly and wear. These pointswere mentioned as being of operational importance in relation to the chosen method of mounting the transmission in the air-frame, and of soundproofing the units. (d) Rotor Noise.—Main rotors rotated at low r.p.m. and theresulting low-frequency disturbances could sometimes be quite clearly heard, but the lecturer had not known them to be trouble-some. Higher frequencies originating from torque-compensating tail rotors could also be heard, but were probably relativelyunimportant. Generally speaking, noise from the rotors of helicopters now in service was likely to be relatively unimportant,but this might not be so later if higher tip-speeds were employed. (e) Local Resonance.—Vibrationary disturbances covering awide band of frequencies were present in the helicopter, and if care was not taken in the region of the cabin, local structureswould resonate and equipment rattle. Such sources of annoyance could be overcome if the manufacturer paid attention to detaildesign and the operator to maintenance details. (f) Radio Noise.—The pilot could suffer discomfort, nervoustension and fatigue from radio disturbances. The operator ex- pected to be provided with efficient and accessible screeningfor the ignition system and other electrical gearing. The author next referred to acceptable noise levels—startingwith the cabin and cockpit—quoting several works on the sub- ject, in particular Human Factors in Air Transport Design, byMcFarland. Curves were used to illustrate findings of noise levels measured in six current types of air transport. Theyreferred primarily to intensity levels above 600 cyc/sec. On the average, and at frequencies above 600 cyc/sec the comfort levelwas classed as " acceptable/comfortable," conversation being possible with raised voice. For future air transport, McFarlandrecommended a drop of some 20 db. However, journeys in heli- copters would be of relatively short duration, and on the scorethat noise nuisance was (within limits) a function of time, it would seem reasonable to accept standards lower than McFarlandrecommended. Mr. McClements suggested a reasonable com- promise (using a curve as illustration) having assumed that cur-rent air transports were not so quiet as the average passenger would like and that McFarland's recommendations appeared tobe unnecessarily severe for the helicopter case. Next, the lecturer considered external noise in relation to thehelicopter. Mr. McClements said: "What acceptable external noise levels will be is difficult to say, because they will dependon a variety of factors, including the position of landing sites relative to dwellings, the approach-path relative to houses, trafficintensity, whether traffic is by day or night, and what the public is willing to stand. Further operational experience is requiredbefore firm recommendations can be made, but as an interim step I suggest that the resulting noise should not exceed that causedby traffic in a busy street. If this is taken as a yardstick, I sug- gest that the external noise should not exceed some 65-70 dbat a distance of 100ft in any direction." He went on to enumerate eight practical points relating tonoise suppression within the helicopter, among them that venti- lating systems should be so arranged that the opening of aventilator did not give access to a sound source. Care should also be taken to provide for the removal and fitting of items—suchas controls—without damage to the soundproofing properties of the assembly through which they passed. The author next turned to the question of mechanical vibra-tions, elaborating on possible causes of discomfort and fatigue already briefly referred to in the second introductory paragraph.The lowest maintained fixed-wing disturbing frequency was usually about 1,200 cyc/min, but in the helicopter there wereperiodic disturbing frequencies of main rotor r.p.m. (approxi-
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