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Aviation History
1931
1931 - 1120.PDF
FLIGHT, OCTOBER 16, 1931 while the balloon method may be suitable for the frequentfogs of low depth found in this country, for the more diffi- cult problem of directing an aircraft in a fog of consider-able depth wireless methods are needed and should be tried out, advantage being taken of the knowledge gainedby the extensive researches in hand in the U.S.A., where large sums of money have been devoted to this end. The Committee welcome the extensive development thatis taking place in Imperial air routes. Following the series of nights for several years by Service aircraft overthe route from Cairo to the Cape, Imperial Airways have inaugurated this route for mail purposes. With extendedflying here and on the India-Australia route, and with the steady reduction in the times of flights between dis-tant points of the British Empire, it is expected that many new problems will come forward in which the Com-mittee will be able to render assistance. One of the more important outstanding matters is the reduction of noisewith a consequent increase in the comfort of passengers. Aircraft Noise The main sources of the noise of aircraft are the air-screw, the engine exhaust and engine clatter. Without an appreciable reduction of each, a saving from any onesource will not produce an appreciable effect in the air- craft cabin. Some idea of the level of noise is of interest ;the scale for its intensity is measured in decibels. As is the case with many physiological phenomena, alogarithmic law is approximately followed by the ear in recording sound intensity. The scale of the instrumentdesigned for measurements of the noise of aircraft has accordingly been arranged .to give approximately equalloudness steps on a logarithmic scale each of 5 " decibels," where the total range of the ear, for a note of the pitchused as the standard of comparison in the instrument is about 130 " decibels.' If the threshold of audition istaken as zero the conversational range of noise from quiet to loud ranges from 40-60 decibels. The noise in thecompartment of a steam train will on this basis be about 60 and the onset of a sense of pain in the ear is 130.At a distance of 80 ft., the noise level of an airscrew run- ning at quite low speed is 70, the silenced Lion engineexhaust is 80 and the unsilenced exhaust reaches the noise krvel of 94. At distances of only 10 ft. the correspondingnoise levels for the engine exhausts are 100 and 110. When these figures are compared with the steam train com-partment at 60 there is seen to be much room for improve- ment. The difficulties of taking measurements relative to amoving airscrew are considerable, and the experiments are expensive. With a reduction in airscrew tip speeds toabout 600 ft. per sec. from the more normal figure of 850, about 20 decibel? can be saved. The silencing of exhaustnoises has proved to be a difficult problem. No silencer yet tested on an aircraft has given a reduction of morethan about 10 decibels. While very effective silencing can be obtained on the engine test bench, the difficulties ofadapting the methods to flight conditions are very great, and have not yet been overcome. No attack has yet beenattempted on the reduction of engine clatter, but in this connection it is interesting to observe that the sleeve-valveengine, whose development for aircraft is desirable from other points of view, will be free from the major portionof the engine clatter present with the poppet-valve engine. If a considerable reduction of noise to the passengers isrequired it must be made at the original source, but there is scope for improving the exclusion of noise from thecabins of air liners. The best type of partition appears to be a double wall packed with light absorbent material.With an approximate weight of 1 lb./sq. ft., a reduction of 30 decibels has been obtained with specially designed filleddouble panels. The reduction of the noise level in an aeroplane cabin to about 75 decibels appears possible, butno measurements in actual air liners have yet given values lower than 80 decibels ; i.e., the noise is very much louderthan in the compartment of a steam train. Accident at Meopham, Kent The accident to the Junkers F.13ge low-wing monoplaneG-AAZK at Meopham, Kent, on July 21, 1930, was referred by the Air Ministry to the Committee, and theAccidents Investigation Sub-Committee have made a thorough investigation, which is described in R. & M.1360. A careful examination of the structure showed that tWewere no defects in the material and no incorrect assembly and in the report a critical examination is made of variousother theories which might have accounted for the a.crf- dent. That the engine cowling blew back and obscuredthe pilot's view was disproved by evidence. The breaking of an airscrew blade in the air, resulting in the applicationof an unbalanced force to the aeroplane sufficient to tear the engine from its mounting, was ruled out since theengine struck the ground with airscrew intact. The wooden bearers under the engine did not fail and permitthe engine to come away from the aircraft. There was no evidence of weakening of the fuselage or tail skid priorto the accident, which might have resulted from the peg- ging down of the aeroplane in the open at Berck duringthe week-end before. An explosion did not occur in the aircraft due to ignition of petrol vapour or to some othercause, since there were no signs of burning or singeing or interpenetration of parts of the structure, and no seriesof breakages radiated from a centre. When these possible causes were eliminated thereremained only a breakage from a too rapid pull out from a dive, or some new- fact that might come forward. Thetail of G-AAZK broke in the air, and yet this part was stronger than the main structure for all the ordinary con-ditions of flight. Had the accident been due to flattening out, the wings must have broken before the tail. Afterthe failure of the wings, the aerodynamic forces acting on the structure were, in the opinion of the Committee,insufficient to break the tail. It appeared, therefore, to be certain that the tail must have broken first, for it didnot come down intact with the main structure nor had it been struck by any other part of the craft. As corro-borative evidence, an analysis of accidents in Great Britain over a large number of years has shown that in no casehas a tail broken subsequently to the failure of the wings. The Committee had, therefore, to search for an explanationwhich would account for the prior breakage of the tailplane. A model of the tail of the Junkers F.13 aeroplane wasconstructed to reproduce the elastic and aerodynamic properties. This model tailplane, together with part of themain wings and centre section, was tested in a wind tunnel to ascertain whether flutter of the tail unit wouldaccount for its breakage, but the critical flutter speed was found to be too high. During these experiments, however,a peculiar phenomenon was observed which was later found to give rise to movements of considerable violence undercertain special conditions of flight. This phenomenon, described as " buffeting," is an irregular movement of thetailplane caused by the eddy system shed by the main wing when nearly stalled. This low-wing monoplane hadpreviously been described as exhibiting a buffeting of the tail organ during landing. At these low speeds duringlanding the amount of oscillation of the tail is quite small, and it is not likely to result in any damage to the struc-ture ; but with a combination of high speed and high angle of incidence that can only occur temporarily in flight, andmay be maintained for only a few seconds at a time, the tunnel experiments showed that large deflections will occurdue to buffeting. The Sub-Committee concluded that in the very turbulent air in which the aeroplane was flyingthe attitude became such that violent buffeting set in at about cruising speed. The tailplane was thereby brokenand the other breakages followed. As a consequence of this accident the phenomenondescribed as buffeting is being investigated very carefully (see Illustration No. 5), and steps are also being takento place the measurement of turbulence in the air, described generally by pilots as bumpiness, on a scientificbasis by the carrying of accelerometers and by other means. In connection with this accident, the Committeehave also stressed the importance of informing the public that by removing parts from wrecked aircraft for reten-tion as souvenirs they may be destroying valuable evidence. Accident to H.M. Airship R.101 The Committee cannot conclude this Report without a reference to the disaster to R.101. The design and construction of this airship had been the subject of most careful thought. Great skill had been bestowed upon the task, and R.101, it was hoped, was to be a triumph of British ability and workmanship. 1050
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