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Aviation History
1934
1934 - 0291.PDF
FLIGHT, MARCH 22, 1934 C i B D Fig. 9 : Method of resisting torsion in box wing. that illustration there were some well-marked shear bucklesin the webs. In that particular specimen the load was being applied towards the right-hand side, so that the left-hand side of the vertical boom would be in compression and the right-hand side in tension. Examining this illus-tration, there was not much difference in the appearance of the buckles. Presumably, if the end load was havingany great effect on the form of the buckles, one would expect the buckles to be running in a more or less per-pendicular direction on the compression side and in a tangential direction on the tension side. Therefore, thatillustration rather suggested that probably the effect on the shear buckles was not going to be of any great prac-tical significance, so far as that type was concerned. A rather strong point had been made of the fact thatany vibration test of complete spars to be of use should include representative fittings, holes and some accidentalirregularities, but the spars shown in the illustration appeared to have loading boxes at the points at which thevibrating mechanism was attached. Could the author say whether the failure he got in these spars was influencedby, or was directly due to, the presence of these fittings, and, if so, was it quite fair to judge the merits of the twoprocesses unless the loading boxes had been specifically designed to suit these processes? Miss L. Chitty (Oxford University Engineering Labora- tories) said it was ten years ago that she was associated with Mr. Pollard in work on methods of construction. In the early work they were feeling their way as regards transferring to metal from wood construction, and she was impressed with the great strides that had been made since then. The field of research was enormous, and it was important that the practical side should go hand in hand with the theoretical consideration of these problems. Dealing purely with the theoretical and mathematical side, there was always the fear that one was being led away from the direction in which it was desirable to go. In carrying out theoretical work, therefore, it was essential always to keep in mind the relationship of theory and practice, and from that point of view the paper was of special value. Mr. M. Langley (De Havilland Technical School) said that Short Bros., if not the first, were among the first people in this country to be interested in the metalclad fuselage. The Bristol Company might have carried out earlier work, but, at any rate, it was satisfactory to find that early work vindicated after something like 15 years. The paper was valuable in that it translated into engineers' language work which had existed for many years in mathe- maticians' language, and as such it would be appreciated by all the technical and engineering members of the Society. One important aspect was not only the bare initial cost, but also the subsequent maintenance cost. If that were taken into account he believed the result would be greatly to the advantage of metalclad. In the case of a multi-spar metalclad wing there was presumably less concentration of load at the roots than was the case with the normal wing structure. In the Junkers the reactions were trans- mitted not into the metal skin but into the braced section of the fuselage, and gradually died out towards the tail as the loads became less. Consequently the stresses could be more easily taken by the metal skin. Similar construc- tions were used, or had been used, by Spartan, Handley Page, and Vickers. In R. & M. 1553, to which the author referred. Mr. Cox appeared to support Pesca's theory that one could take 50 times the thickness of the skin as work- ing in tension with the structural members. Mr. Pollard, however, did not mention Pesca. Was it to be taken that he had compared Pesca and Von Karman and had come to the conclusion that Pesca's formula was much too crude for aircraft? As far as he had been able to_ work out from various information which came to him, if one followed the old directions and concentrated loads into as few structural members as possible, with a considerable amount of secondary structure in the way of fuselage, fairings, etc., which were doing no primary work at all,the results were almost inevitably greater weight, and he hoped that this paper—indeed, he was sure of it—woulddo a lot towards killing the prejudice that had existed1 against the metalclad fuselage. Mr. H. A. Mettam (IVestland Aircraft Works) said thatit still appeared that the design of stressed-skin structures was a matter for the engineer, and in that connection kesaid he wished to define an engineer as the man who was capable of making reasonable assumptions based uponinsufficient data! The author had restricted the appearance of buckles toone-third of the fully factored load, but it would be useful to know if that had been based on the three-quarters yieldrequirements insisted upon for military aircraft, and could it be assumed that buckles, on the basis of one-third ofthe fully factored load, would be fairly elastic. In using Von Karman's formula, did the author con-sider the values given for constant C in his Ref. 4 were correct, or had the author any different experience on thepoint. Since this paper had been prepared, Technical Note No. 479 had been issued in America, and thatappeared to give a rather different law for the variation of permissible stresses with thickness-radius ratio. Inconnection with stressed-skin constructions, was it desir- able to go in for multi-spar construction? Quoting frommemory the figures of the Dormer aircraft, he said, he believed the weight added by skin covering in the case ofthe small aircraft Dornier-Libelle, and later the Dornier- Waal, was about 50 per cent., whilst in the case of alarger machine the addition was only 20 per cent. One of the points it was desirable to find out was, at whatstage in the size of aircraft would the stressed-skin con- struction become worth while? Mr. D. L. Hollis Williams (Fairey Aviation Company)remarked that fabric was about the only material which had never given any trouble, and thus we had stuck toour braced structures and biplanes which hitherto we had thought more efficient. A point that had not been men-tioned was that, with a fabric cover, a thin wing machine relieved itself at high speeds from the point of view ofvery high local fabric pressures. A thick wing machine had not the same capacity, due to its heavy camber, Ofrelieving itself. There were very big opposing forces on the top and bottom surfaces and the fabric wing probablybecame a corrugated wing, and it was because of that that we should, eventually, have to have recourse, on the high-speed machine, to the metal cover. One objection to the metal cover, which must be got over before it could beadopted on a service basis, was the lack of facilities for inspection. That did not, perhaps, apply to the fuselage,because it was possible to crawl down it for the purposes of inspection, but in the case of the wing the tendencywas to build a box which was rivetted up and became more or less like a salmon tin, and the only way to getinto it was by means of a tin-opener. Mr. Pollard, in the course of a short reply to the dis- cussion, referring to atomic hydrogen welding mentioned by Mr. Redshaw, said that so far as he was aware this had only been applied successfully to steels. He believed it had been tried on 18-gauge alloy, but generally speaking not very much progress had been made with it in this application. As to the combined shear and end load effect on the wing dimensions, so far as it had been possible to ascertain, this did not affect the appearance of the buckles to an extent that really mattered. These buckles appeared somewhat appreciably less than was indicated by the formula. Whether there had been end load present or not, the waves would appear in every case, but at a rather lower value than that calculated. Concerning the theoretical stresses being practically independent of the length of the specimen, there were conditions in which one would not care to make the length less than twice the diameter. He did not quite follow Mr. Douglas's comment with regard to Fig. 7. Actually the end load was carried on the vertical carrier at the side. The question of cost, mentioned by Mr. Langley, was. of course, the bugbear at the moment. The substitution of all-metal construction for wooden construction was exceedingly costly when the work was begun 15 years ago, but since then the disparity between the costs of the two methods was nothing like so great. There was nothing intrinsically expensive in shell construction. It was, of course, necessary to get the jigs together, as well as the riveting tools and welding machines, but he did not think it would be more costly eventually. As to the formulae, 291
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