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Aviation History
1936
1936 - 0145.PDF
JANUARY 16, 1936. Reinforcing smooth skin to enable it to resist compression loads. A multi-spar wing with intermediate stringers is shown in I. Sometimes a corrugated skin under the smooth is used, as in 2. A type of fuselage construction in which one edge of each longitudinal '' plank '' is curled over to form an integral stringer is indicated in 3. The behaviour of a curved member under compression and tension loads is shown in 4 and 6 respectively. The rectangular bay shown in 5 may be regarded as a bay in the top of a fuselage. Under torsional loads the tendency will be for the bay to distort, the wire BD going slack and the wire AC stretching. If used in such a bay, the curved members would tend to distort as shown in 4 and 6. is to be kept down to a reasonable figure. In the De Havil- land Comet the strength was obtained by many layers of wood. ftf recent years American aircraft designers have made great progress in all-metal aircraft-construction, and more particularly in the so-called " stressed-skin " construction. A thin shell of metal has very little strength in compres sion, as it soon crinkles under compressive loads long before the real strength of the material is reached. A sheet of paper has practically no strength in compression, but if it is bent into the form of a tube it at once becomes able to support a considerable weight endwise. If the piece of paper is corrugated fan-fashion, and then bent round into a corrugated tube, the strength is still further increased. It is this fact designers make use of in re inforcing their stressed-skin constructions with stringers and frames, the former resisting the compression loads which the flat or slightly bent metal sheet is not itself able to resist. Stressed-Skin Construction By way of an example of the principles involved, the accompanying diagrams may be examined. In Fig. 1 is shown, quite diagrammatically, a wing section with smooth skin covering reinforced with spanwise spars and light stringers. Even when the unsupported panels have been reduced in area by these means they will not sup port without crinkling any really great intensity of strtss unless the gauge of metal is increased or (what amounts to the same thing) the thickness is brought up by using several layers or laminations. But thickness means weight, and in order to get the same stiflness with less Material the method shown diagrammatically in Fig. 2 ]s sometimes adopted. This lies in reinforcing the outer smooth skin of trie upper wing surface with a corrugated 'ayer riveted to it. The corrugations are used on the compression side only. In fuselage construction a similar state of affairs exists. "• plain shell of thin metal would fail by local crinkling nr buckling long before the full strength of the material "ad been developed. Reinforcement by hoops and ^nngers is adopted to stiffen the shell against bucklins:. ln some American machines another method is followed. 67 Instead of stringers riveted to the skin plating, the outei plating is cut in the form of longitudinal " planks," and one edge of each plank is curled over, thus forming a stringer integral with the plank. (See Fig. 3.) Even when reinforced by hoops and stringers to form fairly small panels, the stressed-skin construction does not develop anything like the full strength of the material. Consequently it has to be made heavier than necessary. It has been found that when the gauge of a tube, for in stance, is gradually increased, the strength goes up at a more rapid rate than the gauge. Whereas a thin-walled Duralumin tube of short length may develop a compressive stress of, perhaps, 6-8 tons per square inch, a tube of three times the wall thickness may develop a strength of 25-30 tons per square inch. When Mr. Wallis got these results from tests carried out at Weybridge he did not quite believe them, but as check tests made at the R.A.E., Farnborough, agreed, there could be little doubt that the facts were as indicated. The next step was how to make use of this result in an actual aircraft structure. When studying stressed-skin structures that have been subjected to tests such as bending or combined bending and torsion, one invariably finds that these test specimens show a series of folds where the material buckled under stress. These folds always run diagonally across a panel, from one corner to the opposite. It was, I believe, this fact which led Mr. Wallis to think of geodetic construc tion. Having ascertained that fundamentally there should be an advantage in using his structural material in a concentrated form like bars and tubes, and realising that the failure of skin covering was due to diagonal com pression loads, Mr. Wallis set out to combine a form of construction in-which the advantage of the stressed skin (greatest possible distance from the neutral axis) should be combined with the employment of the material in con centrated form, as diametrically opposed to the stressed- skin dispersed or spread out form. The result was the so- called geodetic construction, many patents on details of which have been taken out by Mr. Wallis in conjunction with Vickers (Aviation), Ltd. At first sight one would be inclined to jump to the con clusion that curved members running diagonally would be just about the least suitable for bracing purposes. Yet it was desired to use curved members as far away as possible from the neutral axis and running diagonally to follow the lines of greatest stress. Bent Struts When a curved member is placed under a compressive load it will tend to close up its two ends and to bow still further in the centre, as indicated diagrammatically in Fig. 4. The fact is very familiar, of course, from the case of the inter-plane strut which has become slightly bent. Its strength is but a fraction of what it was when the strut was straight. Yet in spite of this Mr. Wallis uses this very principle. However, he does not use it by itself, but in a manner which can only be described as remarkably ingenious. Let us, for a moment, turn to the case with which every one is familiar from some twenty years of girder fuselage construction : The rectangular bay braced diagonally by wires or tie-rods. When the fuselage is twisted it can only "give " by one of the bracing wires going slack and the other stretching. This is illustrated diagrammatically in Fig. 5. The bay ABCD was originally a right-angled square or rectangle. Under torsional load it deforms into the rhomboid shown by the dotted lines. The wire BD has gone slack and the wire AC has stretched. To understand Mr. Wallis' objective, let us assume that he intends to use two curved members for AC and BD. In Fig. 4 it was shown what hapoens to BD under compressive loads. Fig. 6 illustrates what happens to a similar member loaded in tension. Its ends tend to spread out and its centre to drop. In other words, it tries to straighten out. Now this is where the designer's clever ness comes in, and we are approaching the real crux of
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