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Aviation History
1912
1912 - 0594.PDF
(/JJGHT general practice to He ash for spars, and general knowledge that ash i^ only alx.ut half as strong in compression as in tension, it follow-, that the spars in tin- upper plane should have about twice as much sectional area as the spars in the lower plane, excepting, of course, the point in the lower plane which carries the engine and pilot. In this case a "local increased bending moment occurs chiefly on one side of the engine only, the other side being probably more than balanced by the lift due to engine torque. It would thus seem that given a vertical distance between the main planes of the biplane equal to the distance between the wing supports and lower wire attachments in a monoplane, and all other details being equal there is nothing to choose between the two types. There ii the important difference in that instead of the usual wires in a monoplane the lower plane sjiars take the tension load, but even then the advantage is doubtful, as wires are stronger for the same weight, and this is accentuated if the lower spars on the biplane are cut into or drilled for strut and wires. The great point in which the biplane is superior to the monoplane as a mechanical structure is in the fact that quite a number of fractures can occur without anything like a collapse of the structure. The reason is explained in the following :—The strength of a biplane as a girder is governed chiefly by the distance apart of the upper and lower planes. Now, in machines as at present built, this distance is so great that there is practically no tendency to bend the fibres in the spars, but there is a pure tension in the one and compression in the other. If these two planes were placed directly on top of one another with no space in between them the spars Would immediately bend under the load, and to bend would have to slide on one another. Sketch 5, of two wooden beams of equal length, shows what I mean. Continuing, the plane spars in a biplane, despite their distance apart, would also tend to slide with respect to one another, and the structure would be robbed of practically alt its strength if provisions were not made to prevent this sliding, which is, of course, done by means of the cross bracing. Now it is fairly safe to assume that the cross bracing is amply strong to prevent this tendency to slide and it would no doubt be quite possible for the planes to support the load with quite a number of the cross bracings wires severed. In fact, if the two tubular diagonal supports for the extension in military-type biplanes and the vertical struts were intact, it is doubtful if anything very serious could happen. Of course, if either of the main spars collapse under the load, disaster is practically certain, but even then, if the machine is JUNE 29, 1912. streneth to the vertical struts, and must be of a section many times greater than the struts themselves, according to the* curvature Coming to the design of fuselages, there are so many different methods in use at the present time that it is impossible to take any but a few of the best known ones. In the first place, any fuselage, no matter what section it be, that is braced with wires in tension is distinctly bad. The chief reason is that initial stresses are put into the structure before any of the loads it is designed to withstand have been applied, and as a mechanical structure it is,'by reason of its very design, robbed m the making of a considerable portion of its strength. Secondly, it is usually found necessary, and especially in old machines, to put considerable tension on the bracing-wires, in order to keep the tail straight, or to remove any sag or twist in the fuselage itself. This considerably reduces the capabilities of the structure to resist shocks, and makes it " short " or brittle. It also fails at any weak spot should there happen to be one because the structure is inelastic, and any unusual stress is concentrated on the first point that will give at all, and this must sooner or later fail. By far the better fuselage is one with sides of sheet aluminium or multi-ply wood. This has the great advantage, as far as resistance to fracture is concerned, in the fact that all the parts are at rest and are not initially stressed so as to have all their elasticity removed as is the case of the wire-braced fuselage. It is not at all necessary with the built up types of fuselage to have diagonal struts or stay wires, as, of course, the sheet sides do all the necessary staying, but it is preferable to have a few vertical stays at intervals, and especially where the aluminium or multi-ply wood sections join, as without these the light sides would bulge and strength is immediately lost. Fig. 6 shows a type of fuselage that will withstand enormous shocks, and one of the best tests of this kind of structure the writer ever saw was the Martin • llandasyde on which poor Gilmour met his death. Despite the enormous crash (and the engine was buried three feet in the ground) the fuselage remained absolutely intact from behind the engine to the tail. A triangular section fuselage is not so strong as is one of square section especially to withstand twisting, but the square section fuselage must he braced diagonally inside (i.e., across from spar to spar), and this should not be done with wires. Undoubtedly the ideal section to withstand the varied loads a fuselage is subjected to is the circular one, but this is useless if made up of four spars boxed in with sheet aluminium or multi-ply wood as shown in the sketch, Fig. 7, because in the case of a bending moment the fibres furthest remote from the neutral axis are stressed to the greatest extent, and if the bending load happened to be either vertical or horizontal the thin covering would have to take nearly the whole load. If the covering has to be designed strong enough for this purpose then the four spars are superfluous. A tubular section fuselage also is the finest possible to withstand a twisting load, but probably this would have to be made in steel, and in fact there are some in existence so made. There is little doubt that behind the wing fastenings, at any rate, a fuselage has to withstand greater vertical loads than horizontal ones, and it is not uncommon to see a fuselage break in the middle, cross-braced from the front, upper, and lower spars to the rear lower and upper spars, the abnormally stressed spar is supported in a certain measure by the other three spars. And another point of advantage in this kind of structure is that as the centre of pressure travels towards the rear spars, the front spars still take a fair share of the load by means of the cross-bracing just referred to and the vertical struts. Of course, the vertical struts run only from the lower plane-spars to the upper plane-spars, and it is a point of .real importance in designing m ^e that substantial ribs are fitted in the planes just at the points where vertical struts are fitted, or the structure loses a great deal of its strength. These ribs should be quite equal in 594 F"i G, . V. simply due to air pressure on the tail planes alone. This happened to Latham's Antoinette when he struck the roof of the shed at Brooklands. For this reason fuselages should be deeper than they are wide, and in the case of a circular fuselage this could have a spar running along the top and a similar one along the underside, pre ferably on the outside, because, as already explained, the extreme fibres are stressed the most. As in the case of the fuselage, the undercarriage should not under any circumstances be braced with tension wires. Probably the most important point in the whole design of an undercarriage is to see that the struts are properly designed to withstand compressive loads, and are inclined at such an angle that they will be parallel to the
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