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Aviation History
1917
1917 - 0296.PDF
MARCH 29, 1917. type of wireless truss, Fig. 18, in which the number andlength of the exposed members is minimised. The construction is especially adapted to the fast small-span racing or scoutingmachines. It is possible to imagine a strutless truss as a combinationof two pairs of monoplane wings with the usual bracing, Fig. 19 ; but the total length of wires is considerable, so thatthe advantages of such a construction would be doubtful. Another form of strutless truss, Fig. 20, is much more pro-mising, and with the appearance on the market of wires having streamline sections, it is possible that the structuralresistance can be reduced even further than is the case with the wireless truss. Fig. 21 shows that this construction canbe also well adapted for the overhang biplane. Triplane Trusses. There were and there are few triplanes. The trusses can be treated here along^the same principles as the biplane Fig. 26—Blfiriot Fl*. 27—Etrich- ...Bracins .,„.,, Taube Bracing Figs. 28 and 29—Commonplane Bracing Y **> " •-•*'" Figs. 32and 33—V-Type ^Figs. 30 and 31—N-Type Bracing 7 Figs: 34 and 35—Single Lift-*" — -.. Truss Figs. 36 and 37—Forces inDouble Lift-Truss Fig. 38 — K - Fig 39—CurtissType Bracing Lift-Truss Strut • Figs. 40 and 41—Forces in .:..«.• K-Type Bracine 4-2 and 43—X-Type Side-Bracing Figs. 44 and 45—Forces i-nX-Type Bracing truss. I mention the triplane because in several instances the problem has been treated incorrectly, as shown in Fig. 22, in which the full height of the truss is not utilised, although it would increase the strength of the truss about four times. Figs. 23 and 24 show examples of correct treatment. The triplane has a mechanical advantage of decreasing the buckling length of struts by half, which makes them relatively several times stronger. Fig. 25 shows a more complicated case of biplane truss for large high-power machines. The more uniform distribution of masses along the truss makes it lighter and stronger. I omit entirely the question of dynamical stability involved in spreading out the masses in this case. Here also different solutions are possible. In the Sikorski method the engines M are placed on top of the lower wing, close to the inner side of the struts. In the French method the engines are put between pairs of straight struts (see the full lines only). In the Curtiss method the engines are fixed between some special crooked struts (see the dotted lines). Each method has somesmall advantages and disadvantages, which can easily be seen from the figure. Until now we have considered only the airplane wing-trussas viewed from the front, or the so-called lift-truss. Inside the wings, however, are placed the so-called drift-trusses.*Both the lift and the drift-trusses are combined to form a rigid three-dimension structure by means of bracing in planespassing through the struts and parallel to the plane of symmetry of the whole machine. This bracing is visible inthe side view of the airplane and I will call it side-bracing. Side-Bracing Monoplane Wings. The subject of side-bracing of monoplane wings does notoffer anything of remarkable interest. Usually each wing has two parallel or slightly converging spars—the front and therear spar. Each pair of spars, together with some central pylon or the landing chassis, taken as kingpost, form thefront and the rear lift-trusses. Both are fixed in case of aileron control, and the front truss is fixed and the rear onemovable in case of warping. A monoplane has usually a double lift-trussing ; the spars are at the same time membersof both the lift and drift-trusses. Fig. 26 shows the side-bracing of the Bleriot monoplane'bus, on which the rear spar is braced to the lower girder of the single lift-truss. This arrangement is more advantageousthan the one on the early type of Etrich-Taube, Fig. 27, because the separate lift and drift-truss (requiring three spars)is uneconomical from the mechanical point of view, and the two wires offer about twice as much resistance as the single-strut brace in Bleriot's construction. The recent type of Taube has side-bracing similar to that shown in Fig. 26. Side-Bracing Biplane Wings. The side-bracing of biplane wings offers many possibilitiesof design. The most common types, Figs. 28 and 29, are based on the same principle, but are adapted to the so-called,straight and staggered biplane respectively. The terms " lift-truss " and " drift-truss " although con-ventional, are not quite correct. None of the trusses takes care of lift or drift alone. Really the resultant air-reactionupon the wings resolved into components parallel to the planes of the lift and drift-trusses gives the external forcesacting on the trusses. These components differ considerably from the drift andlift, not only in case of a staggered biplane, but also in a straight biplane, in which, for instance, at slow flight andlarge angle of incidence the forces acting on the drift-truss are frequently opposite in direction to the drift. Figs. 30 and 31 show the N-type side-bracing, with whichagain the resistance of the wires of the ordinary bracing (Figs. 28 and 29) can be decreased by half. This bracing,especially when combined with the wireless lift-truss (Figs. 16 and 17), offers new and interesting possibilities for heavy large-span aeroplanes. It was applied by the Albatros Co. in the form of a triple lift-truss, which seems to be an unnecessarycomplication, as simpler combinations are possible. Fig. 33 shows the V-type side-bracing, found in the modernNieuport scouts. The two converging struts are fixed in a special socket fitted between the spars of the lower_ wing.This construction is also adaptable to straight brplanes (Fig. 32), but in both cases is especially good for an unequal-chord biplane. Although the trussing in the Nieuport machine is treated as of the double-lift type, there is noreason why it (preferably the rear one) could not be treated as a single lift-truss with front struts acting as braces. Development of Single Lift-Truss. The first single lift-truss was used in one of the first Chanute gliders, which was a quintuplane. Chanute, how- ever, did not seem to appreciate the advantages of the single lift-truss system, as he adopted the double lift-truss for his subsequent machines. It was Breguet who (1909) produced and advocated the single lift-truss biplane, his main object, however, being to vary automatically the angle of incidence of the wings, which were hinged to the steel tubular spars (Fig. 34). A more perfect and elegant construction of the single lift- truss, which can be called I-type side-bracing, or simply I-strut, was used in Dorner's flying boat (1913). The struts, Fig- 35. were fixed in sockets having long bases that reached from the front spar to the rear spar, and were fixed to the latter. An almost identical construction was adopted in 1914 ' for the R.A.F. fast scouting machine. Mechanically the front and rear parts of the socket bases can be considered as a cantilever subject to bending, accordingly as the centre of * The drift-truss, being enclosed by the wing, does not involve aerodynamicalproblems, and can be treated in any desirable way from the structural point ol view only. A discussion of it is therefore omitted. 296
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