Metal Wing Construction.
Of two machines, equal in air performance, the one which can be most easily produced has an obvious and, especially at the present time, a very important superiority. Rapidity of production is a most cogent argument in favour of metal construction, for once the necessary machines are set up, and the jigs and dies made, and given a constant supply of material, output is only limited by the speed of the machine. In addition, there are the very exacting demands of interchangeability. Now, it is infinitely more easy to obtain exactitude in metal than in wood, and, moreover, assuming that it is possible to produce woodwork to the nearest ·01 of an inch, what preventive is there against shrinkage, which occurs even when using the dryest of timber. By the more extensive use of metal there should be a considerably reduced proportion of scrapped parts, and erection would be accelerated. It is significant that the planes of some of the most recent German machines are constructed largely of steel tubing, which is at present the most practicable form in which steel can be used. Of course, steel tube spars are quite an old detail, although the more general English practice is to core them with spruce or ash, as in [Fig. 42]. One remembers a
Fig. 42.—Steel tube spar with wood filling.
monoplane, built some time before the war, in which the spars and ribs were of steel and the covering of thin aluminium sheet. In flight this machine was particularly fast, which may be accounted for by the reduction of skin friction, which a smooth surface such as aluminium would afford. In addition, the tendency of a fabric covering to sag was also obviated. Another example of metal construction is afforded by the Clement-Bayard monoplane, exhibited at Olympia in 1914. The plane construction of this machine, as shown by [Fig. 43], consisted of channel steel spars, steel leading and trailing edges, and thin steel strips replacing the usual wooden stringers. However, steel construction in modern English machines is restricted to the various organs of the empennage, and occasionally one finds ailerons so built. There seems no valid reason for the continued use of wood as the material for the construction of such items as the fin, rudder, and elevators, as a considerable saving of labour and time can be effected by using the various forms of steel tubing; moreover, the tendency which most controlling organs built of wood have to warp and twist with variations in temperature is prevented by the steel frame. One frequently sees such items as the ailerons and elevators distorted, which must result in excessive drift, if not erratic flying. At the present time it is difficult to obtain aluminium alloy in any large quantity, and this, in conjunction with the present high prices, precludes its extensive use. When this material is procurable in quantity, and when design is reasonably standardized, rolled or lattice spars and stamped ribs may come into vogue.
STEEL SPARS
Fig. 43.—Rib construction with metal spars.