Metal Construction.
The question of the aircraft materials of the future is not so much a problem as a matter of gradual evolution. In view of the dwindling supplies of suitable timber, it certainly seems more than probable that some form of metal construction will one day constitute the structure of the aeroplane. The manufacture of the various components in wood does not necessitate an extensive plant, the labour necessary is comparatively cheap and easily available, and moreover the transitory nature of the whole business, and the ease with which essential changes in type can be made without the wholesale scrapping of the expensive jigs associated with the use of steel, all strengthen the case in favour of wood. The conclusion of hostilities would introduce another state of affairs, and it is conceivable that the various types will then be standardized for different purposes, which may necessitate the greater use of steel. Certainly the advantage of steel would be better realized under some system of standardized design, but this unfortunately is not possible while present conditions obtain. The advantages of metal as a material considered briefly, are that it permits of design to close limits without the allowance of so-called factors of safety, which are now necessary through the great variation in the strengths of wood, manufacturing procedure would be expedited, while one can reasonably expect a greater degree of precision in the finished machine, due largely to the increased facilities for accurate manufacture of components which metal affords. It is quite possible, of course, given a uniform grade of steel, to design to extremely close limits without fear of collapse; but the human factor in the shape of fitter, welder, or operator introduces the unknown element, and one for which some allowance must always be made. One cannot assert that any very decided indication exists of a trend in modern design towards metal construction, and it is quite possible that this will not arrive until it is rendered imperative by reason of the scarcity of timber. The precise composition of the metal is rather a controversial matter, some authorities favouring steel, and others some alloy of aluminium, such as, for instance, “duralumin.” The production of a suitable alloy constitutes a real problem and one upon which the Advisory Committee for Aeronautics have already made investigations and experiments. A disadvantage with steel is that, although it is quite possible to produce, say, a fuselage entirely of this material to withstand easily the greatest stress encountered in flying, such a structure, owing to the thin nature of the various components, would suffer damage through shocks induced by rolling over rough ground, and also by handling. In addition the effects of crystallization would require some considerable study. These and other reasons indicate that an alloy of aluminium, which for a given weight would be considerably more rigid than steel, offers possibilities as a material. It might prove advantageous to combine both metals, using steel for the more highly stressed parts, such as, for instance, wing spar attachments, interplane bracing lugs, and indeed any part where the load to be carried is one induced by tension.
The foregoing is indicative of some of the more important directions in which improvement and development are possible, and certainly ample scope yet exists for the attention of the student, or indeed any one interested in the future of the aviation industry.
CHAPTER II.
MATERIALS.
Seeing that wood constitutes the material for the greater part of the structure of the aeroplane, that is with very few exceptions, some notes on the characteristics and qualities of those woods most commonly used may prove of interest. The choice of a suitable wood for aircraft construction is a matter of some difficulty, engendered by the variety of considerations of which at least some observance is essential. The fundamental principle of aircraft construction, that of obtaining the maximum strength for a minimum of weight, affords one standpoint from which a particular wood may be regarded, but this does not constitute in itself a sufficient reason for its choice. Of almost equal importance are such considerations as the length and size of the balks obtainable from the log, the total stock available, the relative straightness of grain and freedom from knots as well as the durability of the wood.