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Aviation History
1922
1922 - 0632.PDF
THE CASE FOR METAL CONSTRUCTION By JOHN D. NORTH (Fellow) ALTHOUGH I have been requested- by the Council of this Society to read a paper on the metal construction of aero planes, there are two phases of the subject with which I do not feel prepared to deal. In the first instance, any paper dealing historically with this matter is likely to give rise to controversy which is of neither scientific nor engineering interest, and I propose to leave this aspect severely alone. Secondly, I do not pretend to expound a process of design. The technique of this new branch of the engineering art is still too fluid ; while so far as experimental results are con cerned, it is probable that more have been published than have been digested. The aeroplane engineer, designer, constructor or user, not unnaturally, is inclined to pin his faith to the system of composite construction, which, brought to a state of high perfection, he has found to serve him well in the past. All the history of engineering relates the giadual displacement of timber by lighter and more durable structures of steel, but such a transition in aeroplanes he feels is difficult, if not impossible, to realise with advantage. Qf the three separate metal aeroplane movements in Great Britain, Germany and France, that in this country, at least, received its principal impulse, not from a realisation of the great engineering advantages attending it, but from the pressure of a world shortage of the limited supplies of that class of timber most suitable for light structural purposes. The cessation of the demand for the mass production of aircraft, coincident with the termination*) f hostilities, deprived the movement of its principal motive power, but not before it had been realised in some quarters that metal, and parti cularly steel, construction, could succeed on its own merits even in competition with a normal timber supply. It may be helpful first to note some general criticisms of metal construction. The principal argument which has been advanced is that metal construction has not demonstrated its advantages over the usual composite form of structure. This is not, strictly speaking, a sound criticism, being in the nature of an argumentum ad ignorantiam, and though it is quite true that in most cases the users of this argument have not had the necessary experience of metal construction to enable them to appreciate it, such an argument cannot be considered as conclusive. It may be inferred from various remarks that in the opinion of critics metal construction is expensive, perhaps heavier and, in temperate climates at least, not likely to be more durable than wood. It has further been criticised by * Paper read before the Royal Aeronautical Society on October 19, 1922. 45 40 35 3 11/ I25 UJ > UJ £ 15 Y TOTAL WEIG 3 • ENGINE ETC.. •8 • PETROL 4i.OI ^*s»^^ """"^T^^ ^^fea^ ^^§ ^ r<i ^ ^< *&& HT PER HP ir/T. PER (-P ^V/TPERH= | FIC >.l. analogy on the grounds that small motor-boats are invariably, or at least most successfully, constructed of timber, and in some cases it has been further put forward that metal con struction must be associated with large aircraft just as it is associated with large ships. The weakness of this argument is immediately apparent when we consider that the first object of a boat is to keep out water, and the adverse experi ence of constructors of steel motor-boats is almost entirely due to the fact that it is difficult to make them watertight. These conditions are hot analogous to those obtaining in aircraft, and it hardly seems necessary to pursue the argument further. With regard to the other criticisms, I hope to indi cate in these notes that the following advantages may be obtained by the use of metal construction :— (1) Retrenchment of weight. (2) Improvement in the reliability of materials. (3) A longer life, particularly under conditions of storage. (4) A better resistance to adverse climatical conditions. ' (5) An improvement in bulk manufacturing facilities. (6) A reduction in some risks and effects of fire. It is, I think, not always sufficiently realised what a very important effect comparatively small differences in the struc ture weight of an aeroplane have upon its general charac teristics ; particularly is this the case when the aeroplane is designed for a high performance or a very long range of flight. There has been some little tendency, and designers of equipment seem to be special offenders in this respect, to imagine that a few pounds here and a few there are negligible in regard to the gross weight of the aeroplane, and that special efforts to economise weight are therefore not necessary. If it is realised that the margin available for military load per unit horse-power is only the remainder after deducting the structure, the engine, installation and fuel weights, it will be appreciated that a saving in weight in an individual part is reflected in an economy on the gross weight, greater by the ratio of the gross weight to the military or revenue load. This is illustrated in Fig. 1, which shows the variation of revenue load—the revenue load in this instance includes pilot, instruments, equipment, etc., so that the influence of structure weight on revenue load is enhanced, particularly in the case of small aeroplanes—or military load with different per centages of structure weight for aeroplanes having various classes of performance. The variations in performance have- been indicated by the power loading in pounds per horse power, since this figure conveys in a simple manner the typical performance, to the aeronautical engineer. It will be noticed that even with a power loading of 15, which is representative 2* £6 28 3ff 32 34 36 7. STRUCTURE WEIGHT 632
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