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Aviation History
1926
1926 - 0531.PDF
JULY 29, 1926 C9 THE AIRCRAFT ENGINEER SUPPLEMENT TO FLIGHT Bearing the foregoing in mind, it is interesting to see how few modern engines are suitable for weight-carrying machines. Assuming that the maximum speed of weight- carriers nowadays should be in the neighbourhood of 100 m.p.h., it will be seen from the accompanying table that by using the most efficient two-bladed airscrews, there are only three engines which satisfy the condition that the tip speed should not exceed 850 ft./sec. It should be men- tioned that the tip speeds of some of the others could be reduced by using four-bladers or two-bladers of small diameter and large blade widths, but only calculation on the particular machine under consideration will show whether the gain in efficiency would offset the loss due to increased slipstream and insufficient airscrew diameter. The conclusion to be drawn from the chart is that most of the modern aero engines are only suitable for high-speed aircraft, and do not enable the best performance to be obtained on slower aircraft, such as commercial machines, and that a reconsideration of the gear ratios used or the pro- vision of alternative ones to suit the requirements of aircraft designers is highly desirable. AIRCRAFT PERFORMANCE. The Influence of Size on Structure Weight. By J. D. NORTH, F.R.Ae.S. (Continued from p. 60.) The question of the structural economics of aeroplanes has always been a vexed one. Direct analytical attack is possible only in a very crude form, and a statistical presenta- tion is vitiated by the paucity of evidence and by the fact that what evidence there is is not sufficiently detailed to permit of satisfactory correlation. It is nevertheless of first-class importance in the determination of performance, which, for practical purposes, must always be referred to the paving or military load and not to the gross weight as is done for purposes of aerodynamiqal analysis. As a very rough 8tartinu point, aerodynamic performance may be considered in terms of pounds p?r horse-power, and this in its turn may roughly be divided up into power plant, structure, and military or paying load. The difficulty of making clear cut divisions is obvious, and has actually been enhanced by the fact that usually three separate persons or organisations are responsible for the weight involved. The engine designer, the aeroplane designer, and the purchaser where he specifies his military or commercial requirements. For a fair pro- portion of the aeroplane weight the responsibility is joint, insomuch as the aeroplane designer actually designs parts the necessity for which is imposed on him by the requirements, direct or implied, of the engine designer and purchaser. The fact that there are three parties responsible for the weight of an aeroplane has had a very real effect on design and an even greater one on statistics. The power plant properly includes engine, airscrew, fuel and lubricant, tanks and pipes, cooling devices, exhaust systems, engine mounting and cowling, starters, engine controls and many additional items required by individual engine design. Only a part of these are usually classed as power plant. Similarly, with military load the consequential installation weight is not generally included. This has inevitably resulted in engines and equipment being designed on the basis of net weight and not gross installed weight, with uneconomical results. The aeroplane designer has to shoulder all these extra weights under the head of structure, with the result that structure percentage, as a comprehensive term, has little or no meaning as from one aeroplane to another. In addition to these points, the structural weight of an aeroplane is affected by its size and by the load factors specified for the design. The question of size may at first be most conveniently considered on the assumption that the load factor remains constant, and, there- fore, that the truly structural parts of the aeroplane are subject to loads directly proportional to the gross weight of the aero- plane. This is true for aerodynamically similar machines. The question of the influence of size on structures and 464 mechanisms is one of first-class importance in all branches of engineering, and its influence in nature is equally notable. The physiological aspect of size has been discussed by several writers, such as, among others, Ray Lancaster, who referred to the limitations from structural considerations of the size of land animals compared with sea animals. In a paper at the Southampton Meeting of the British Association, a writer (I believe, Professor Julian Huxley) gave a paper on the functional significance of size, wherein he referred to the problem of animals of various sizes dropping down a mine shaft, and the hydrostatic pressure in a giraffe's foot. It is not proper to discuss here the question of physiology, but many of the problems of animal structure and mechanism have features which are common to all structures and mechanism. The reasons which make the bulk of the whale tO times as great as that of the elephant are, in man 7 respects, similar to those which determine the size of the motor onmibus and the ocean liner. The comparative size of the pterodactyl, the largest of nature's flying creatures, with the whale, has a definite relation to the comparative sizes of aeroplanes, airships and submarines. ,<i/<-.-:.> If we discuss two bodies of different size "b\it which are geometrically similar, that is to say, two bodies! which are in all their dimensions exactly proportionate, we have a change of scale usually denned by the ratios of the linear dimensions, that is to say, that when we refer to one body as being twice as large as another we generally mean that it is twice as long, has four times the surface and eight times the bulk, and if the density be the same, of course, eight times the mass. In the case of an aeroplane, however, conditions of aerodynamical similarity make the gross weight of the aeroplane proportional to the surface. This naturally demands a change in average density. We can show, however, that the weight of the structural members must vary as L::. In the case of ties this is easily understood. The forces acting on the ties are pro- portionate to the gross weight, which is in its turn proportionate to the surface (L2). and consequently if the stress in the material is to remain constant as it must do, on the assump- tion that the same materials are appropriately used in all cases, then the cross-sectional area of the tie will be propor- tionate to the gross weight of the aeroplane. In other words, when the tie is enlarged on the same scale as the whole aero- plane, it will still be subject to the same stress, but the member is now longer by virtue of the change of scale of its length, and the weight of the tie is the product of the cross-sectional area and its length, which weight would vary as L3. In the case of struts the same conditions apply ; provided conditions of similarity are maintained, the radius of gyration of the struts will vary as L and consequently the slenderness ratio would remain unchanged. Hence the mean average permis- sible stress remains unaltered, and the struts follow the same law as ties. Similar conditions can equally be shown to apply to struts with lateral loading and continuous beams. In consequence, the whole range of truly structural members. fittings, etc. (truly structural in the sense that the forces on them are proportionate to the gross weight of the aeroplane), become a steadily increasing proportion of the gross weight. It might be imagined that thin hollow members could not follow on similar lines because with reduction of scale they would become so thin as to be unstable, but the conditions for elastic instability depend on the ratio between thickness, curvature and equivalent free length ; thus structural members geometrically similar will sustain the same stresses from con- siderations of sta bility. The lower limitation with regard to the dimensions of members may be affected by having to resist damage in handling, and this as a human factor ma ' be nearly independent of size.or alternatively thickness limitation may be determined by questions of corrosion where the surface volume ratio is an important factor in the structural influence of corrosion. A further restriction on small sizes comes from the problem of manufacturing difficulties, which arise with very small absolute limits. On logical grounds, limits should be on a percentage basis, and, where dimensions become very small, reasonable limits on a percentage basis may be too small as absolute figures to be a practical manufacturing proposi- tion. Generally speaking, however, the influence of these factors is not so large as is often imagined. The fact that the
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