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Aviation History
1918
1918 - 0171.PDF
FEBRUARY 14, 1918. "A POST-WAR AEROPLANE." V." By F. W. HALLIWELL, A.MJ.I.E. [Concluded from page 151.) «« Points Affecting Aerodynamical Efficiency." SPEAKING broadly, the aim of the designer is to obtain efficiency, that is, to be able to carry the maximum load over a given distance with the least expenditure of energy. For fighting machines two points only are of primary importance, e.g., speed and rate of climb. Unfortunately, the essentials for these two are diametrically opposed to one another inasmuch as the first requirement for maximum climb is low velocity in order to secure the best lift/drift ratio. Since both of these are desirable characteristics in the majority of machines, it will be seen that the solution of the first problem must be in the nature of a compromise. Considering an aeroplane in horizontal flight, the necessary conditions of equilibrium are maintained by balancing the forces acting upon it. The two which have to be overcome in order to obtain flight are W, the weight of the machine acting vertically downwards, and R, the total resistance of the machine to forward motion. W is balanced by L, the " lift " of the wings, and R is balanced by T, the thrust of the propeller. It will be seen, therefore, that if we wish to better the performance of any machine we must reduce either W or R. Whatever the purpose of the machine the reduction of head resistance, or more correctly " drift," is of great importance. The total drift of an aeroplane may be conveniently divided into two parts. Firstly, the drift produced by lifting surfaces, With the object of weight reduction in view, there are many reasons for giving particular attention to the weight per h.p. figure of the engine. It is the heaviest single item, and including radiator, exhaust pipes, &c, probably weighs about 40 per cent, of the total weight; therefore any successful attempt to reduce its weight appreciably brings down the weight per h.p. of the whole aeroplane. It may be argued that the limit in this direction is nearly reached, but progress is undoubtedly being made, and up-to-date machines show more signs of becoming lighter in this than in any other component. Some of the more recent six-cylinder in line engines show considerable reduction in weight per h.p. with the additional advantage that they are more easily adapted to a well stream- lined fuselage than is the Vee type. It would seem advisable to consider making the radiator an integral part of the body or wings, with a view to reducing resistance, as well as econo- mising weight; attempts have been made at this in several German machines. The possibilities of lightening the fuselage or wings do not appear to be great, but something may be achieved with an entirely new method of construction. The weight of the bracing wires and turnbuckles, &c, used in present-day machines is considerable, and new designs show a tendency to eliminate these as far as the body is concerned LLLVATIOM WITH nALr or" SHELL BCMCVED i rUSLLAQL CONSTBUCTION-. SUCTION or WiNG SPAR. Fig. 7. rMD VTW TAkTM SX A.'K i.e., the wings ; secondly, the drift produced by detrimentalsurfaces, such as fuselage, struts, under-carriage, &c. Once the wing section is fixed, the resistance of the wings cannot be reduced, and being a component part of the re- action, increases as the square of the velocity (approx.), as does the lift; so the efficiency of the wings remains the same at all speeds. The drift, due to detrimental surfaces, however, also increases as the square of the velocity (approx.) without any attendant increase in lift; thus it will be seen that any means of decreas- ing the detrimental surface or drift due to same is of great value, more especially so in high speed machines. In a machine of orthodox design about 50 per cent, of the total drift excluding wing resistance is due to the engine and fuselage ; and about 20 per cent, due to wiring, e.g., cables and controls. It will be seen at once that here are two serious factors to be taken into account. Considerably less resistance is set up by streamlined objects. The "finess," e.g., the proportion of length to breadth of the streamlined shape is dependent upon the velocity and is found by experi- ment. Obviously, therefore, it is necessary, after having brought down the projected area of all detrimental surfaces to the minimum, to carefully streamline same, and it is in this respect that recent machines have shown improvement. Upon investigating the weight of up-to-date machines it is difficult to see how any great progress can be made without lowering the factor of safety, which, under war conditions, is not desirable. either by substituting 3-ply bracing, also doing away with doped canvas, or alternatively employing a monocoque type of fuselage. Suggesting a Method of Construction. From the foregoing it may be seen that in setting out to arrive at the best possible construction of an aeroplane, we must, consistent with the necessary strength, consider before everything else the questions of streamline shape and weight reduction. Taking first the fuselage ; to the author's mind the elimination of wood, canvas, and to a lesser extent wire bracing, is the keynote of improved construction. The stresses in the body are many and varied, and in extreme circumstances the whole of the fuselage may be under severe combined bending moments and torsion. An all-metal construction, preferably of duralumin, would seem to possess many advantages over the existing type, in addition to being non-inflammable. Duralumin is suggested here because it is the metal which most nearly approaches the standard of strength for weight required, and should not necessarily be regarded as absolutely suitable material. Proceeding along these lines the method shown in Fig. 7 would appear to commend itself. In this system we still retain equivalent members to the longerons, but in all probability these need not be more than three in number, and would be of channel or tee section, built up as indicated, running the whole length of the body. At intervals would occur trans- verse stiffening members lightened out and flanged at all edges; these are riveted to the shell and connected also to> 167 "" G 2
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