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Aviation History
1912
1912 - 0832.PDF
(/jjGHT] 70 per cent., and for this reason I call the other constant •, the " anticipated efficiency " of the machine. In the Military Trials there were machines with « = 45 per cent. and t = 120 per cent., but those that did best among the monoplanes had «-8o per cent., and the Cody biplane had «=6s per cent. It is clearly a question to determine, therefore, what efficiency may reasonably be anticipated for any class of design ; and this data, obviously, can only result from the careful compilation of accurate BASIS-.GLIDE«1.n6. Er FICIENCN' E«TO> practical information such as the Military Trials afforded. At the moment I should say that a good monoplane may anticipate 80 per cent., if its main object is to fly very fast; and that a biplane, as ordinarily constructed, is most likely to realise its full virtue as a flying machine when designed for an anticipated efficiency of about 65 per cent. If the machine is designed to an unreasonably high value of «, the wings will be so small for the weight that the climbing and speed range will both be curtailed ; while, if the value of e is unduly low, the large wing-surface that results will give unusual facility for slow speed flying and manoeuvrability, but at the expense of absolute maximum flight speed. So it would appear that there is a particular speed that is best for the particular purpose, and that there is a particular relationship between weight per sq. ft. and weight per h.p. that is best suited to attain it. The question is how to arrive at these proper values when, instead of a machine that is already built, we seek for assistance from the figures to help us in starting upon a design. In the first place it is obviously necessary to determine the object of the design with greater precision of detail than mereVy to say BA51S: SLIDE "-1 in 6, EFFICIENCY €--SO% 4S 70 NPH 50 6O FLIGHT 5PEED *' I want to build the best aeroplane." It may be said that every entrant in the Military Trials regarded the adjective as applicable to his own machine, althought it is still more certain that the judges did not. For the purposes of this article I must assume that the object can be specified by deciding upon the efficiency to be anticipated, i.e., to fix in advance the value of t, and, indeed, at the moment, it seems to me that in the selection of a suitable e the designer sums up his view of the situation in the most concise SEPTEMBER 14, 1912. practical form that is possible. The selection of e must, of course, itself be based on personal experience and study of the subject. Thus, it does not follow that the machine that is designed for the highest t is the best for the purpose in view, as witness the selection of the Cody with 4 = 65 per cent, for first prize in the Military Trials when there were other machines with t = 80 per cent, which also realised their anticipations. Another point of importance, too, is that the anticipated efficiency c is not necessarily the limit of the machine's attainments, provided that the anticipation has been intelligently moderate, e.g., a machine designed for 1 = 65 may perhaps demonstrate 70 per cent, or more. On the other hand, over great expectations or an unduly small claim in the matter of t does prejudice the opportunities for high speed unfavourably. With these explanations let us proceed to try and learn more about the relations that exist between the four factors of weight, power, speed and area, on the assumption that a definite value of e has already been selected as the basis for the design. In order to facilitate this inspection, I have prepared a number of diagrams, each of which relates to a different value of 1, but all of which assume that the machine has a gliding angle of 1 in 6, and that the resistance remains constant independent of the speed, which is not true in itself, although it is a convenient basis for the purpose in hand. On any chart, it is permissible to select any point on any one curve as the origin of the design, but, having done so, then the point on the other curve that lies on the same ordinate (vertical line) must not be exceeded. Thus, suppose we have in mind a fast monoplane with reasonable all round qualities that will weigh in flight about 1,900 lbs. and the purchase of a certain engine that is known to give say 80-h.p. (The fact that engines are only BASIS' GLIDE -\ m 6, EFFICIENCY € sr 100 °/c 45 70 SO FLIGHT SPEED M-PH available in certain sizes necessarily limits the choice, and makes it necessary to have some particular motor in mind from the very first—the area of the wings is really the only factor that can be adjusted precisely to the requirements.) The origin of the design is thus a load of 24 lbs. per h.p. approximately. Being a speed machine, the object of the design will be to realise the highest possible overall efficiency, which for the moment, and in the light of the Military Trials, we will assume to be 80 per cent. Turning, therefore, to the t = 80 per cent, chart, we find that a load of 24 lbs. per h.p. may be accompanied by 6*25 lbs. loading on the wings and should result in a flight speed of 75 m.p.h. The wings, therefore, should be 304 sq. ft. in area and have an effective angle of 12J0. To make them of less area than this is to anticipate a higher efficiency than 80 per cent., which, while it may produce a slightly higher speed, is certain to curtail the range and all round merit of the machine. If speed pure and simple is the basis of the design that includes a given motor there is, of course, no other solution than to strive after the highest possible value of e, but whenever the anticipation exceeds the best values established by experience, the design courts failure and even, perhaps, disaster. Progress indeed must be encouraged, but the proper way to develop is from the sure foundation of a machine defined for an efficiency that is attainable by all, but so well designed that its actual performance outstrips its anticipations. In effect, this is to say that a designer ought not to try and gain speed by increasing his wing loading until he sees his way clear to make an equivalent reduction in the total weight of his machine such as will leave e unchanged. This principle is, of course, itself open to abuse, because it is affected by that very vital matter of the 832
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