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Aviation History
1940
1940 - 0813.PDF
MARCH 14, 1940 251 DYNAMIC SIMILARITY Saunders-Roe Produce a " Flying Scale Model" A IRCRAFT production has become r\ an expensive business. Time *• *• was when a new aeroplane could be (in the early days of flying it had to be) produced for *a few- hundred pounds. But size has in- creased, materials have become ex- pensive, equipment .has multiplied a hundredfold, and modifications have become almost impossible once a tvpe has gone into production. One result of all this is that designing "by eye" or "by guess" no longer meets the case. Not only so, but the most elaborate calculations, the most careful tests of scale models in wind tunnels, and even tests at high Reynolds Number in compressed-air tunnels and with airscrews running are apt to leave unknown, or at least doubtful, features which may assume important dimensions in the finished machine. Even the so-called " dynamic model," that is to say, the model in which not only the external shape but the e.g. position and the moments of inertia are truly represented, cannot be relied upon to provide all the information sought in the early stages of a new design. This, of course, is particularly so in the case of a design which departs considerably from any previous ones of which the firm in question has had experience. During recent years there has been a tendency among air- craft manufacturers to go to the trouble and expense of building actual scaled-down models of any important new type proposed. In this way all features which might be doubtful can be thoroughly examined and tested, and modifications are nothing like so expensive to make as would be the case if the full-size machine had gone into production. To be truly comparable with the large aircraft which it represents, the flying model must be in every way a replica flight ' photograph. The somewhat unusual shape of the steps is clearly visible in this view of the SARO . . Flying Scale Model. . , • ';.• . • • "•-..' - K •..,- •• . '*'-•' .--••• •-••.- -. ,-., ». o'f the full-size machine. In the airframe itself this "does not present very great difficulties. It is different with the engines, however. Strictly speaking, these should be exact scale models of the "real" engines which it is proposed to use, and the airscrews should be to scale also. That is an ideal which obviously cannot always be attained. It so happens, however, that the Pobjoy engine of some go b.h.p. comes very close to being a scale model of modern large radials. "What helps is that, in spite of its relatively low power, the Pobjoy engine has reduction gearing for its airscrew. The latest firm to build a flying scale model is Saunders-Roe, Ltd.; Of Cowes. The new machine was demonstrated last week by Fit. Lt. Ash, and created a very favour- able impression, not only being fast and very manoeuvrable in the air, but showing good sea qualities, clean running and absence of porpoising. That the actual Saro flying model is not a very tiny machine is shown by the fact that the overall length is 42ft. 8|in. and the wing span 50ft. If it is assumed that the large fly- ing boat of which this is a model is to be two and a half times as large, we arrive at a four-engined flying boat of 125ft. wing span. There is, of course, no reason why the full-sized machine should not be much larger than that. The similarity, except possibly for percentage structure weight, holds good. " flight " photograph. Fit. Lt. Ash, who hascarried out the test flying of the SARO Flying ScaleModel. This " Flight " photograph.view of the SARO Flying Scale Model at moorings gives a good idea of the relatively narrow beam and the tumble-home sides. In the full-scale machine the outboard floats will be retractable.
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