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Aviation History
1923
1923 - 0761.PDF
DECEMBER 20, 1923 LIGHT AEROPLANES AS FLYING SCALE MODELS OF LARGE MACHINES Good Beginning Made in France WHILE on this side of the Channel we appear to have got intothe habit of thinking of light aeroplanes as pleasure craft— intended mainly for sporting flying and, perhaps, later on foruse by the private owner-pilot—there is another sphere of utility which it seems reasonable to suppose can be usefullyfulfilled by the latest type of aircraft. We refer to the use of light 'planes as flying scale models of larger machines.Hitherto it has been customary, when a novel design was contemplated, to have small scale models tested in a windtunnel, and while the usefulness of the wind tunnel is not denied—indeed, most of our present knowledge rests on theresults of wind-tunnel tests—the results so obtained are not always entirely trustworthy. For one thing, there is thematter of " scale effect," or the correction to be applied to model tests in order to make them applicable to the full-sizemachine. Sometimes these corrections are of considerable magnitude. That in itself would be no great matter, butthere is in addition a good deal of uncertainty which way the scale effect will go. Generally speaking, there is an improve-ment in stepping up from model to full scale—that is to say, the actual machine usually has a smaller relative resistance,and often a greater lift, than that indicated by the model tests. in the model, while tests are carried out on stability andcontrollability features as well as on the simpler lift and drag measurements, it seems likely that the total cost might veryeasily reach the cost of building and flying a light 'plane incorporating the main features of the larger machine. Now that the " Million Pound Monopoly Company " isbeing formed, and specifications have been sent out for three- engined machines to be capable of non-stop flights fromLondon to Malta, it might be well worth while to build light 'planes incorporating the three-engined arrangement, andconforming in geometrical shape and proportions, but on a reduced scale, to the larger machine. We admit that thereare difficulties, chiefly connected with the question of power- plants, but these should not be unsurniountable. For instance,if the large machine contemplated were to be fitted with, for instance, a Napier " Lion " in the nose of the fuselage and aSiddeley " Puma " on each wing, it might not be very easy to find two types of small engines that would imitate in relativepower and weight the Napier and Siddeley engines mentioned, but it should be possible to come somewhere very near tothe right proportions. For instance, the Napier " Lion " might be represented by a Bristol " Cherub " and the Siddeley THE DE MONGE TYPE 72 : This photograph shows a wind tunnel model of the machine. The engines willbe Lorraine-Dietrichs of 375 h.p. each. Before building the large machine M. de Monge has had a twin-engined ^g light monoplane built to test out the general arrangement. In some'eases, however, it has been found that the scale effect" goes the wrong way," in other words, that the full-size figures are less good than the model figures. The very factthat designers know that this may be so causes a certain amount of distrust of model figures, although in the greatmajority of cases the scale effect is in favour of the full-size machine or component. Nevertheless, so long as any doubton the point exists there will be a certain amount of hesitancy in accepting model figures. Then, apart from the question of scale effect, there are anumber of things which model tests in wind tunnels cannot very well give, mainly relating to controllability, etc. It istrue that in the latest Duplex wind tunnels at the N.P.L., in which can be tested models complete with airscrews runningat various slip ratios, the actual conditions of free flight can be very fairly imitated, but even so the conditions cannotquite compare with an actual machine flying under its own power and going through all manner of manoeuvres. Itwould appear that it is along these lines that the light 'plane could, and should, be employed to provide at fairly low costa flying laboratory from which much might be learned. The scale effect, even supposing that the light 'plane were butone-third the linear scale of the larger machine which it represented, would be extremely small compared with thatinvolved in stepping up from a model measuring, for the sake of argument, 3 ft. in span, and which represented a full-sizemachine measuring 100 ft. in span. Controllability and other similar problems could be examined by flight on the light'plane much better, or, at least, much more convincingly, than on the most elaborate model, and, last but not least, thecost would probably not be so very much greater. It is a fairly expensive matter to build a small but very carefully andaccurately made scale model of a machine, and if such refine- ments as airscrews driven by electric motors are incorporated " Puma " by a smaller motor-cycle engine, such as theDouglas 500 c.c. The question of speed of revolution and propeller efficiency would complicate the problem, but theactual conditions could, we think, be very fairly imitated. As this use of light 'planes undoubtedly comes under theheading Research, it would seem reasonable that the Air Ministry should, in future Air Estimates, set aside certainsums to be devoted to this particular form of practical experi- ments. Thus, the policy followed at present, of orderingcertain experimental machines so as to keep abreast of the times and having always actual practical experience of goodmodern types, even if financial considerations preclude the building in quantities of such types, could be extended toinclude a greater variety to types without greatly increasing the cost. We are, of course, aware that certain types cannotbe represented by light 'plane flying models, and that the information desired can only be obtained by building theactual machine, but there arc a number of other types con- cerning which it would appear the flying scale model couHprovide most valuable data, and at a small fraction of the expense of the full-size machine. A Case in Point A very good example of the manner in which the smallflying model of a large machine can be used for collecting certain data is provided by the de Monge twin-engined lightmonoplane, illustrated in the accompanying drawings and sketches. For the latter we are indebted to our excellentFrench contemporary L''Ahonaulique. This machine repre- sents a large three-engined commercial monoplane now underconstruction at the works of the French Buscaylet-dc Monge firm. At the last Paris Aero Show, it may be remembered, awind-tunnel scale model was exhibited on the stand of this firm. This model is shown in the accompanying photograph. 761
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