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Aviation History
1928
1928 - 0228.PDF
MARCH 29, 1928 has been designed as a three-seater day bomber, and that therefore certain specified loads had to be carried, loads consisting partly of equipment, partly of machine gun arma- ment, and partly of bombs. What percentage of each is involved we are not in a position to state. Aerodynamic Design Those of our readers who have followed his interesting series of articles on " Aircraft Performance " in our monthly technical Supplement THE AIRCRAFT ENGINEER will have obtained a fairly good idea of the general design policy of Mr. J. D. North, Boulton & Paul's Chief Engineer and Designer, and in examining the " Sidestrand " one looks for such features as Mr. North has advocated in his articles. Among these perhaps none was more prominent than the reduction of induced drag by having a high value of the ratio Span2of —^TT +> and a glance at the general arrangement drawings and some of the photographs will show that the " Sidestrand " has a very large span for its area, or, as we used to say before modern aerofoil theory became the fashion, high " aspect ratio." While Mr. North drew attention to the importance of large span, he also pointed out that for large machines it is difficult to obtain a high value of the span2/-wt. ratio because of the increased wing weight which quickly puts a limit to the span which it is economic to employ. In the " Side- strand," therefore, one may take it that an endeavour has been made to get the best compromise between wing structure sideration of the operational conditions to be met, and then, taking as a basis a good streamline shape, curving its centre line to give the required aerodynamic characteristics, the original streamline section being chosen of such a thickness that it will accommodate spars of sufficient depth. Thus, in any Boulton & Paul machine one is not likely to find any stereotyped wing section, although some of those in use majr, more or less accidentally, have a fairly close resemblance to certain " accepted " sections. Incidentally, the original streamline shape taken as the basis is generated by the general- ised Schoukowsky theory. The wing cellule having been carefully designed to meet the particular operational conditions of the type in question, great care is again taken in the design of fuselage and engine nacelles. In the case of the " Sidestrand," for instance, a start was made with a body of very good streamline shape, generated as in the case of the wing sections, a model of which was tested in the wind tunnel. The cockpits were then added one by one, the drag being measured after each such addition. If a certain cockpit shape or arrangement was found to add unduly to the drag, modifications were made until the figure had been reduced to what appeared to be the lowest practic- able value. Take for example the prone gun position under the fuselage. Obviously this might easily increase the body drag to a very high figure, but by persistent experimentation the drag caused by this gun emplacement was ultimately reduced to a very low value indeed. The engine nacelles were the subject of similar research THE BOULTON & PAUL ["FLIGHT" Photograph SIDESTRAND " : Three-quarter rear view. The careful streamlining of the engine nacelle can be seen in this photograph. weight and aerodynamic efficiency, and it will be of interest to examine briefly how far the wing arrangement of the " Side- strand " may be expected to have reduced that part of the wing drag which is due, as Mr. C. C. Walker puts it to " carry- ing a certain weight on a certain span at a certain speed." The total loaded weight of the " Sidestrand " is 8,850 lb., and the span is 72 ft. Span 2 The value of -~r- is therefore 0-518W and the monoplane value of the ratio of lift to induced drag is, at 70 m.p.h. for instance, 20-31. As the gap .'span ratio of the " Sidestrand " is about 0-14, this value is increased to 25-9 for the biplane arrangement used. Thus at 70 m.p.h. the induced drag is only 342 lb., which is remarkably low and corresponds to a thrust horse-power of 64 b.h.p. only for over- coming induced drag at that speed. Since at this low speed (corresponding probably fairly well with the climbing speed of the machine) the induced drag is a large percentage of the total wing drag, it is seen that the " high aspect ratio " wing arrangement does appear to have proved extremely beneficent. The span-/weight value of 0-518 is quite high for a machine of this weight, and in a number of machines this ratio only reaches a value of 0-3 or so. We believe that actually in the " Sidestrand " the extra wing weight which was the " price paid " for the higher value of Span2/W amounted to some 200 lb., but at that it paid to carry the extra weight. While on the subject of wing design, a few words concerning the method used by Mr. North and his staff in the choice of wing section may be of interest. The method was outlined by Mr. North in his series of articles to which reference has already been made, and consists in starting off with a con- and the form finally chosen, which is well shown in several of our photographs, has given about as low a drag as it is possible to attain with engines placed outboard. The research included wind tunnel tests with model airscrews running, and at large angles, it having been found that the " interference drag " is largely an induced drag and liable to be greater at large angles, thus affecting performance on climb, etc. Undercarriage design, although perhaps more of a structural than an aerodynamic problem, also shows this striving for aerodynamic " cleanness," the undercarriage of the " Side- strand " being of remarkably low frontal area for a machine of this size. Altogether the Boulton and Paul " Sidestrand " is a machine which well repays a close study, the results of the very great care taken in its aerodynamic design being reflected in the performance figures which will be found at the end of this article. Structural Design If the aerodynamic design of the " Sidestrand " is of more than usual interest, the same applies at least as much to the structural features. Although in what follows the reference is particularly to the " Sidestrand," most of the constructional details are now standard Boulton and Paul practice, and would apply fairly closely to any machine built by that firm, since a process of standardisation without cramping the freest possible development has been evolved by the firm during the last few years. It will be known to most of our readers that Boulton and Paul were among the very first aircraft firms in England 208
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