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Aviation History
1932
1932 - 1319.PDF
DECEMBER 29, 1932 91 THE AIRCRAFT ENGINEER SUPPLEMENT TO FLIGHT even if very thin panels were available at a commercial price. But for large frames having heavy shear loads, such is not the case. Thus, ia general, it may be said that at the present time, for lightly-loaded structures, fabric covering is the more economical, whilst rigid panelling can be used economically where the shear loads are large. There may be, and probably is, an intermediate class of craft in which plywood covering may be used more economically than either. The number of machines of medium size in the Show which had plywood panelling seemed to support that view. In the case of cantilever wings, however, where torsional stiffness and strength are of about equal importance, the use of rigid covering must receive every considera tion, and reasonable sacrifices in weight for small air craft appear to be fully justified. Before delving further into this subject, the reader should recognise that the desire for great torsional stiffness has been the spur behind the development of rigid-covered parts, and not the economy of weight that might be effected by its use. The problem before the designer is the development of economic stresses in the skin under forces in the plane of the covering. In the case of a girder member the problem is easy. The load on the member is known. If the shape of the member renders the mathematical determination of its strength an impossibility, or if uncertainties arise, them the member is put in a test ing machine, the necessary observations as to its strength and stiffness under load are recorded, and one or two such tests may cover the whole wings of a machine. In the case of the panel-braced frame, curved or otherwise, the problem is nothing like so simple (apart from the difficulty of making a few panel tests representative of a complete monocoque body). Con sider a simple sheet, loaded as shown in Fig. 2. Under quite small loads the panel deforms, wrinkles appear in the manner indicated by the broken lines, whilst a sec tion of sheet viewed at A—A takes the form shown in Fig. 3. By the use of stiffeners secured to the sheet, the formation of these wrinkles can be delayed. Within certain limits this wave formation is elastic, and the flat surface of the sheet is restored on the removal of the load. The question at issue is what degree of elastic deformation can be permitted on the surface (body, wing or other component) of an aircraft so covered, i.e., what depth of elastic wave is to be considered the maxi mum. Obviously plastic deformation could not be per mitted, and it would appear that the answer to the question will only be found through prolonged actual use and experience with this class of structure. The best sections to use for the framing, the best disposition of these parts, and best interconnection combined with the most economical thickness of material may, and probably will be determined in the laboratory. Doubtless such experimental results will subsequently be verified mathematically. The case of the plain sheet has already been exhaustively examined by H. Wagner. The static loads at which buckles, etc., appear may be accurately predicted by experiment and theory, but the essential question as to what elastic deformation can safely be allowed will only be settled by users and operators. This is analogous to the old question of structural flexibility. Many an aircraft has attained a bad reputation, though perfectly airworthy, by a definite lack of torsional stiffness in the fuselage or tail, and the same thing would inevitably happen if por tions of covering of wing or fuselage were seen to pucker under varying loading conditions. Only experience will show to what extent this can take place without causing trouble. At present, when the panel-braced structure, particularly in metal, is a comparative rarity, this puckering is not unduly criticised, but in the future, when many different types and methods will render comparison easy, this non-scientific matter of appearance will be a matter of first importance in design. It seemed that this factor was, apparently, already anticipated to some extent by several French constructors, notably Wibault, who had spaced his in ternal reinforcements, particularly in the wings extremely close together. No measurements were made' but the free panel area did not seem to be more than about 3 in. by 3 in. The question of reinforcement is also bound up with external appearance from the point of view of dis guising initial buckles in the sheet, as well as bad appearance due to the formation of the elastic waves previously mentioned. Stiffening of sheets against local handling must be kept in mind also. How much the spacing of stiffeners is bound up with stress require ments, and how much with other considerations, one is unable to say, but the stiffener spacings on the Wibault wing-covering agreed, approximately, with that already established as necessary for the development of the yield strength of the material used as panel-bracing in a plane frame when subjected to, and reinforced against, shear loads applied in the plane of the frame. The provision of a panel of the correct thickness and properly reinforced for sustaining the shear load is, of course, only part of the problem. Direct loads have also to be supported, and this matter likewise has so far received little attention. The only published literature on this subject that the writer has know ledge of is a paper by Capt. A. B. Miller, entitled " The Effective Width of a Plate supported by a Beam," published by the Institution of Civil Engineers (Publication No. 83). It is understood also that papers bearing on the subject have also been presented to the Institution of Naval Architects, but references are not available. It is stated in the above paper that " . . . . it appears to be the usual practice to take 40 to 80 times the plate thickness as the effective width . . . . " that is, to the constants for the framing members is added the moment of inertia, etc., of the above width of plate, taken about the appropriate axis. Taking a mean figure of 60, i.e., 30 times the thickness on either side of the centre line of the frame-member, we see then that the development of high stresses in approxi mately flat covering is a hopeless proposition. The weight and amount of work involved in fixing the necessary stiffness puts such a design out of court. Another aspect of the problem is, however, the extra strength or reinforcement that the covering gives to the stringers, and from that consideration the use of rigid covering has been proved to be a practicable proposition. Undoubtedly the lightest and most effective method of reinforcing a sheet against shear, and particularly against longitudinal forces, is by corrugation. It has, however, been definitely established that deep corruga tions, even those parallel to the direction of flight, are detrimental to performance, while small corrugations cannot be of much assistance from the strength aspect. Such corrugations, one imagines, were introduced into aircraft at the exhibition for the purpose of disguising initial buckles, and usually they were ineffective in doing so. It is unlikely that such creases would affect the performance of the aircraft, least of all those on the Nieuport-Delage, which might have been \ the depth of those on a Junkers. What is continually in dispute regarding the matter of increased drag due to non- plain surfaces are the ridges often used for con venience in manufacture. As already stated, the P.Z.L. exhibited one set of wings having ridges of the Wibault type and one plain set. Information was furnished that the difference in performance was very small indeed; consequently to split what difference there was and still retain ease of fabrication of the wing, it had been decided to use ridges on the underside of the wing only, which appears to be good common sense. The use of deeply-corrugated coverings is not obso lescent, although no aircraft so covered were exhibited. For slow-flying aircraft, say up to 125 m.p.h. or so. 1232 c
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