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Aviation History
1928
1928 - 0511.PDF
JUNE 21, 1928 47- THE AIRCRAFT ENGINEER SuPPLKMKNT TO FLIGHT shape of the final rolls or dies in order that the material may spring back to the designed shape. The formula generally used is :— 1 1 2/ R^R^ET where Ro= the required radius of the portion of the roll or die gap curve under consideration. R = the designed radius. E = Young's modulus of the material. t = the strip thickness, and /= a figure dependent on the strength of the material. The problem of spring-back is of extreme complexity, and the writer can give no mathematical justification for using this formula, but where / is taken as the ultimate tensile strength of the material, a fairly close approximation is obtained to the desired shape. If / is taken either as the elastic limit or the yield point, the material will certainly spring back far too much. As a matter of fact, the value given for/should be varied with the thickness of the material. In cases of very thin strip it is often found that the value of / for exact results exceeds the ultimate tensile strength of the material, also the spring back is governed to an appreciable extent for any particular arc by the shape of the section at the extremities of the arc. A typical variation in / with thickness is shown in Fig. 4, and in Fig. 5 the relation between R and R_ is shown graphi- cally for constant values of t and /; it will be noted that for very small radii the spring-back is small, but it is rarely negligible. The designer of the corrugated sections should also be responsible for the manufacture of the tools. Then by careful observation, and by having templates of finished sections made and taking measurements of these, he will quickly collect data enabling him to obtain results with all the precision that is necessary for this work. If the material is oversize, or where considerable variation of strength occurs along the length of a strip, irregularities will ensue in the formed section. The technique of the manufacture of heat- treated steel strip has greatly improved during the last 18 months or two years, so that now trouble due to either of these causes is of rare occurrence. Where reduction in thickness of strip is brought about between rolls, or in a die due to either faulty tools or oversize strip, then bad forming results will be obtained. It is not desired to give the impression that the production of corrugated strip is an extremely difficult or fortuitous matter. On the contrary, it is an operation in metal construction which, given the proper conditions, causes very little trouble. (To be continued.) THE APPLICATION OF AERODYNAMIC DATA TO THE STRUCTURAL DESIGN OF AIRCRAFT. By FRANK KADCLIFFE, B.SC. A.R.Ae.S. Fifteen years ago the science of aerodynamics was slowly taking form from the accumulated experiments which had been performed and made public by the various aerodynamical laboratories of the world. In many ways much that had been observed was undigested and the inferences to be gleaned were far from clear. Even so, aircraft had been built during the whole of the war period and many of the designs were certainly extremely successful. But the design of aircraft up to this period of ten years ago was essentially one of development rather than the try-out of new ideas. Experience in past types was the chief factor in the development of a new type, and the general application of scientific principles was viewed with great misgivings. The past fifteen years appear to the writer to have been years of consolidation in aerodynamic knowledge with the result that, today, as never before, aircraft design is becoming more scientific and less the product of rules of thumb and empirical formula. The scientists of Germany, America, and our own country have been largely responsible for giving shape to these present- day accepted principles and for establishing some degree of ordered experience from an otherwise state of chaos. The work is by no means completed, but it is believed that the whole conception of aerodynamic theories is now resting on a sure foundation. What is still absent, however, today, which would greatly accelerate the framing of further principles, is co-operation in and an international discussion on theories and views in aerodynamic ideas. It is not by any means easy to see how such a pooling of ideas and a discussion thereon could be effected, and consequently the way it is being done at present is by the assimilation by the keen student of the other countries' points of view on certain matters, and the adoption, wherever practicable, of what proves to be the best inter- pretation of phenomena. This is a field in which all keen students of aeronautics cun actively take a part in research work and help thereby a speedier adoption of the best explanation of aerodynamic principles and phenomena. The object of this series of articles is to place before the younger members of our design staffs a few applications of aerodynamic data and to make reference to published reports where further information can be gleaned for a deeper study of the subjects referred to. The junior members of our designing staffs are often keen enough to improve their knowledge and experience, but are handicapped from the beginning because they do not know where to begin, what is important and what is not. Technical notes are issued at short intervals by several countries, but the practical man finds the application of the same difficult unless he has first the preliminary guidance. If these notes serve in making R. & M.'s hitherto useless of real practical value, then the writer responsible for these notes will consider his time to have been amply repaid. The primary object of these articles will be the application of aerodynamic data to structural design and it will be as well to leave for a later article the consideration of the choice of an aerofoil and concentrate entirely on the derivation of lift, drag and moment curves of a given aerofoil for application to stressing the wings, body and tail of an aircraft. These will be derived both graphically and algebraically, as both have their useful application. b U_ O LJ VA 0 •b •b •<L DRAG VALUES \ •05 10 j oi 1 i •15 INCIDENCE of BIPLANES T«E FUNCTION :F -R &M723 •u. g GAP S1+S2 * MEAN SPAN •20 25 <T (AFTER PRANOTL 5, RATIO OF SP ' ^ " OF TWO WINC VNS S 30 -35 40 1 Fie •4S 1 1 1 ==« 1. •50 An examination of aerofoil data will reveal the fact that the lift curve plotted against angle of incidence as base, is for all practical purposes straight from the no lift angle up to about 0-9 A-Lmax. This being so. the lift curve is con- veniently expressed in the form kh = Aot-f B (1) Where a is the angle of incidence in degrees and A and Bare constants which can easily be determined. The km curve which in this country expresses the non- dimensional moment coefficient of the wing referred to its leading edge is also conveniently expressed as a linear equation in the form Jfcm=C*L + D (2) or km = Ea + F (3) where C, D, E and F are constants and can be determined from the lift curve equation. (In passing, it should be noted that the American practice is to give the, moment coefficient about a point in the wing which lies at a quarter of the chord length behind the leading edge of the aerofoil.) 464c
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