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Aviation History
1933
1933 - 1111.PDF
S3 NOVEMBER 30, 1933 THE AIRCRAFT ENGINEER SUPPLEMENT TO FLIGHT (Ref. 9), where N.60.R. is a reflexed section obtained by reflexing the rear portion of N.60, the sections have been plotted out on squared paper to obtain the centre line by the N.A.C.A. definition. The resulting centre line to datum and the calculation of the no-lift charac teristics are given in Table 8 and Figs. 11 and 12. •06 •05 •04 •03 •02 •01 °< -01 -02 ) •: 1MX) •; %\W 5 X SV f Iu„F,(x)dx. -01737 J'ij°fj(x)dx -01*10 7 J ^/' \. FIG.I2 -> i-3 Section N. 60 R. Calculation of No-lift Characteristics Method of R. and M. 910 by The summary of the calculated values of R. & M. 910 functions are given in Table 9. • TABLE 9 Section N.60 . . N.60 11 ' 0-1245 0 1245 L.E. Slope = <(, f0-279 + 0-251 I T.K. Slope *V yofl(x)dx ! F2= j/o/2(-t)^ o lo -0-115 fO-0368 0-07134 0-01737 0-00702 0-0141 Taking first N.60 section and applying the empirical correction of equation (9) Kw!0 = 1 -4-9 X 01245 X 0-07134-f-0-00702) + 0-0036 X 0-279 + 0-0453 X 0-115 = 0-924 (- 005604 + 0-00702 + 0-00101 + 0-00521) = 0-924 X (- 0-0428). = _ 0-03955 The measured value for Kw0 was — 0 04 so that the igreement here is good. Similarly, for Section No. N.60 R. = 0-924 (— - X 001737 + 00141 + 00036 X 0-251 -0-0453 X 0-0368) = 0-924 (- 001363 + 0-0141 + 0-00090 - 0-00167) = - 0-924 x 0-0003 = - 0-00028 Km The measured Km„ was -0.0005, so that the accuracy of the empirical correction is about the same for reflexed as for non-reflexed sections. The suggested empirical correction to the calculated moment at no-lift gives results which are in keeping with the measured moment at high Beynolds Number, for both reflexed and non-reflexed sections. Further checks may be necessary as more information becomes available, but for all practical purposes it is thought the correction will be sufficiently accurate. REFERENCES ^ "No-lift Characteristics." W. R. Andrews. AIRCRAFT ENGINEER, Dec. 27, 1929. 8 Aviation Handbook. E. P. Warner and S. P. Johnson. 9 N.A.C.A. Technical Note No. 388. " Wind Tunnel Comparison of Three Normal and Three Reftexed Aerofoils." G. L. Defoe. (To be continued.) TECHNICAL LITERATURE "THE STRESSES IN AEROPLANE STRUCTURES"* THERE are three more or less independent methods of improving the efficiency of our aeroplane structures, and these may be classified as the engineering way, the scientific way and the mathematical way. The engi neer's task is to build lighter and even lighter struc tural members to carry loads of a given magnitude. The scientists should contribute more exact knowledge of the types and magnitude of the loads imposed by- various conditions of flight and landing—and this they do, but unfortunately it is very rarely that they advo cate any reduction in load and factor requirements. Lastly, the mathematicians can help by devising more correct methods of calculating first the distribution of the loads throughout the structure and second the effect of such loads on the individual members of the struc ture. Here again it is unfortunate that more correct methods usually involve more complex calculations. It is most refreshing, therefore, to be able to pay tribute to Mr. H. B. Howard as the originator of the most important simplification in stressing methods which has been introduced since the late Arthur Berry- tabulated his famous functions. The method of utilis ing polar diagrams for determining the stresses in beams under compression was first described by Mr. Howard in R. & M., 1233, and is now more amply described in the book under review. The avowed object of this book is to collect together " those portions of the theory of structures which are of particular interest to the aeroplane designer "—and this object has been well and truly achieved. It is, however, definitely the mathematical side of the sub ject that is under consideration, and readers must not expect to find guidance on how to proportion their aeroplane structures or to find any but slight informa tion on the aerodynamic loads imposed on the struc ture. These omissions are not defects, but the natural outcome of a considered policy which is in itself a striking comment on the development of the science of strength calculations. Mr. Howard's book will be an essential part of the equipment of all those engineers whose task it is to deal with the mathematical side of aeroplane structural design. By reducing many com plicated problems to comparatively simple formula?, the author has laid a debt of obligation on the shoulders of those general practitioners of stress-work who do not wish either to puzzle such matters out for themselves or, in the more advanced problems, to browse among the pastures of Phil. Trans. Roy. Soc. Chapters I and II deal with the elementary subjects of " Stress and Strain " and " Bending of Beams." Chapters III and V cover the cases of uniform and tapered struts very completely—hut naturally do not include curves of strength of actual tubes. Chapters IV and VI deal with both analytical and graphical methods for stressing single-bay and continuous beams. In addition to many applications of the polar diagrams already mentioned, we have an account of those im provements in the Berry analytical method which have been devised by either Mr. Howard himself or his colleagues at Farnborough. Chapter VII summarises the standard elementary work on simply stiff frameworks and introduces the Southwell tension coefficients, whose virtues cannot be too often impressed on any who are not yet familiar with them. Chapter VIII on Strain Energy is very- important in these days when all redundancies of struc ture may be taken into account in calculations. The sections "on the bow strain of struts and on the appli cation of strain energy to members which do not obey Hooke's law are both unusual and important. They suggest the possibility of approximating more closely * " The Stresses in Aeroplane Structures," by H. B. Howard, B.A., B.Sc, F.R.Ae.S. Published by Sir Isaac Pitman and Sons, Ltd. Obtainable from FLIGHT Offices, price 20s. 9d., post free. 1198
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