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Aviation History
1919
1919 - 0643.PDF
MAY 15, 1919 IfidUD DRAWING OFFICE DATA By E. O. WILLIAMS, B.Sc.Eng. (Lond.), Assoc. M. Inst. Civil Engineers, Assoc. Fellow R.Ae.Soc.G.B. Concluded from page 599.) VI.—THE NEWALL STANDARD TABLES OF LIMITS. IT IS, of course, well known that in engineering certain limits have to be imposed to insure accuracy and to make provision for the proper fitting together of two or more parts. Absolute accuracy is not humanly possible, and some limit must of necessity be put to the amount by which a certain article may differ in size from that called for in the specification. This margin, providing for what is considered in modern practice reasonable error in workmanship, is usually termed Tolerance, while the necessary difference in the sizes of two pieces that have to go together is known as Allowance. Fig. 38 is a table of these Tolerances and Allowances, collectively termed limits, and are known as the Newall Standard Tables of Limits, which are employed in aircraft construction. These tables will be self-explanatory, and do not, I think, require any comment. VII.—ON PLOTTING GRAPHS OF WING SECTION DATA. In an aircraft firm intending to build machines of their own design, it is a matter of the greatest import ance that some degree of uniformity should be estab lished and maintained in the matter of planning out the various types of machines which it is intended to build. As the choice of a suitable wing section for the purpose in view is of paramount importance, it would be well to decide beforehand on some sys tematic method of plotting curves of the lift coefficient, L/D ratio, travel of the c.p., etc., for all the wing sections of which these data are available, so as to have always ready at hand all the particulars required for purposes of design. In deciding on the form these records are to take, it should be kept in mind that their primary object is to facilitate the choice of the section giving the best results for the particular purpose aimed at, and that, therefore, the method which combines the easiest comparison with full particulars will be the most convenient for this purpose. There are in general use several methods of plotting wing section data. Thus, we have the Eiffel method, which is, I believe, pretty generally used in France. In this country the N.P.L. system has been generally adopted, although alternative systems have been suggested. For example, Mr. Handley Page, in his lecture before the Aeronautical Society, entitled " The Case for the Large Aeroplane," outlined a method which lent itself better to a comparison of different wing sections than the systems more gener ally employed. In Fig. 39 is shown a method of plotting wing section data evolved by the writer, which he has found in practice to answer the requirements of the '*. itUCH.41 I 5 ST I 4 r 2 3 a Si '•O J> 2 PI J ita U I 1 I ro b 01 0 pie 3 <>l,<4 Mt> *• *8 r- a: 2 S H o 2? P) 2 Pi* 4 a Pi ta I ro 1 b 2 1 5: r J 3 n » 1 r ft* I 1 I 01 & r- r o r> w * * •v O C t* ?j (A ! o ft tT 3 15 15 3r 1 I, N 1 SI r- Pi 5 3 6 PI tn Fig. 38. 6*J
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