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Aviation History
1919
1919 - 0562.PDF
square 3,700 x -21.68 1538 5,215 lbs. Compared with the 3,700 lbs. strength found for an I section strut of the same length and area (which, of course, is merely another way of saying of the same weight), this figure promises exceedingly well for the X section. Up to now we have only considered the advantages of the X section from the point of view of strength. No less important is the question of manufacture, and a few brief notes on this subject may not be out of place. Reference to the section in the upper left-hand corner of Fig. 26 shows that as the X section is symmetrical the four hollowed-out portions are all alike, i.e., of an angle of 90 deg. and a radius of -2 in. at the bottom of the channelling. This means that one set of cutters for the spindle machine will suffice for the four sides of the strut, a considerable advan tage, since a number of cutters can be made from the same template. It will further be seen that if the cutters are origin ally made sufficiently large, they can be used for struts of all sizes, the angle being 90 deg. The thickness y can, with the same cutters, be made any thing desired by choosing suitable stops when spind ling, thus pushing in the cutters to a greater or smaller extent. As a manufacturing proposition, therefore, the X section easily holds its own against the I section, so that any way one looks at it the X section may be said to lend itself extremely well to aeroplane con struction. Before leaving the subject of the X section, it may be advisable to point out that where this section is employed for very long struts there might be a danger of the flanges y (Fig. 26) buckling before the load on the strut reaches the value calculated by the usual strut formulae. To prevent this, webs should be provided at intervals, the distance between webs MAY I, 1919 clearly depending^ on the thickness y. The writer suggests that a web distance of not greater than 40^times y will be sufficient to prevent buckling. For^the numerical example just given this web distance would amount to -26 x 40 = 10-4 ins., so that for safety there should be left a web in the centre of the free length of strut, which was 20 ins. (c).—Square and Circular Sections (Solid and Hollow). The comparisons made have, up to now, only included the I and X sections. To compare every conceivable section possible would be out of the question in an article like the present, but there are some which are in general use, and which will, therefore, be included in the comparison of sections. These are the solid and hollow square sections, and the solid and hollow circular sections. In Fig. 25 curves have been plotted for these sections. As a basis for comparison were chosen solid, square and circular sections, ranging from -8 in. square and •8 in. diameter respectively to 1-8 in. square and 1 • 8 in. diameter respectively. In the case of the hollow sections the thickness of the walls of both square and circular sections ranged from -2 in. to •28 in. in the manner shown in the table in Fig. 25. From these curves it will be seen that the solid square and solid circular sections are very inferior to both I and X sections, the two curves indicating that there is little to choose between the square and the circular section. So also in the case of the hollow sections, the curves for which practically coincide. Both are, however, superior to the others considered. This indicates the advantages attending the employ ment of hollow sections for struts of the overall sizes considered. Of the two, the hollow square section is already very extensively used, whereas the hollow circular section is not greatly favoured where wood * s N & f h 53; * £> h> 5 ft *> 5 i3* P c a i« <» 5ft £31 1 r 5 * • - t 0 , 7S ': 2 M 0 fe § . C5 V. •ft to 09 (AI °5> t <> 6- -^ •S) to •5 U 0. U to to. •3 8*- *•. vj <« 6* to 0. v. •V. * »o G~ "3 to to • ^ -". «•. to *• to to i ^. *• to *• to °5 G- fi m ••" ^* On to N> •O •0 -0 la m •^ N to to to 6^ to •0 HIT •**. - N> to to "1 ^ 6-- to <s» ^ to b •0 •0 §• -* *o to 3? . 0 C> °3 °3 K> O, In to >fc. to. 5 <*> s a x 3 3 r 1 5 1 a K 5 K 3* S P 3 1 5 Ki * 3* if ? 1 11 «, i :» - * 1 %\ - « \V" 0 A w< nr ... / i lb O to 5 2 0 H 0 * WWW 5 \V3\i\ » (b Is, r> Pri <5 r> ki Hi Kr, to ft b \ioy r y |y ^ ^ 1 ! •i i 1 •L Fig. 26. 562
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