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Aviation History
1918
1918 - 0206.PDF
J^j^fissssi FEBRUARY 21, 1918. cube scale is one-third (Linear Scale :—full size), therefore I of whole section about XX is given by :— 3-4 X 4_ 13.61 (ins.)*, which corresponds to the figure given in the particular pocket book of standard sections for this size of section, i.e., 5 ins. x 3 ins. As a further example, take an oval-shaped tube. (Fig. 2.) This was drawn 4 x actual size, but because of symmetry again only one quarter was drawn. The cube figure was drawn in the usual way, i.e., the actual ordinates are cubed and then drawn to scale 100 times full size for convenience. The area of the cube figure for the whole section = 5.59 x 4. FIG. 2. TUBE SECTION .4R.EA "j OP • y CUBE ,X|FIGURE. Jr Now linear scale = 4. Cubes scale = 100. .•. I of whole section about axis XX _ 5-59 -4— = .01863 (in.)*- and to save time in getting out the values of I for such sections, the accompanying graph (Fig. 3) has been made, showing values of I of circles of diameter .2 in. up to diameter 2.0 ins. They are plotted in such a way that a good degree of accuracy may be obtained by mere inspection of the graph. It will be seen that the scale of I is not constant, and consequently the values of I are written on the curves themselves, but the curves themselves are all successively continuous. As an example of one of the many uses of such a graph, take the case of a metal tube having an outside diameter of .875 in., and gauge thickness = 20 = .036 in. Therefore inside diameter = .803 in. From the graph the I corresponding to a diameter .875 in. = .02876, and I corresponding to diameter .803 in. = .02038. - Hence nett I of tube = .02876 — .02038 = .00838 being the difference. Conversely knowing the value of I for a tube, which will suit the particular job in hand, then the diameter and gauge can easily and quickly be found by reference to the graph. If the polar moment of inertia of any circular section be required, this is equal to twice the I about a diameter. The usefulness of such a graph like the one illustrated in Fig. 3 is considerable, but for those who would prefer to inter- polate, the tabulated values of I are here given, together with the corresponding diameters. This system of graphing has wide application where accurate readings are required by mere inspection and it is for the readers themselves to apply the method to their own particular needs. 3 x 100 x 4 If the section be not symmetrical then the C.G. must be found, through which the neutral axis is then drawn and the cube figure constructed in two separate portions, the one above the axis and the other below it. This method can be used for any shape of section, to find the I about any axis of bending ; it is quick and accurate, simple and straightforward, and is certainly not so well known as its usefulness would warrant. For circular sections of rods, tubes, &c, the I of the sections is easily calculated from the ordinary rule :—I = ^- (D4 — dx), where D = outer, and d = inner, diameters respectively. Many pocket books give a list of values of I for circular sections which is undoubtedly of much use. The usefulness of such a table is, however, considerably lessened when the gauge of a tube, say, is such that the inner diameter becomes an odd size, although the outer diameter may be quite an even figure. To overcome this difficulty Diam. of Circle. Ins. .150 • Jot) .225 .250 .275 • 300 • 310 • 325 •335 •35O •365 •375 •385 .400 .410 •425 •43O .450 • 460 •475 .480 . 500 .510 •525 •53O .560 •575 .580 Values of I. .0000248 .0000785 .0001258 .0001918 .0002807 .0003976 .0004533 .0005475 .0006182 .0007365 .0008711 .0009701 .001079 .001256 .001387 .001601 .001678 .002013 .002198 .002498 .002606 .003068 .003321 .003729 .003874 .004487 .004828 .005358 •005555 Diam. of Circle. Ins. .600 .610 .625 •635.650 •675.700 •725 •75O •775 .800 .825 .850 •875.900 •925 •95O •975 1.000 1.025 1.040 1.075 1.100 1-130 1.160 1.200 1.230 1.250 1.270 Valuesof I. .006362 .006796 .007489 .007981 .008754 .010190 .011790 .013550 .015570 .017700 .020100 .022740 .025610 .028760 .032180 .035910 .039960 .044340 .049090 .054160 .057420 .065580 .071780 .080040 .088880 .101780 . 112360 .119830 .127690 Diam. ofCircle. Ins. 1.30 1.32 1 -35 i-37 1.40 1.42 1.45 1.48 1.50 1.52 1 -55 i-571.600 1.625 1.650 1.670 1.700 1.720 I-75O . 1.780 1.800 1.820. 1.850 1.875 1.900 1 -920 1.950 1.970 2.000 2.020 Values of I. . 14019 . • I4903 .16304 .17291 . .18800 • 19959 _ .21698 •23550 .24849 .26200 .28332 .29824 - .32169 •343io - .36382 .38181 •40998; ' .42962 .46040 .49277 •51523 •53857: •57496: .60670 .63967 .66697 .70988 •73929 .78540 .8T726 Air Raid Precautions. ON the invitation of the Home Secretary, a conference of representatives of the local authorities of London was held at the Home Office on Feb. 13th to consider questions relating to shelters and other matters connected with air raids. Mr. Brace, the Parliamentary Under-Secretary of State, presided, in the absence of the Home Secretary, and the Lord Mayor and representatives of the London County Council and most of the metropolitan boroughs were present. The Commissioner of Police explained that about 1,650 emergency shelters have been scheduled, but he laid stress on the point that the great majority, though affording good cover against splinters, were not bomb-proof, and were pro- vided primarily for persons who are in the streets when the warning is given. The first consideration was to provide shelter for those who were not able to reach their own homes. The Commissioner mentioned that notices advising the public not to leave their homes had been issued by several local authorities, and the suggestion that this advice might be generally given and followed was unanimously approved by the representatives present. The general opinion of the conference appeared to be against the continuance of unofficial patrols. As regards warnings, the representatives of local authorities were unanimously in favour of warning being given by the maroons up to midnight. A few representatives were irt favour of the use of sirens to supplement the maroons, but the majority, especially those of the authorities whose areas adjoin the river, where sirens are constantly heard, were against the use of sirens at any time. The conference, with only a few dissentients, resolved against the use of maroons after midnight. In view of the resolutions of the conference the Com- missioner has decided that the maroons (which are at present used up to 11 p.m.) shall be used till midnight, and after that hour the warning will be given by the police whistles and displaying the " Take Cover " notices as heretofore. The conference unanimously approved of the decision that bugles should be used for the " All Clear " signal, and that sirens, even if used by local authorities to supplement the warning, should in no case be used for " All Clear." 202
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