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Aviation History
1910
1910 - 0752.PDF
[/ySRrj SEPTEMBER 17, 1910. TWISTED ELASTIC MOTORS. By W. LANG DON - DAVIES. THIS article, which is published especially for the benefit of those of our readers who make working model aeroplanes, contains the data that is necessary to enable them to calculate the amount of elastic that will be required for the motor. The figures and conclusions have been drawn from an exhaustive series of tests conducted by Mr. VV. Langdon-Davies, who has made a special study of this subject, and is, we believe, the first to carry out any such experiments. To obtain exact figures of the torque, number of revolutions, variations of torque during untwisting, &c., of the twisted elastic motors used for driving model aeroplanes, I have made the following tests, and as the figures obtained may be of use to others I venture to offer them to readers of FLIGHT. The elastic used consisted of -jV in. square strands, 1 ft. long, well lubricated with French chalk. Six sets of 2, 4, 6, 8, 12, and 16 strands were tested in the same manner ; each set was twisted until it broke, and the torque pro duced measured at intervals of the twisting. The torque at the moment of breaking could not of course be taken, and was found by continuing the curve. In no case did the elastic break at the ends, or at the place where the last strand was joined ; the break almost always occurred near the centre. The torque increased rapidly from zero while the elastic was twisting in a straight rope about its pwn centre ; the increase was very much slower when the first knot had formed, and only increased slowly until the length of the whole rope was filled with the first series of knots; the torque then increased rapidly again until the second series of knots commenced, when the increase again became less rapid. A complete second series of knots was never arrived at, for more than four strands, before breaking occurred. Curve III shows that the total work obtainable is in direct pro portion to the cross section for equal lengths of elastic, and is therefore directly proportional to the weight of rubber. From these data we can calculate that if a model allows of only 1 ft. length of elastic, and requires an average torque of '158 oz. at 2 ins. r, the necessary torque can be obtained with 11 strands of TV in- sq. (see curve II), and this will give 166 working revolutions (curve I). If more revolutions are required the strands cannot be increased, as the revolutions will be decreased (curve I). If the strands are decreased more revolutions will be obtained, but the torque will be decreased. It should be remembered that the total number of revolutions of the screw, all other things being equal, controls the distance flown for a given average torque. Three further sets, all of four strands of the same elastic, 1, 1*5 and 2 ft. long, were tested in the same manner. The result is given in Table II. TABLE II.—Four strands iV in. square. Length in ft. Average Torque. Revolutions. 1 ' 1-5 TABLE I. •355 ••• 283 •371 ... 423 2 ... '377 ... 544 From this table it is seen that for variations of length for constant cross section the torque is practically constant. The revolutions T x R. 101 157 205 Strands •h. »n. square, 1 ft. long. 2 4 6 8 12 16 i. 75 per cent. of Breaking Twists «= Revolutions. 402 283 234 203 .. 156 143 ii. Average Torque for WorkiDg Revolutions m ozs. at 2 in. Radius. •13 • •355 • •713 • •975 • .. i'75 • 277 1. iii. Working Torque x Working Twist. 52 100 167 198 273 393 400, Table I is derived from these six tests. Column i gives 75 per cent, of the breaking twists, and these are taken as the working twists, or the number of twists the elastic can be safely wound up to, or the total possible number of revolutions obtainable from the screw. Column ii gives the average torque in ounces at 2 in. radius, over 75 per cent, of the curve from zero to breaking point. Column iii gives the torque x revolutions not per zco unit of time), and shows the total work obtainable. The curves I, II and III are plotted from columns i, ii and iii respectively ; the points from which they are derived are shown, for I by circles, for II by crosses, and for III by a cross in a circle. The vertical scale gives, for curve I the revolutions, for curve II one hundred times the torque at 2 ins. r in ounces ; the real torque is therefore the scale figure divided by 100, or multiplied by -oi. This has been done for convenience of drawing only. For curve III the scale figures give the total power obtainable from.the various cross sections, and therefore weights of elastic used. The horizontal scale gives the numbers of strands, and therefore cross sections, used, all strands being 1 ft. long. Curve I shows by its shape that the possible working revolutions decrease as the cross section of the elastic is increased. The numerical value of any point on the curve gives the number of working revolutions obtainable, and depends, among other things, on the elasticity, i.e., quality, of the rubber. Curve II shows that the torque increases as the cross section increases. rv Langdon Davies 750
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