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Aviation History
1920
1920 - 0104.PDF
JANUAKY 22, I02O AIRCRAFT UNDERCARRIAGES "\. BY JOHN D. NORTH, F.R.A.E.S., F.R.MET.SOC. MESSRS. LUKE TURNER "AND CO., of Leicester, have made some interesting discoveries in their researches on the best form of rubber shock-absorber. They say :—In the case of a ring, if thoroughly wetted when under a load sufficient to produce about 50 per cent, extension, it will be found to have a permanent extension of 8 per cent, measured on the straight length, but it returns to its original dimensions as the moisture evaporates. To obtain a uniform initial tension on each ring the internal diameter should be calculated from the minimum position of the spools and 8 per cent, deducted from the circumference. The ring is then made to this internal diameter, saturated with water when under tension and then placed on the spools in a wet condition. When dry the initial tension will correspond with the load required to produce an 8 per cent, extension, which load can easily be found by a static test on the ring when in a dry condition. Mr. Turner hopes in the near future to improve ' on this by treating the ring so that it retains the 8 per cent, extension, and when required for use it will be immersed in some liquid solution which will allow it to shrink when on the spools. This method, if successful, will be very convenient for spare rings for renewals. Their test results show in the case of a ring the enejgy absorbed in hysteresis is a larger percentage of the total energy than in the case of the ordinary cord. In a ring it varies from 15 to 20 per cent. These hysteresis figures have been obtained by static tests, i.e., the load gradually increased or decreased. On the other hand, the few experiments that have been made with rapidly alternating loads seem to indicate that the hysteresis energy shrinks to a very small percentage of the total, both for the ring and ordinary cord. As a rapidly alternating load re- Initial test After 500 repetitions After 10,000 repetitions presents the condition when an aeroplane lands, it would seem that it is doubtful whether rubber, either as ring or cord, will act as a shock dissipater. (Fig. 22.) The effect of repetition of loading is very marked. Messrs. Turner have investigated the effect of repeated appli- cation of load to produce 66 per cent, extension. After 10,000 repetitions the energy absorbed in stretching to 66 per cent, has fallen off nearly 23 per cent., whilst the hysteresis expressed as a fraction of the energy expended has fallen off from 32 per cent, to 17J per cent. The figures in the repetition tests are as under :— • ---.;• Work done .. . • in stretching " - 66 per Per cent. cent. Hysteresis, hysteresis. 79-0 25-3 32-0 62-5 15*0 24-0 61 *o io-6 17*4 Experiments carried out by Boulton and Paul show that with a ten-inch ring subjected gradually to a load of half a ton the hysteresis, if the time taken to apply the load is half a minute, is less than a fifth of that when the time taken is four minutes (Fig. 23). A ten-inch rubber shock-absorber ring f in. diameter was attached to hooks in the testing machine and a load of half a ton applied at different rates. The time taken for the full load to be exerted was £, 1, i\, 3 and 4 minutes. In each case a number of observations of the extensions corresponding to different fractions of the full load were noted, from which a series of hysteresis curves were plotted. The areas enclosed by these curves were determined and plotted on a rate of loading base. The curve drawn approxi- mately through the points gives at once the effect of changing the rate of applying the load. In the case of an undercarriage fitted with pneumatic tyres and rubber shock-absorber a sufficiently near approxi- mation for the shock-absorbing capacity may be arrived at by plotting the load deflection curve of the rubber shock- absorber and tyres, and integrating with the maximum permissible load as superior limit. More accurately the kinetic energy of the wheels and tyres and axle may be considered separately (since it does not affect the springs) and account may be taken of axle deflection. In most cases, u orUJ h*t I 0 t> id u. Ul ul IZ a II at Ulft 1 <S •c 0 0 & '0 • - <a <o + • * 6 CNOi. H3MI <\i V!) •"•»- - - OJ 6 UT C z I n a j •J N i e 102
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