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Aviation History
1920
1920 - 0162.PDF
This corresponds to a free drop of 2-24 ft. and a rebound to 1 -05 ft. The damping after the first oscillation is natur- ally less owing to the lower piston speed in the oleo gear. It is distinctly shown that the rebound energy largely comes from the tyre. Capt. Barn well has suggested to me the use of solid tyres, and there certainly seems to be a possibility of development in this direction. It must be borne in mind, however, that the tyre is called upon to absorb the kinetic energy of the wheel, and the tyre and axle that of the moving parts of the undercarriage, hence a definite shock absorbing capacity is required. At the same time the distribution of stress on the wheel is not so good as with a pneumatic tyre, and considerable increase in wheel weight may be expected with solid tyres. Perhaps some form of cushion tyre may provide a satis- factory solution of the difficulty. Taxi-ing—Periodic OscillationsOscillations set up due to taxi-ing on uneven ground may of course be estimated on an assumed ground condition.These oscillations may be very serious, and if the period of the machine coincides with the " ridge and furrow " pitchan undercarriage might easily be broken. This is, of course, obviated by changing the taxiing speed. Shock absorbing intaxi-ing must be considered, and defects such as rolling, excessive bouncing, or hard running may occur with anundercarriage which is quite efficient for landing. Landing on One Wheel Before attempting to draw deductions from the evidence submitted, there is one more case of landing to be dealt with, i.e., landing on one wheel. In stressing an undercarriage for side load, it is usual to assume either that the C.G. of the machine is over the wheel in contact with the ground or that one wing tip is just touching the ground. Fig. 39 shows 1 f •'" • FIG. 39. these conditions occurring simultaneously. An arbitrary load-factor of two on the normal loads, with the machine resting in the position discussed, has been customarily used. Thisis not really satisfactory, and some breakages of under- carriages have occurred under side load which are difficult toexplain. There is no doubt that the majority of the shock absorption falls on the tyre under these conditions, and con-siderable experimental data will be required for an investiga- tion into the actual loads induced in the structure. It cer-tainly seems likely that if a general improvement is to be made in undercarriages a definite capacity for absorbing sideshock will have to be provided. Wheel Position To reduce " bucking " the wheels should be as nearly aspossible under the C.G. of the aeroplane when the thrust line is horizontal, thus necessitating the use of leading wheels.The introduction of leading wheels permits the use of brakes, and when provided with proper shock absorbing mechanismwill protect the machine against turning over as a result of striking obstacles on bad ground. The load on the tail isalso relieved. To reduce side loads and the danger of turning over side-ways, the wheels should have a wide track. This helps to overcome the difficulty introduced by the change of wheelinclination due to big movement, It is clear that a really efficient undercarriage will bet heavier and offer more resist-ance than the elementary types in use, but probably this increase would not be excessive. • • • # # On examining the evidence set out above, one is particu- larly struck with the large amount of additional data necessary before satisfactory conclusions can be reached. I have drawn as far as possible on such sources of information as were ^available to me, and^while another six months' work 162 FEBRUARY 5, 1920 would undoubtedly have enabledjme to present a more com- plete set of results, I felt that probably the discussion follow- ing this paper would achieve the same object in a more rapid manner, and that at the same time many people might be sufficiently interested to extend the work already done. I feel sure that there are many untapped sources of informa- tion, and I hope that the admittedly controversial nature of this paper may result in disclosing some of these. SUMMARY AND CONCLUSIONS "The following points seem to be indicated, though not of course in any way conclusively. 1. Wheels and Tyres Wheel " strain energy " is negligible. Shock absorbing capacity increases with rate of loading. Energy dissipated (" hysteresis ") increases with rate of loading. v The value of " static "/" dynamic " absorbing capacity ratio, as deduced from the Palmer Tyre Co. tests, ranges from 72 per cent, for a i-ft. drop to 95 per cent, for a 6-ft. drop. The value of the same ratio obtained from C.I.M. 744 is 70 per cent. There is considerable difference in method between the two sets of tests. From some special tests by the Palmer Co. it is deduced that the percentage'dissipated out of the total energy absorbed in static and dynamic tests is as follows :— (a) Static hysteresis .. .. .. 8 per cent. (b) Dynamic hysteresis .. .. 20 per cent. The dynamic cases correspond to actual conditions. Solid Tyres Solid or rather cushion tyres might possibly be used with advantage to reduce oscillations. They must have a shock- absorbing capacity capable of dealing with the kinetic energy ., of the moving parts of the undercarriage. Considerable increase of wheel weight is to be feared with this system. 2. Rubber Shock Absorbers Even tensioning and consistent quality of material are important, and individual rings seem to give the best promise of fulfilling these requirements. Shock absorbing capacity remains constant with varying rate of loading. Energy dissipated diminishes with rate of loading. (Boulton and Paul experiments.) Mr. Turner states that with rates of loading corresponding to actual conditions any hysteresis of rubber shock absorber appears from preliminary experiments to be negligible. The details of these experiments are, however, not available. Shock absorbing capacity decreases after repeated ex- tensions. Vertical Velocity.—Value of u is indeterminate, but should not be less than V sin y. Load Factor.—Landing factor should be of the same order as flight factor. Oleo Gear Design.—Since to get high shock absorbing capa- city a big movement of the leg is required, only a small part of the piston movement should be coincident with movement of fte rubber. With this condition the damping is increased. To take full advantage of the oleo gear, a relief valve is necessary, in addition to the constant leak holes, opening just before the maximum load on the structure is reached. The effect of using air cushions requires further investigation. In order to get large travel certain obvious difficulties due to angular motion of struts, etc., have to be overcome. A large wheel base will assist this, and at the same time improve conditions for landing canted or side to wind. A large wheel base is also necessary with big travel to avoid " rolling " or swaying in taxi-ing. Leading wheels have definite advantages, but must have a proper shock absorbing capacity. The possible undercarriage arrangements are innumerable, -but the advantages of various systems may be examined quantitively by the methods outlined. It is reasonable to expect that a considerable improvement can be made on present practice by a slight sacrifice of flight performance. In conclusion I have to thank those who have so kindly furnished me with information and assistance in the pre- paration of this paper. Particularly Mr. Thomas Sloper and the Palmer Tyre Co., Ltd., for valued information and special experiments on tyres ; Capt. F. S. Bamwell, B.Sc., F.R.Ae.S., and the Bristol Aeroplane Co., for details of the Braemar Gear ; Capt. F. M. Green, A.M.I.C.E., F.R.Ae.S., and the Siddeley-Deasy Co., Ltd., for details of the Siskin Gear ; Mr. Tu ner, of Luke Turner and Co., for information dealing with rubber shock absorbers. The Director of Research, for per- mission to quote several confidential information memo- randa. My assistant engineers, Mr. O. Glauert, B.A., for the work on the complete Oleo Gear Analysis ; and Mr. H. J.
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