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Aviation History
1963
1963 - 1706.PDF
f"LIGHT International, 19 September 1963 517 INDUSTRY International Products Flight Systems Company News Great Britain Undercarriage Drop Testing A vast amount of knowledge is available on the design and performance of aircraft under carriages. Performance prediction has also reached a high level, and plenty of informa tion regarding fatigue life is available although further information on the latter subject is still being built up. The difficulty with ad hoc fatigue testing is the time con sumed (to say nothing of the high cost). Friction and wear are also difficult to pre dict. Friction affects the performance; wear may reduce a part beyond the tolerance limit or it may initiate stress-raisers which in turn affect the fatigue life. Calculation of oil flow through orifices involves an orifice coefficient which is greatly influenced by the local geometry. The choice of the orifice is therefore determined by experience, and may be sufficiently wrong to influence the perform ance materially. Even if computers are used in the performance predictions the possible error due to the wrong choice of orifice coefficient or wrong friction assumption Smo// drop test machine, with a levered- suspension nose unit installed. The white elastic cords increase the acceleration of the falling mass to give the desired contact velocity will persist. It is therefore necessary to check the initial performance calculations by drop testing. From simple beginnings, drop testing has developed into a scientific procedure demanding elaborate apparatus and, for large, heavy-duty undercarriages, sub stantial structures. The falling masses in volved vary from about 3001b to over 100,0001b. Obviously such a range cannot be embraced by one drop-test machine, and at the Arle Court (Cheltenham, GIos) works of Dowty Rotol Ltd there are four machines—the smallest of which has two falling carriages, one heavy and one light. The range of each machine overlaps its neighbours, so there are no gaps in the coverage. No two undercarriages are alike, so each has an adapter or bracket to mate it to the machine. The actual drop takes only a fraction of a second, and the recoil slightly longer, so the whole process is over in one to three seconds from release to final static condition. During this time complete records have to be made, so the next important consideration is instrumentation and the number of variables to be recorded. In the simplest case—that of the single wheel on a telescopic leg—the following information is necessary: falling mass, total travel, axle travel, acceleration (nega tive), and maximum tyre travel. Two attitudes of a main undercarriage must be taken into account, "tail up" and "tail down." If the unit cannot be mounted in the bracket in both positions, the differ ence in angle must be simulated by introduc ing a wedge under the wheel(s). To represent drag the wheel can fall on a suitable wedge, but this can require correction factors especially in levered-suspension designs. To represent transient drag, the wheel can be spun up (backwards) and stopped on contact with the ground, after about one revolution, to simulate spin-up when an aeroplane makes contact with the runway. The coefficient of friction of a rotating wheel on a concrete block starts at about 0.25 and rises to approximately 0.35 just before the relative motion stops. If a higher coefficient is required a steel plate can be substituted; and the highest coefficient, about 0.8, is realized by substituting smooth aluminium tread plate, because aluminium is such a good conductor of heat that the contact surface remains cool and the "dimples" of the tyre tread clean off the rubber crumbs. This subject of contact surface leads directly to another piece of apparatus, the "ground reaction platform." This can Intermediate drop test machine, with a levered- suspension bogie undercarriage (for a Vulcan) installed. The drums on either side spin-up the wheels before the drop be used instead of the accelerometer for determining the vertical reaction. It gives a clearer reaction curve, because that from the accelerometer is often ill-defined through secondary vibrations causing "fuzz" —or "grass," as our electronic friends call it. The ground reaction platform consists, essentially, of a top table and a base plate joined by links with strain-gauges mounted on them. Other links, in the horizontal plane, provide stability and take care of the drag and side loads. It is a compar atively easy matter to calibrate the platform in a compression testing machine. It is essential to mount and protect the strain- gauges with the utmost care, the arch enemy being the ingress of moisture. The other important instrument is the potentiometer, which is used to record linear movements. These are of various lengths, from the long (4ft to 6ft) down to the short (2in to 3in). The long ones are for total travel and the short ones for shock-
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