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Aviation History
1950
1950 - 0527.PDF
16 March 1950 349 to the cardan shaft from a 15 h.p. variable-speed elec motor. As the bobweights rotate, their out-of-balam moments produce an alternately sympathetic and anti- pathic reaction upon the gearbox, and this reaction is directly transmitted, via the tubular shaft, to the airscrew. The frequency used is normally of the order of 80 eye/sec. A somewhat similar rig is used for the determination of torsional characteristics, but the method of inducing the vibration is different in that an epicyclic gear is used with eccentric bobweights on the planet pinions. Fre- quencies of approximately 230 eye/sec can be obtained. In order that the largest airscrews can be accommodated, although the main body of the rig is, of course, inside the shop, the mounting shaft pierces the wall and the airscrew to be tested is mounted outside. In addition to the various test-rigs described, there are, in the laboratory, a variety of general-purpose rigs for the fatigue-testing and vibration-characteristic deter- mination of components and small sub-assemblies. An i^uii'estiug example of one of these smaller test units is a vibrating table: essentially, it comprises an electro- magnetic vibrator connected to a spring-mounted table-top. Objects on the table can be " bounced " at about 90 eye/ sec and accelerations up to 80 g induced. In concluding this brief dilation upon the work and equip- ment of the vibration laboratory, it must be stated that, not only does the department make all of its own strain gauges but, indeed, it makes virtually all the scientific equipment used, simply because no one else in this country manufactures suitable equipment for the work. It is, in fact, as a result of its eminence in this highly specialized field that the laboratory has, from time to time, been called upon to undertake work in connection with marine, auto- mobile and structural engineering. One of the latest pieces of equipment which has been designed and built by the department is an electrical linear-computor which is capable of solving twelve simul- taneous equations in twelve vari- ables. The process of computing the vibratory characteristics of an airscrew blade from drawings is both complex and tedious in the extreme, and whereas manu- ally it takes virtually months to ptoduce the requisite answers, it id expected that, by means of the computor, the whole process will take not more than a week. It should be emphasized in this connection that the calculator is in no way a substitute for vibra- tion testing; it will merely per- mit the nodal points for known orders of vibration to be calcu- lated expeditiously but, as already pointed out, stress load- ings as such cannot be calcu- lated ; they must be determined by test. The production of an entirely new airscrew entails (i) a general design - study including aero- dynamic and stress investiga- tion; (ii) specific research and development of major compo- nents and assemblies; (iii) manufacture of prototype equip- ment ; (iv) development and approval testing, including pro- totype flight trials, and (v) clearance to production. The ex- tent of initial development de- pends upon the preliminary results obtained, but can be likened to an abridged form of the final approval test, the main requirements for which are mandatory and are appropriately laid down both by ;-boat are undergoing spinning J centrifugal twisting moment. Strain-gauge set-up for flight trials of the Hermes IV airscrew. the Ministry of Supply and /or the Air Registration Board. The airscrew, the fan (if fitted), the spinner and de-icing gear are given their initial spin tests at R.A.E., Farn- borough, electric motors being used to spin the units at speeds 10 and 25 per cent above the maximum r.p.m. of the engine to be used. These tests are made for periods of 30 minutes at various blade pitch-angle settings, the purpose being to confirm that there is adequate strength in blade retention, that there is no excessive blade flutter and that the spinner, fan, and so forth can withstand over-speeding; without distortion or collapse. Having established that the airscrew is structurally and functionally sound, the next stage is a strain-gauge test of-the airscrew assembly on the engine for which it has been designed, and this can take place on the test-beds Blade-root loads are measured on this rig, with hydraulic pres- sure in the cylinders simulating c.f. and eccentric bobweights driven from variable-speed motors providing the excitation. \ \
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