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Aviation History
1948
1948 - 0494.PDF
412 AIRSCREW TESTING critical, and although the artificially in- duced vibration, loads are not imposed truly in accordance with the full-scale flight loads, the test conditions are more rigorous in that the blade specimen is being subjected to distortion in the direc- tion offering the greatest resistance to such treatment. The capacity of this rig provides for the simulation of centrifugal force up to about 120 tons and frequencies of vibration in bending up to 1,000 /min. can be imposed at any required amplitude. Measurements are taken through the medium of resistance strain-gauges cemented on the specimen, visual indications being given on cathode-tube oscilloscopes and permanent records taken by an oscil lograph camera. We will examine this equipment later. Next on the list of tests is the rig spinning of the hub. For this purpose an old Kestrel 30 engine is used to provide the motive power, the shaft extension transmitting drive direct from the crankshaft (the reduction gear has been dismantled) to a bearing head; this is carried on a pedestal base and incorporates a versatile mounting whereby a dummy airscre,W-shaft of any required size from 1 to 7J S.B.A.C. standard can be attached on which to mount the hub. One is very apt to regard an airscrew hub as being merely a shell in which the blades are mounted and quite to take for granted, if not com- pletely overlook, the tremendous strains under which it has to operate. Nothing could dispel this facile view more thoroughly than to see a hub on this spinning rig. To simulate the blades, huge steel weights, rather like monstrous cheeses, are mounted in the blade barrels. Further, since the bending, twisting and plain centrifugal loads imposed on a blade in flight are necessarily transferred through the blade root into the hub, these, too, have to be allowed for in the tests ; to achieve this, the steel weights are mounted eccentrically relative to the barrel axes, so that the requisite offset moments are applied. The degree of angular eccentricity is, of course, deter- mined relative to the speed at which the hub is to be rotated. A typical schedule for this test would call for rotation of the hub for a total of five hours in convenient periods at 1,360 r.p.m. to produce a centrifugal force of 237,320 lb and a bending moment of 108,000 lb in." Translating these values to 106 tons and 48^ ton inches respectively, lends, perhaps, increased emphasis to the magnitude of the loads. Before a hub is put up for rig spinning, it is carefully checked dimension- ally and also undergoes magnetic crack detection. Having completed its spinning test, the hub is again checked dimensionally and for cracks. The spinning rig is housed in a cell with several feet thickness of earth banked all around it in the plane of rotation. Nevertheless, when hubs have burst, fragments have cut through all the protection like the proverbial knife through butter. Distortion, Strength and Functional Tests Number three on the test schedule is concerned with distortion tests of the piston and cylinder assembly. The rig for this is very simple and consists merely of an appropriate mounting for the particular type of assembly to be tested and with the requisite oil connections for fine and coarse pitch opera- tive movement. Operation is first checked at normal oil pressures and then at 50 per cent overload pressures, the units being stripped at various stages during the test and any distortional set measured. Special strength tests and functional tests respectively take fourth and fifth place in the test programme. There are no standard rigs for strength tests, however, since the conventional appliances to be found in the normally equipped materials laboratory perform quite adequately such strength investigations as are required. Special functional tests are also most usually performed on those test rigs which are employed basically for a different purpose. This is exemplified by the functional test of the automatic fine pitch stop locks which is carried out on the piston / cylinder assembly dis- tortion test rig. The actuation check of the fine pitch stop is incorporated in the same test runs as those for distortion. There is, however, a separate rig for the functional testing of transfer bearings used with contra-rotating airscrews. This rig provides attachments for the fore and aft parts of the transfer bearing, each rotating oppositely to its complement. The attach- ments can, in addition, be moved axially so that the thrust duty of the transfer bearing can be ascertained. Bearing temperatures are registered by thermocouples. De-icing for airscrews is still a subject of research in itself and experi-
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