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Aviation History
1950
1950 - 0526.PDF
VWvW Blade fatigue test. Excited by a moving-coil vibratory the blade is subjected to 500 million reversals with continuous monitoring. QUEST for EFFICIENCY . ... The inquisitors of the vibration laboratory make use of vibration rigs of various kinds and, in conjunction with these, the extensive use of strain gauges permits a detailed picture to be obtained of the stress distribution and magni- tude. Blades, for example, are vibrated in a battery of six brick-walled wells, each of which is capable of taking a blade up to ioft in length. The blade is hung downward from a stirrup-yoke anchored over the root, and a con- necting rod from the stirrup-head is coupled to an electro- magnetic, moving-coil vibrator, so that the blade is vibrated substantially at right angles to its mean chord, that is to say, normal to the plane of rotation. Mounted close behind the blade toward the tip is a pick-up unit (microphone) which senses the tip vibration and accordingly transmits a signal which is fed into the power amplifier of .the vibrator. In this way, a regenerative feed-back circuit is established whereby resonance-locking is obtained. Knowledge of the particular use for which the blade has been designed means that some close approximation to the main frequencies involved can be made and, accordingly, the apparatus is set to produce the required amplitude in the required mode of vibration. The usual range of frequencies lies between 80 and 140 eye/sec and the normal duration of a fatigue test investigation is 500 million reversals—therefore, at 100 eye/sec 24 hours per day, the test would continue for 58 days. Strain gauges at strategic points on the blade trans- mit to a cathode oscilloscope stress-measuring unit which continuously and automatically records and monitors the stresses. For vibrating blades edgewise, i.e., substantially in the plane of rotation, the laboratory is furnished with four test-rigs. Blades are mounted two at a time (purely for balance) in opposite ends of a cylinder, being retained therein in a similar fashion to that used in the appropriate hub. In the bore of the cylinder are two free pistons, the outer faces of which each bear against a dolly transferring thrust to a duralumin plug in the taper bore of the blade root. Hydraulic pressure fed into the cylinder, by forcing the piston-dolly-plng outward, can thus simulate Wntri- fugal loading on the blade root and means of retention. By itself, this can be considered as a steady stress loading, but for the fatigue-testing of the blade root, vibration stress must also be reproduced. A mechanical out-of-balance vibrator employing eccentric bobweights is therefore attached below the centre of the cylinder, with a cardan- shaft drive from a variable-speed electric motor, the cylinder being suspended by loops of elastic cord. Here again, stress distribution and loading are sensed by strain gauges and, too, the normal test duration is 500 x io* cycles —or, of course, prior failure. A somewhat similar rig system is used for the testing of three- and four-blade airscrew hubs. Dummy blade-shanks are retained in the hub barrels by a means precisely similar to that used for the true blades. High-tensile-steel pins pierce the dummy-blade shanks at right angles to the blade axis, but the pins are offset slightly from that axis in order to simulate air-load bending forces. Flexible steel ropes wrapped round the pins act as tension members for the simulation of centrifugal force, the tension being derived from a hydraulic jack giving 50 tons pull per blade on the three-blade rig, and 100 tons pull per blade on the larger four-blade rig. On each of these rigs, use is similarly made of an eccentric bobweight vibrator unit and, as in the pre- viously quoted example, the supporting medium is elastic cord. Both the hub rigs have a capacity of excitation forces up to approximately half a ton, and the normal practice is to run at a resonant frequency so that, for a required vibra- tory force, the power input is diminished. The rig for torsional fatigue-testing is extremely interesting, for it enables the requisite torsional modes of vibration to be reproduced without rotating the airscrew. The hub is mounted on an arbor which is flange-bolted to one end of a tubular shaft. At the opposite end of the shaft is bolted a gearbox in which eccentric bobweights are rotated in opposite directions from a bevel gear coupled Shown here on the universal spin-rig, the hubs of a contra-prop tor the Pri trials. Offset weights on the dummy blade-shanks simulate both centrifugi
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