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Aviation History
1956
1956 - 1495.PDF
FLIGHT, 19 October 1956 651 Strain-gauging Helicopter Rotors Saunders-Roe Electronics Specialist Describes Pre-amplification Technique AT the first meeting of the Helicopter Association's 1956-57lecture programme, held at the Royal Aeronautical Society- on October 5, a paper entitled Strain Gauge and Blade Motion Recording Systems for Helicopters was presented by Mr. P. D. MacMahon, electronics engineer with Saunders-Roe, Ltd. Dr. G. S. Hislop was in the chair. Mr. MacMahon began by outlining the problems specifically associated with rotating-wing-aircraft instrumentation and review- ing some of the present methods of recording strain-gauge and blade-motion signals. The problems that had to be dealt with concerned:— (a) Magnitude, nature and distribution of stresses in rotor blades. (b) Dynamic geometry of blades about their hinges, which requiredfull distribution of typical cycles. (c) Stresses in rotating mechanisms: for example, control andpower-distribution linkages. (d) Structural vibration. (e) Cyclic stresses fed back into pilot's or other control linkages.(f) Torsional vibrations associated with rotor drives and other power-transmitting shafts. Present helicopter instrumentation techniques were based almost entirely on those developed for fixed-wing aircraft of the last 20 or 30 years. This necessitated equipment modification and adapta- tion to a greater or lesser degree, and it was his contention that these methods were not an entirely satisfactory solution to the problem. It was essential to recognize that a very different range of problems was met and, while the aircraft type of instrumenta- tion could be made to yield answers, these were not often in a convenient form. Furthermore, a great deal of uneconomical labour was required to bring into prominence data which it should be the fundamental design-task of the instruments in use to provide. The problem of recording steady stresses (which normally changed only with change of flight conditions) simultaneously with dynamic stress was particularly difficult in helicopter work. This was because the fatigue-inducing vibratory stresses were not usually very large in comparison with the loads which induced steady stress. Consequently, for any given recording field a choice had to be made between accurate recording either of relatively low-amplitude high-frequency detail or of large-amplitude static detail. Until recently it had not been possible to do both satis- factorily at the same time. Signal amplitudes rarely exceeded a few millivolts and signal- frequency band-width was from approximately 2 c.p.s. to about 300 c.p.s. The lower figure corresponded to a once-per-rev pheno- menon at 120 rotor r.p.m.; the higher corresponded to possible excitation due to engine vibration harmonic orders at average engine speed. In general, it could be said that direct current was preferable to alternating current for energizing the strain gauges. Recording could be by mirror galvanometers to photographic film or paper or, alternatively, to magnetic tape. The last-mentioned would undoubtedly be used widely in the future when current development work had improved the method. It was not practical, except with certain limitations, to drive the mirror galvanometers direct from the strain-gauges, and electronic amplification of the signals was therefore necessary. Apart from conventional forms of amplifier, two newer types, the magnetic amplifier and the transistorized amplifier, were worthy of consideration for use in the future. They could be used to great advan- tage where miniaturization techniques were desirable, but more experience was needed with them before they could be applied satisfactorily to work on helicopters. As far as the recording of rotor blade movements was concerned, amplification of signals was not necessary, since for this work the strain-gauges were replaced by high-accuracy linear potentiometers Ener- gizing voltage for the potentiometers was obtained from the H.T. supply, via suitable dropping resistors, and connections from the potentiometers were made direct to the turret. The angular displacements of cyclic- (Left) The pre-amplifier mounted above the rotor head of the Skeeter. (Right) The £. panoramic display unit, installed on a struc- ture in the cockpit in front of the flight observer's position pitch change and blade flapping to be measured were generally of the order of several degrees. Therefore, the angular resolution obtained from commercially available toroidal-wound potentio- meters of plus or minus j deg was adequate. It was not adequate, however, for motion of the blade about the drag hinge—the "niggling" present at the blade root in a cruising flight condition— and accuracy had to be improved by step-up gearing in the mechanical linkage between the blade root and the potentiometer. Care was necessary in the design of the step-up gearing, in order to avoid any fundamental resonance excitation in the mechanism. The lecturer then gave a detailed description of recording equipment. There were available a number of different galvano- meter cameras, such as those of Miller, de Havilland and Savage and Parsons. Confluent pulse techniques were exemplified com- mercially in the Elliott-Yates ten-channel display equipment, while telemetry was an exceptionally promising method for the future— particularly so when the vast amount of experience now being accumulated in guided-weapon establishments became more generally available. The Mervyn system of data recording and analysis, in which use was made of magnetic tape as the transfer medium, was another equipment of great value to the research engineer. Mr. MacMahon went on to describe the lightweight six- channel equipment designed and produced by Saunders-Roe specifically for helicopter application. The original proposals for the equipment had been made to the Ministry of Supply in 1953 and a design target of 70 lb all-up weight had been set so that the equipment could be used in the smallest helicopters. To avoid the practical difficulties associated with the use of slip-rings at low signal levels it had been decided that pre-ampliners should be mounted on the rotor head. The relatively high signal output would then be passed through slip-rings to a panoramic display unit in the cockpit below. The display unit was to provide a visual assessment of the oscillatory stress characteristics with complete control of signal level by individual attenuating facilities for each of the six channels. The signals were to be recorded in a small multi- galvanometer camera. Steady state stresses were to be measured by reference to a wide-sweep galvanometer mounted in the display unit, the signals feeding this galvanometer being proportional to the unbalance present in the pre-amplifiers which, in turn, would be directly related to the strain-gauge bridge or transducer connected to it. Attenuating facilities would also be incorporated. Power supplies were to be derived from a three-phase 400 c.p.s. rotary inverter running off the aircraft 24-volt system, each phase of 115 volts A.C. being stepped up or down by transformers to provide regu- lated, unregulated and e.h.t. voltage. Excitation of the strain-gauge bridges (or other transducers) and the pre-amplifier heaters was to be by means of a Venner accumu- lator, the lightweight, small bulk and straight-line discharge characteristics of which were eminently suitable for the purpose. The accumulator was to be housed in the pre-amplifier structure
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