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Aviation History
1950
1950 - 0528.PDF
35© FUGHT QUEST for EFFICIENCY . . . of the engine manufacturer, or in the modern, well-designed test-tunnels at Hatfield. It is justifiable to regard this as a major stage in the development of the airscrew, in that the object is to establish the magnitude and frequency of the vibratory stresses induced from the engine or the surrounding environment. Smothered in strain gauges, the airscrew is run over the complete speed range in positive and reverse pitch, the recordings from the strain gauges being taken, via a slip-ring '' pineapple'' mounted in front of the hub, to cathode-oscillograph equipment. The duration of the test generally varies between five and ten hours according to the extent of checking required. In the event of the stresses being excessive, it is some- times possible—depending upon the order, amplitude, fre- quency, r.p.m. condition, and position on the blade of the high-stress vibration—to remedy the matter by reducing diameter, thinning and/or narrowing the blade. In so doing, however, a small loss in performance generally occurs, as the blade no longer complies with the original optimum design. In addition, any change in blade design necessitates further strain-gauging of the whole airscrew in order to establish that the alterations axe successful. Similarly, any subsequent alterations to the power unit which may affect the vibratory characteristics of the air- screw also demand a strain-gauging re-check. Whilst this testing of the airscrew is in hand, the process of unit type-testing the ancillaries is in progress, and the constant-speed unit, reversing pump and feathering pump are all subjected to 50 hours of rig-testing designed to reproduce as nearly as possible the variety of conditions likely to be met in service. All these tests can, however, do no more than prove the mechanical and electrical efficiency of the units under simulated working conditions. Individual tests of all the airscrew equipment having been completed, the various units are then brought together for the 115 hours' mandatory testing on the appropriate engine. If it so happens that engine approval is also under consideration, the required test duration is 155 hours, but in both cases the schedule of testing follows an inter- nationally agreed set of requirements. In addition to periods of endurance running at cruising, climbing, maxi- mum and minimum take-off and high-speed conditions, the principal operational requirements include 1,500 operations of the c.s.u. control over the effective range. Steel-blade development. The blade shells are withdrawn from the furnace, inserted in theadjacent 2,000-ton press and are then blown out against the dies by high pressure nitrogen. Tunnel-testing in progress with a Mamba turboprop 50 feathering and unfeathering operations and 200 reversing and unreversing operations. On completion of testing, and removal from the engine, each part of the equipment undergoes the normal accept- ance tests as for a new unit, the purpose being to detect any falling-ofi in performance. These subsidiary checks are followed by complete dismantling and the close examination of every component. Subject to all parte being in a satisfactory condition, flight-testing can then go ahead without delay. Despite the fact that flight-testing is nominally the final phase in development of the airscrew, it is frequently the most difficult and prolonged, because it is by no means always possible to predict the cor- rect constant-speed-unit character- istics for all conditions of flight. Performance during the tunnel-tests is not an accurate guide, since the absence of flight-speed airflow through the airscrew disc gives un- representative loadings on the blades. Furthermore, entirely different conditions of control are created by the various attitudes and altitudes of the aircraft. As a consequence, it cannot be too strongly emphasized that it is en- - tirely erroneous to imagine that an airscrew installation is fully proven and developed operationally by the time it reaches the first prototype flight stage. The preliminary function of flight-testing is to establish satis- factory governing and synchroniza- tion, and many hours of intensive 'development involving numerous changes to the governor and pro- longed flight-testing are at times necessary to achieve complete satis- faction. Feathering and unfeather- ing trials are also carried out at an early stage, these being followed by checks of the de-icing equip- ment. De-icing might appear at
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