Abstract:
High-speed wind tunnel tests of various model propellers were required as part ofa general research programme dealing with propellers for high-speed aircraft. A two-blade 4 ft 6 in diameter Clark Y section propeller of 6 per cent. thickness ratio and 7 per cent. total solidity was tested at three fixed blade angles over a range of forward Mach numbers up to 0.8 and rates of advance up to J = 4. In addition, the forward Reynolds number based on 1 ft. chord, was varied from 1 million to 4 millions at one blade angle, the forward Mach number being held constant at 0.3. (i) The experimental technique employed for measurement of overall thrust and torque of model propellers in the Royal Aircraft Establishment High Speed Tunnel was proved successful and capable of yielding reasonably consistent results. (ii) No appreciable scale effect was present on the tests made at low Mach number, but this does not necessarily hold at high Mach numbers, for which condition no evidence is available. (iii) The variation of thrust and torque coefficientsand propulsive efficiency with increasing Mach number at constant rates of advance show no serious departure from the variations to he expected from such a blade section operating at high Mach numbers. (iv) A maximum efficiency of 0.9 was attained with this propeller at low forward speeds and tip Mach numbers. With increase in Mach number the efficiency fell slowly but steadily until some critical Mach number was reached when the rate of decrease became serious. The critical tip Mach number varied between 0.9 and 1.2 depending upon the operating conditions. At a forward Mach number of 0.7 and upwards the rate of decrease in efficiency became large, though the maximum efficiency at M = 0.7 was still quite high at 0.76. It might be possible to reduce this rate of decrease at a given Mach number by operating at still greater blade settings.
