Wind tunnel tests of high pitch airscrews

Abstract

The main series of tests of the original family of airscrews described in R&M 829 consisted of measurements of overall thrust and torque on 5 two-bladed and four-bladed airscrews of pitch diameter ratios 0.3, 0.5, 0.7, 1.0 and 1.5. These tests have now been extended to much higher pitch values and the original tests repeated at a uniform Reynolds number. The additional tests were made with the blades of P/D 1.5 rotated to the equivalent pitch values 1.0, 1.25, 1.8, 2.2 and 2.5. Some of the tests on the low pitch screws were made in a closed 7 ft. tunnel, but the tests of the highest pitch screws were made in the new open jet tunnel No.1 in order to use the higher maximum tunnel speed. Thus a comparison was obtained between observations in the closed and open jet tunnels for a number of airscrews and these support the standard methods of correction for tunnel interference. New apparatus was used including a new 15 H.P. induction motor of 9 in. diameter to drive the airscrew. The effect of the airscrew boss was eliminated by using a cylindrical guard body of 0.27 airscrew diameters with faired nose and tail of sufficient length to give a uniform flow in the absence of the screw. The thrust readings were corrected by pressure plotting the airscrew boss, so that the recorded thrust and torque coefficients refer to the exposed portions of the blades only. Instructions are given for correcting the performance data for the effect of interference when the screw is mounted on the fus elage of an actual aeroplane. The results show that the maximum thrust coefficient for the higher pitches is limited by the stalling of the blades, so that after reaching a value of about O.135 for the two-bladers and 0.26 for the four-bladers, the value of kT remains very roughly constant and independent of pitch for all smaller values of J. These values are however subject to a scale effect on maximum thrust coefficient of 5 to 10 per cent. for an increase of Reynolds number from 1.8 x 105 to 3 x 105 but there is some evidence to suggest that the full scale values will not differ greatly from those of the model. The torque coefficient increases with increase of pitch at all working conditions. The maximum efficiency for the two-bladers increases slightly from 88.4 per cent. at P/D 1.5 to an absolute maximum of 89.7 per cent at a P/D rather less than 2.5. For the 4-bladers the corresponding figures are 84.8 and 86.8.