Chord Width Research Articles

Boundary Layer Control on Helicopter Rotor Blades

THE possibility of applying boundary layer control by suction at the trailing edge to reduce the profile drag of a helicopter rotor blade using centrifugal forces to provide the suction was investigated. It was shown theoretically that it is possible to use this method of suction, and that it is advantageous to suck at the trailing edge if the drag reduction obtained was greater than a certain value which depended on the amount of suction. Experimental work was carried out on a model wing with different types of slots to determine the profile drag reduction possible using theoretically predicted suction flow rates. The experiments were carried out at an effective Reynolds Number based on wing chord of 0.62 x 108. It was found that a single long slot of 2 per cent chord width spanning the whole blade, as close as possible to the trailing edge on the upper surface, gave the best results. The maximum drag reduction occurred at high values of lift coefficient and was of the order of 20 per cent. Using the experimental results it was shown that a helicopter employing this type of boundary layer control would require less engine power.

Supplementary Notes on Drzewiecki's Theory of Airscrews

In the previous contribution and in the reply to the discussion justice was not done to several points.It was made clear that a circulation of air was established round the airscrew at standstill, creating a certain velocity before the airscrew disc in such a way as to modify fundamentally the thrust on the blades. But it was not explicitly stated that at full speed (of the aeroplane) this component of velocity was ignored altogether and each blade was assumed to be attacking “ still ” air.This does not mean that the air was still until the blade actually drew level with it, for, if the analogy with lifting surfaces holds, we know that at a distance of twice the chord width in front of the blade the air is appreciably disturbed, and this disturbance may be by no means negligible even at four or five times the chord's width in front of the instantaneous position of the blade.