Boundary Layer Control on Helicopter Rotor Blades

Abstract

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.