Higher Harmonic Rotor Control

Abstract

The elimination of the retreating blade stall speed limitation for helicopters by means of an appropriately programmed feathering input is studied for the general case of a rigid flapping blade with hinge constraint (thus making the results applicable to conventional, offset‐hinged or cantilevered rotor blades). It is concluded that second harmonic feathering alone will not be particularly effective in delaying the stall limit, but that a suitable programme of several higher harmonic inputs will enable the retreating blade stall limit to be pushed beyond the advancing blade compressibility limit. In the course of the investigation generalized equations were developed for blade flapping to the nth harmonic under the influence of feathering to the nth harmonic. The resultant matrix is symmetrical and checks with the few available limit cases derived by other workers. Because of loose coupling in the matrix generalized equations can be derived giving the effect of any particular harmonic of feathering upon flapping and angle of attack distribution around the disk. The effect of higher harmonic feathering upon rotor stability derivatives is not discussed in this text, but examination of the equations indicates that an improvement in stability could be obtained by the application of second harmonic control. This paper does not discuss the mechanical details of obtaining a higher harmonic feathering input, nor is it suggested that this is necessarily the best means of obtaining higher forward speeds. In certain cases it may be the only means however.