Application to Rotary Wings of a Simplified Aerodynamic Lifting Surface Theory for Unsteady Compressible Flow

In a recent paper, Jones and Moore have developed a simple numerical lifting surface technique for calculating the aerodynamic coefficients on oscillating wings in subsonic flight. The method is based on the use of the full lifting surface theory and is not restricted in any way as to frequency, mode of oscillation or aspect ratio when M < 1. In this study, this simple but general method of predicting airloads is applied to helicopter rotor blades on a full three-dimensional basis. The general theory is developed for a rotor blade at the Ψ = π/ 2 position where flutter is most likely to occur. Calculations of aerodynamic coefficients for use in flutter analysis are made for forward and hovering flight with low inflow for Mach numbers 0 and 0.8 and frequency ratios p/Ω= 1 and 4. The results are compared with values given by two-dimensional strip theory for a rigid rotor hinged at its root. The comparisons indicate the inadequacies of strip theory for airload prediction. One important conclusion drawn from this study is that the curved wake has a substantial effect on the chordwise load distribution. The pitching moment aerodynamic coefficients differ appreciably from the results given by strip theory.