A possible unsteady thickness noise mechanism for helicopter rotors

Abstract

An alternative theoretical approach to the low‐speed helicopter rotor noise problem is proposed. The traditional treatment of this problem is to model the situation by a system of unsteady thrust and drag dipoles. The dipole strengths are either estimated empirically, or related to the inflow distortion via an unsteady aerodynamic response analysis. An analysis is presented which suggests that an alternative model consists of a system of unsteady lift dipoles, plus a distribution of inplane quadrupoles. The strength of these quadrupoles is related directly to the product of the blade thickness and the distortion inplane velocity components. It is shown how this mechanism is likely to be the cause of the subjectively important higher harmonic rotational noise observed in or near the rotor plane. Some comparisons with experiment are presented.