Correlation of unsteady pressure and inflow velocity fields of a pitching rotor blade

Abstract

Predictions from four different analytical methods are compared with measurements of unsteady inflow velocity and surface pressure distributions on a pitching rotor blade in hover. The test case is a stiff two-bladed teetering rotor constructed from full-scale tail rotor blades, subjected to w/rev simple harmonic pitch oscillations under incompressible flow conditions. The chordwise distributions of unsteady pressure at three radial locations on the blade are compared with Theodorsen's and Loewy's two-dimensional incompressible unsteady aerodynamic theories and with Kaladi's pulsating doublet distribution method. Inflow velocity is predicted successfully using Peters' modal theory for steady as well as dynamic pitch conditions. The effect of dynamic inflow on rotor unsteady surface pressure is studied. At inboard radial locations, Loewy's two-dimensional theory for even harmonics of forcing frequency and Theodorsen's two-dimensional theory for odd harmonics provide efficient and reliable predictions of unsteady blade surface pressure. At outboard radial locations, panel or modal methods have to be used to predict amplitude and phase of unsteady pressure. Tip effects, mean pitch angle effects, and effects of rotation have been demonstrated. Nomenclature b = number of blades C(k) — Theodorsen's lift deficiency function, F + IG C'(k) = modified lift deficiency function for the rotor, F' + iG' Cp = pressure coefficient, 2(p - p y)/pfl2r2