Two-Dimensional Unsteady Aerodynamic Application to Higher Harmonic Control Test Data

Abstract

In 1982, flight tests were conducted on a modified OH-6A helicopter that incorporated an active higher harmonic control system to reduce vibratory loads on the rotor system. While reducing vibrations up to 90%, open-loop higher harmonic control data showed significant reductions in the main rotor shaft torque and engine power indicating that higher harmonic control may be also providing performance improvements. In this study, unsteady, rotary-wing, potential flow theory for a thin, oscillating airfoil is used to explain the mechanism behind the performance improvements seen on the OH-6A. Equations for the 2-D, unsteady, rotary-wing aerodynamic forces and moments with a finite number of wakes, or shed layers of vorticity, beneath the rotor are developed and applied specifically to the pitch and flap oscillations seen in the hover flight-test data. A modified finite-wake lift deficiency function is developed for this finite-wake theory that is analogous to the classical Theodorsen and Loewy lift deficiency functions, and is used in used to calculate the horizontal time-averaged propulsive force (negative drag) derived by Garrick to show that unsteady aerodynamics can provide the order of magnitude effects necessary to quantify the performance improvements seen on the OH-6A.