Alleviation of dynamic stall induced vibrations in helicopter rotors using actively controlled flaps.

Abstract

This work presents a treatment of the helicopter vibration reduction problem at high advance ratios, taking into account the effects of dynamic stall. Dynamic stall is associated with separation of the airflow in the leading cadge region of the blade, which results in a lift coefficient overshoot, large pitching moments, and a drag penalty. Under separated flow conditions, vibratory loads are much larger than under attached flow. Among the available tools for the simulation of dynamic stall, semi-empirical models offer the best compromise: between efficiency and accuracy. The ONERA model is used to describe the loads during stall, in conjunction with a rational function approximation for unsteady loads for attached flow. Single and dual actively controlled flaps are used to reduce: vibrations. Several control laws are considered in this study: a controller based on the steepest descent method widely used in rotary-wing research, an algorithm resembling simulated annealing, time domain control laws and an algorithm where the relative importance of vibration components is allowed to vary. Successful multicomponent vibration reduction, between 30% and 99%, is demonstrated over the entire range of advance ratios considered (0.3 ≤ mu ≤ 0.45). This study represents the first successful implementation of vibration reduction in presence of dynamic stall, and physical explanation for the vibration reduction process is also provided: the angle: of attack is reduced over the entire rotor disk. Finally, saturation limits on the control deflections are imposed, which keep flap deflections in a practical range. Effective vibration reduction is achieved even when imposing practical saturation limits on the controller. A drag model that includes the effect to flap deflections is incorporated in this study. The increased drag caused by the actively controlled flap does not affect, the vibration reduction capability of the ACF in the presence of dynamic stall, and results only in a moderate helicopter power increase. Finally, the, robustness of the vibration reduction process in the presence of a freeplay type of nonlinearity is also demonstrated.