Active Reduction of Unsteady Rotor Noise via Redundant Control Allocation

Abstract

This paper demonstrates the development and implementation of active noise abatement flight control laws that make use of redundant control allocation. A periodic equilibrium of the coupled rotorcraft flight dynamics and acoustics is first found at a desired flight condition using a modified harmonic balance trim solution method. Next, the nonlinear time-periodic dynamics are linearized about that periodic equilibrium and transformed into an equivalent higher-order linear time-invariant system in harmonic decomposition form. Composite aeroacoustic measures are included as an output of this system. To speed up runtime and make control design tractable, the order of these harmonic decomposition models is reduced via residualization to an 8-state model where the states are representative of the rigid-body dynamics of the aircraft. This 8-state model is shown to provide accurate acoustic response predictions for small-amplitude pilot inputs and to abate runtime by a factor of approximately 10⁵, thus enabling acoustic predictions in generalized maneuvering flight that are significantly faster than real-time. The 8-state model is subsequently used to synthesize an Explicit Model Following (EMF) control law with pseudo-inverse allocation to redundant control surfaces. In this study, the redundant control surfaces taken into consideration were an active horizontal stabilizer (or stabilator). Results demonstrate that while affinity in the controls yields little potential for noise abatement, the use of redundant control surfaces is an effective method for active reduction of unsteady rotor noise. In fact, redundant control allocation was shown to be increasingly effective with increasing aggressiveness in maneuvers. Within the context of future-generation rotorcraft, noise abatement through redundant control allocation will be particularly effective for Future Vertical Lift (FVL) configurations featuring high levels of control redundancy and capable of aggressive maneuvering flight.