Reducing Rotor-Body Coupling Using Active Control

On the RAH-66 Comanche, there is an occasional undesirable coupling between the whirling of the effective rotor center of gravity caused by the Regressive Lag Mode, and the body roll dynamics. Several attempts have been made to reduce the severity and frequency of occurrence of these oscillations but an undesirable coupled response was still sometimes encountered at high load factors. An innovative bandpass filtered cross-feed methodology was proposed by Rotea and Weller, and subsequently studied in simulation and wind-tunnel testing with mixed results. The goals of the effort described in this paper are: (a) to implement the cross-feed methodology in nonlinear simulation to verify its advantages and (b) to understand the physics behind the methodology so as to rationally develop a preliminary design for possible implementation in the aircraft. The paper first presents an empirical investigation of the crossfeed controller using a high fidelity non-linear GenHel model of the RAH-66 Comanche that shows significant reduction in the rotor-body coupling. Then, a first principles based simplified coupled body-flap-lag model is derived that compares well to a linear model derived from GenHel through numerical perturbation. The simplified model is used to show that the lag response of the aircraft is strongly affected not only by the cyclic inputs but also by the flap response of the aircraft through Coriolis coupling. The model shows, using simple approximations, that these effects are almost equal in magnitude. The original cross-feed control concept is based purely on a mathematical construct that involves finding the principal input direction and no real reference is made to the underlying physical mechanism. The simplified model developed here shows that there indeed is a physical explanation for the effect, and the success of the cross-feed is a direct consequence of this mechanism. This removes the criticality of the precise numerics of the cross-feed solution and suggests that the cross-feed methodology might in fact be quite robust. This is an important consequence of the effort described in this paper that helped set the stage for a more confident flight test exploration of what essentially is an active rotor control technique. Representative results from the testing are also shown.