A multibody study on single- and multi-variable control algorithms for tiltrotor whirl flutter stability augmentation

The results of a numerical study on prop-rotor whirl flutter stability augmentation are presented in this paper. Based on a 1/5 scale semi-span aeroelastic wind tunnel model of a generic tiltrotor concept, a numerical simulation is set up using a general purpose multibody simulation tool. The model possesses a gimballed stiff-in-plane rotor with a constant velocity drive system. Three different control algorithms are presented and evaluated. Firstly, there is a single-input single-output controller using conditioned wing deflection feedback on longitudinal cyclic swashplate input. Secondly, two types of multi-input multi-output control that are based on time domain identification of the model are presented: A Linear Quadratic Regulator with a Kalman-Bucy state estimator and a Generalized Predictive Control algorithm. Both of them use measured wing deflections in order to calculate appropriate swashplate input. The control studies are done with a windmilling rotor. Detailed results of the closed-loop behavior of three wing and two gimbal natural modes are presented up to the closed-loop stability boundaries. Robustness analyses of the algorithms with respect to wing natural frequencies, structural damping, flight speed, rotation speed, flight altitude and signal noise are an integral part of the article. The rotor shear force is shown in the open-loop condition and in presence of a controller in order to illustrate the whirl flutter mechanism and the action of the controller. The three control methods yielded substantial gain in stability and critical speed and they showed good robustness margins.