'Smart spring' identification for hovering rotor aeroelastic-stability augmentation

This work deals with tailoring of adaptive material included at the roots of hingeless helicopter rotor blades to be used in individual blade control (IBC) strategies. Usually, IBC strategies involving the use of adaptive materials either consider adaptive material embedded in the blade structure for inducing strain deformations, or apply adaptive actuators for controlling segments of the blade (e.g. for moving trailing-edge flaps). Here, the adaptive material is used to provide augmentation of modal damping in a passive control approach, that can be conveniently tuned so as to make it the most suitable for the actual rotor configuration under examination. The presentation of a procedure for tailoring this `smart spring' is the aim of the paper. The aeroelastic blade model considered consists of a cantilever slender beam undergoing flap, lead-lag and torsional motion, coupled with a strip theory approach for the prediction of the aerodynamic loads, based on the very low frequency approximation of the pulsating-free-stream Greenberg's theory. Starting from this model and applying the Galerkin method, generalised mass, damping and stiffness matrices of the basic blade, as well as the incremental generalised mass, damping and stiffness matrices due to the `smart spring' have been determined, the latter depending on the `smart spring' inertial and elastic characteristics. It will be shown that the application of an optimal control criterion, followed by a low frequency-approximation observer, yields the identification of the most suitable `smart spring' characteristics for augmentation of rotor blade aeroelastic stability. The validity of this procedure will be demonstrated by numerical results concerning the stability analysis of two hovering blade configurations.