Aeroelastic response & stability of helicopters with elastomeric lag dampers

An analytical model capable of studying the aeroelastic response and stability of helicopter rotor systems incorporating elastomeric dampers has been developed. The rotor is idealized as a rigid blade with flap, lag, torsion, and damper degrees of freedom. The elastomeric damper is modeled using a recently developed time-domain finite element technique based on the method of Anelastic Displacement Fields (ADF). The damper model preserves the strain-dependent nonlinear behavior, characteristic of elastomeric materials. The potential of the new approach is explored through a single element model of an elastomeric lag damper in simple shear. The blade degrees of freedom are augmented by the damper degrees of freedom. The blade and damper responses are discretized about the azimuth using a finite-difference formulation. The nonlinear coupled blade and damper equations are solved by the Newton-Raphson iteration process. Linearized stability of the rotor system is evaluated using an eigenvalue analysis based on Floquet theory. Results are presented for both soft-inplane and stiff-inplane configurations. The nonlinear behavior of the elastomeric damper is seen to have a significant effect on lag mode stability in hover and forward flight. The details of damper response in forward flight vary significantly with blade loading and advance ratio; damping appears to increase with dynamic lag amplitude. This work provides a framework for continuing research on elastomeric dampers and their effects on rotor performance and stability.