Analysis and Testing of a Model-Scale Magnetorheological Fluid Helicopter Lag Mode Damper

Elastomeric and Fluidlastic® dampers are used for mitigating aeromechanical instabilities in advanced rotor systems. Magnetorheological (MR) fluid damper performance is compared with that of a similar sized Fluidlastic® damper for purposes of augmenting lag mode damping in bearingless rotors. The MR fluid dampers are tested for two conditions wherein the magnetic field is turned on and off. Benchtop experiments were conducted to evaluate the hysteresis characteristics of the two dampers. For the Fluidlastic® damper, the experiments show that the stiffness modulus decreases from a high of 18kN/m at 0.1 mm amplitude and remains nearly constant (at 16kN/m ) above 0.4 mm amplitude. For the MR damper (ON condition), the stiffness modulus shows a wider variation from 16 kN/m at 0.1 mm to 10 kN/m at 1.2 mm. The MR damper (OFF condition) stiffness modulus value remains constant at around 8kN/m. The damping capacity of the MR fluid damper (ON), as quantified by the loss factor, is comparable to that of the Fluidlastic® damper for amplitudes above 0.6 mm . For lower amplitudes, the Fluidlastic® damper has the higher loss factor. The stiffness and the loss factor of the MR damper (OFF) form the lower bounds for the three configurations. A nonlinear model is proposed to describe the damper behavior. The model predicts the damper behavior fairly well except for cases of low amplitude excitation where the nonlinearities are more pronounced.