Hysteresis Modeling of Semi-Active Magnetorheological Helicopter Dampers

Abstract

MR dampers in soft-inplane rotors such as hingeless and bearingless rotors have potential benefits including semi-active control of aeromechanical instabilities, such as ground and air resonance. An experimental performance characterization of hybrid elastomeric/MR dampers for the 1/6th scale Comanche wind tunnel model rotor is presented. These dampers are similar to prior fluidelastomeric Comanche wind tunnel model dampers, except that the passive fluid is replaced by an MR fluid. MR lag dampers were tested under a compressive preload with the magnetic field turned on (ON condition) and off (OFF condition). Damping was characterized for single frequency sinusoidal excitation at the lag/rev (10 Hz) frequency; that is, the lightly damped inplane rotor blade bending mode that plays a dominant role in aeromechanical instabilities. Dual frequency testing was also carried out at 10 Hz and 15 Hz corresponding to the model rotor lag/rev and 1 /rev or rotor RPM frequencies respectively. In all of these tests, the force versus displacement hysteresis cycle or energy diagram was measured for the MR dampers. Two nonlinear models are compared: (1) a stiffness plus viscoelastic-plastic model, and (2) a stiffness-viscosity-elasto-slide model. These models were developed to capture the nonlinear behavior of these dampers. The model parameters were identified by minimizing the mean squared error between the predicted and measured MR damper force time histories due to a lag/rev harmonic excitation. Model validation for both single and dual frequency data was carried out. A key conclusion is that both models accurately predict damping performance, which suggests that the underlying hysteresis model is not unique when only damping is the performance metric.