Damping augmentation of helicopter rotors using magnetorheological dampers

This dissertation describes an investigation exploring the use of magnetorheological (MR) dampers to augment the stability of helicopter rotors. Helicopters with advanced soft in-plane rotors are susceptible to ground resonance instabilities due to the coupling of the lightly damped rotor lag modes and fuselage modes. Traditional passive lag dampers, such as hydraulic or elastomeric dampers, can be used to alleviate these instabilities. However, these passive dampers suffer from the disadvantages that they produce large damper loads in forward flight conditions. These damper forces increase fatigue loads and reduce component life. Thus, it is desirable to have lag dampers controllable or adaptable, so that the damper can apply loads only when needed. MR fluid based dampers have recently been considered for helicopter lag damping augmentation because the forces generated by these dampers can be controlled by an applied magnetic field.; In this dissertation, control schemes to integrate MR dampers with helicopters are developed and the influences of the MR dampers on rotorcraft ground resonance are studied. Specifically, the MR dampers are incorporated into the ground resonance model in two ways: using a linear equivalent viscous damping and using a nonlinear damper model. The feasibility of using MR dampers to stabilize ground resonance is studied. The open loop on-off control is utilized where MR dampers are turned on over RPM where ground resonance occurs, and turned off otherwise. To further explore the damping control ability of MR dampers, the nonlinear semi-active closed loop feedback control strategies are developed: feedback linearization control and sliding mode control. The performance of the two control strategies is evaluated using two examples: to stabilize an unstable rotor and to augment the stability of a marginally stable rotor.; In addition, the robustness of the closed loop control strategies is studied using two cases: damper degradation and parameter perturbation. In first case, it is assumed that some MR dampers are damaged which lead to an unstable or marginally stable rotor. The use of the remaining operational MR dampers to recover the stability margin is investigated. It is shown that using the developed control strategy, the MR dampers can successfully recover the stability and stability margin of the rotor in most studied cases. The robustness of parameter perturbations are studied by perturbing mass, damping, and stiffness parameters. The method to improve the robustness of the feedback linearization control is suggested and is approved feasible.; To evaluate the damping provided by lead lag dampers and predict the aeromechnical instability, damping identification algorithms from rotor stability test data are developed. First, the linear damping identification is researched. Three existing methods: Hilbert transform, moving block method with time domain window, and wavelet transform are evaluated and compared. Hybrid algorithms combining the advantages of the three methods are developed and applied to experimental test data. A nonlinear damping identification algorithm designed specifically for the system with MR dampers is developed. The envelope of the free response of such system is derived and the damping identification problem is transformed to envelope detection problem, so that algorithms used in linear damping identification can be applied. For the single degree of freedom system, all three methods accurately identify dampings: Hilbert transform, moving block method with time domain window, and wavelet transform. For the system with persistent excitation such as rotor stability test data, the hybrid methods again show better performance than other methods.