An analytical model for nonlinear elastomeric lag dampers, and its effect on bearingless rotor dynamics and aeroelasticity

An analytical model for an elastomeric lag damper is developed, as a combination of linear and nonlinear springs and dashpots. Its behavior is described in the time-domain by a nonlinear differential equation. This representation is independent of frequency- and amplitude-dependent complex modulus components (G;A methodology is developed for the aeroelastic analysis of bearingless main rotor helicopters in forward flight. The large elastic twisting in the flexbeam due to pitch control is independently evaluated as the structural response to a prescribed swashplate motion (in vacuum), and its inclusion in the calculation of strain energy, results in modified nonlinear equations for aeroelastic analysis. Since the large elastic control deformations are purged from the total elastic deformations, normal modes can be used for the aeroelastic analysis in forward flight. The present analysis has good convergence behavior in evaluation of blade periodic response. Introducing an elastomeric damper between the flexbeam and torque tube results in an increase in equilibrium lag displacement as the damper stiffness increases the lag frequency and brings it closer to resonance with the forcing frequency (1/rev). Using normal modes which consider elastomeric damper impedance in the calculation of blade modal matrices and load vector, results in better convergence behavior for the calculation of rotor-fuselage-damper trim (equilibrium) response. For a larger (stronger) damper, the lag mode stability decreases with increasing forward flight velocity, due to the reduced damping of the elastomer at higher amplitudes of excitation. For a smaller (weaker) damper, the lag mode stability may increase slightly at higher forward flight speeds due to increased aerodynamic damping.