Aeroelastic analysis of bearingless rotors using large deflection beam theory

Abstract

The aeroelastic response and stability of bearingless rotors are investigated using large deflection beam theory. The bearingless rotor configuration consists of a single flexbeam with a wraparound-type torque tube and pitch links located at the leading edge and trailing edge of the torque tube. The outboard main blade, flexbeam, and torque tube are all assumed to be an elastic beam undergoing arbitrary large displacements and rotations, which are discretized into beam finite elements. The finite element equations of motion for beams are obtained from Hamilton's principle. Two-dimensional quasi-steady strip theory is used to evaluate aerodynamic forces in both hover and forward flight. For the analysis of hover flight, the nonlinear equations of motion are solved for an equilibrium position through an iterative procedure. The modal approach method based on coupled rotating natural modes is used for the stability analysis. For the analysis of forward flight, the nonlinear periodic blade steady response is obtained by integrating the full finite element equation in time through a coupled trim procedure with a vehicle trim. After the coupled trim response is computed, the aeroelastic response is calculated through a time-marching solution procedure under small perturbations assumption. A stability analysis is then performed using a moving block analysis. The results of the full finite element analysis using the large deflection beam theory are quite different from those of a previously published modal analysis using the moderate deflection-type beam theory.