Drivetrain Influence on the Lead-Lag Modes of Hingeless Helicopter Rotors

Structural couplings between the flexible main rotor and the flexible drivetrain of the Bo105 helicopter are investigated by numerical simulation. For this purpose, the rotor hub constraint Omega=const. is dropped and a drivetrain model, consisting of discrete inertia elements and intermediate flexible elements, is connected to the hub. By use of the multibody-software SIMPACK, the coupled rotor-drivetrain system is linearized and the eigenmodes are compared to those obtained with a constrained rotor hub. The drivetrain has a significant influence on the shapes and eigenfrequencies of the collective lead-lag modes. While the first collective lead-lag eigenfrequency is raised by the finite drivetrain inertia, the second is lowered due to drivetrain flexibility. To assess the influence of modeling inaccuracies on the observed couplings, the study is complemented by a sensitivity analysis. Rotor blade mass axis offset, blade pitch (causing elastic coupling) and blade precone angle have only weak influence on the coupled modes. In contrast, variations of drivetrain inertia and stiffness strongly affect the eigenfrequencies of the coupled rotor-drivetrain modes