Modeling the bifilar pendulum using nonlinear, flexible multibody dynamics

This paper deals with the modeling of the bifilar pendulum, a hub-mounted self-tuning vibration absorber used on certain rotorcraft. The bifilar consists of a tuning mass that acts as a pendulum and is connected to a support frame by means of two cylindrical tuning pins. The tuning pins roll without sliding on curves of cycloidal shape machined into the tracking holes on the support frame and tuning mass. In this work, a detailed model of this device is presented, which involves nonlinear holonomic and nonholonomic constraints. The formulation is developed within the framework of finite element based dynamic analysis of nonlinear, flexible multibody systems, and features energy preserving and decaying time integration schemes that provide unconditional stability for nonlinear systems. Numerical examples are presented that demonstrate the efficiency and accuracy of the proposed approach.