Multiphysics modelling of a faulty rod-end and its interaction with a flight control actuator to support PHM activities

Spherical joints, or rod-ends, are critical components employed in flight control systems to connect the actuator to the aerodynamic surface and to the airframe. As any mechanical component, rod-ends are subjected to a wide range of failure modes, which can cause catastrophic effects at the actuator level and hence at the aircraft level. Despite this criticality, they are often overlooked in studies concerning the definition of new Prognostics and Health Management systems for flight control actuators, mainly due to the lack of sensors to monitor their behavior. The definition of a physics-based model of the rod-ends is hence a required preliminary step to pursue the definition of novel Health Monitoring schemes able to detect the fault inception in these components and track their evolution in time up to failure conditions. In this paper, a model working under few simplifying assumptions is proposed, complete with a Finite- Difference Method solution for the behavior of the lubricating film. The proposed model is then applied to a pre-existing, experimentally validated model of the flight control actuators of an in-service, wide-body commercial aircraft in presence of excessive backlash. Comments on the rod-end model are presented and further work is laid out.