Effect of pitch angle and rotor speed on bending-bending elastic coupling of the CH149 Cormorant helicopter tail rotor blades

The control and understanding of vibration issues faced by helicopter rotor blades are essential to ensure safety of flight. In some cases, the dynamic response experienced by a system is hard to predict, such as for the limit cycle oscillation (LCO) experienced by the tail rotor of the CH149 Cormorant helicopter used by the Canadian Forces. While the origins of limit cycle oscillation are still unclear, the analysis carried out by the manufacturing company, AgustaWestland, identified the flapping S-mode as a potential contributor of the instability problem. An adequate understanding of the frequencies and mode shapes of the tail rotor system can provide valuable insight and shed light on the LCO recorded.;The complexity of the design and the aerodynamic loads encountered during flight can all have a significant influence on the system dynamics. To concentrate on the frequencies and mode shapes without the external forces applied, this research focuses on the impact of the stiffness characteristics of the half-hubs and the blades of the tail rotor on bending-bending elastic coupling between the flap S-mode and lead-lag C-mode at different rotor speeds and pitch angles. This thesis present a simple and valid model of the CH149 Cormorant tail rotor used to examine the effect of the rotor speed and pitch angle values. This research also highlights the intermodal coupling between the flap S-mode and lead-lag C-mode before the operating speed is reached, as well as the change in the in-plane behaviour of the S-mode shape for a pitch angle around 8° at operting speed.;To investigate the influence of the rotor speed and the pitch angle on the frequencies and mode shapes of the tail rotor system, a Matlab computer program was developed to numerically model the system and study the uncoupled and coupled behaviour of the modes based on a finite element approach. The program was designed to provide a close representation of the actual half-hub and blade, based on the physical data provided by AgustaWestland and using approximation methods. To validate the model, it was tested against published results first, and then was used to obtain the frequencies and mode shapes of the tail rotor of the CH149 Cormorant helicopter. Those results were compared to the expectations based on theoretical concepts and to recorded experimental data.;The simulation obtained by the modelling program was successful. While the model designed was validated as a reliable tool to explore the dynamic of the tail rotor system, bending-bending coupling between the S-mode and the C-mode was also identified. The computer program revealed the influence of the rotor speed and the pitch angle on the frequencies and mode shapes of the coupled modes. In conclusion, the behaviour of the mode shapes was established and can now be used in a more elaborate model, which includes the effect of the aerodynamic loads. This may lead to a better understanding of the origins of the LCO recorded on the CH149 Cormorant helicopter.