Coaxial rotor aeroelasticity is complex due to the counter-rotating wake system, rotor lift offset, periodic blade passage loads, unsteady rotor-wake interactions, reduced rotor speed and stiff hingeless blades. In this study, the aeroelastic stability of a coaxial rotor is examined in hover and forward flight. The rotor wake is modeled with the Viscous Vortex Particle Method, a grid-free approach for calculating vortex interactions over long distances. The spanwise blade aerodynamic loading is calculated using a computational fluid dynamics based reduced order model in attached flow, and the ONERA dynamic stall model in separated flow. Two propulsive trim procedures are developed: one with the propulsor not operating, and the other with the vehicle at level attitude. An aeroelastic stability analysis based on Floquet theory is applied to the periodic system. A novel graphical method is developed to identify coupling between blade modes of the two rotors. The effects of lift offset and advance ratio on the hub loads, inflow distribution and aeroelastic stability are examined to provide an improved physical understanding of the aeroelastic interactions. Results indicate that the blade passage effect is caused by the bound circulation induced inflow. The first and second lag modes are the least stable modes in hover and forward flight.
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