Whirl flutter and rotor hub center’s motion image of tiltrotor aircraft

Abstract

Based on the Johnson's model of tiltrotor aircraft with nine degrees of freedom containing the assumptions of articulated rotor, rigid nacelle, elastic wing and clamped connections between the nacelle and wing, and considering the elastic connections between the nacelle and wing, the aeroelastic model of tiltrotor aircraft with eleven degrees of freedom for analysis of the whirl flutter of rotor/nacelle and instability of wing in forward flight has been established in this paper. The influences of the nacelle pitching and swing on the rotor inertial force and Coriolis force, the gyroscopic moment produced by the nacelle motion and rotor rotation, the supporting stiffness and damping of nacelle, the influence of nacelle motion on change in inflow of rotor and the influence of nacelle motion on wing motion have been considered in establishing the equations of motion of the rotor/nacelle/ wing coupled system. By using the system parameters of experimental model, the modal frequencies and damping ratios changing with the flight speed were calculated to determine the critical instability speed of the system. By influence of change in the supporting stiffness of nacelle on the critical instability speed, the dynamic instability form of tiltrotor aircraft in forward flight such as the vertical divergence motion of wing flutter or the whirl flutter of rotor/nacelle can be determined. According to the drawn motion trajectories of rotor hub center, the motion images of rotor hub center in the conditions of whirl flutter and wing instability of tiltrotor aircraft can be revealed, intuitively describing the motion patterns of rotor hub center in occurrence of the whirl flutter.