Reduced Order Aerodynamics and Flight Mechanics Coupling For Tiltrotor Aircraft Stability Analysis

This manuscript outlines the production of a flight mechanics and reduced order aerodynamics coupled model for stability analysis of tiltrotor aircraft. The coupled model allows for the dynamic nature of the wake to be included in the stability analysis rather than reducing the dynamics to a set of stability derivatives as found in classical stability methods. The flight mechanics is based on standard 3D rigid body equations of motion which is coupled with the aerodynamic system obtained by the Unsteady Vortex Lattice Method. The aerodynamic model was reduced using two methods, the eigensystem realisation algorithm for a linear time invariant approximation, and a periodic reduced order model for the full linear time periodic system. The methodology is applied to a Bell XV-15 type tiltrotor aircraft in hover and forward flight. The wake effects are found to be very important to the stability of the tiltrotor as evidenced by significantly more unstable eigenvalues of the coupled system when compared with literature, as well an equivalent classical analysis carried out using the same aerodynamic system. The two model order reduction methods are compared, showing a close correlation indicating that the time-averaged approximation is adequate to represent the periodic dynamics. The results show that if the time-averaged behaviour could be obtained without a full aerodynamic simulation of the aircraft, then the coupled stability could be assessed with a much lower computational cost, which is ideal for preliminary investigations.