The use of Periodic Reduced Order Models in Coupled Flight Mechanics Analysis of Tiltrotor Aircraft

This manuscript outlines the production of a flight mechanics and reduced order aerodynamics coupled model for stability analysis of tiltrotor aircraft. By including time history effects this improves on classical stability analysis where a truncated Taylor expansion is used. These effects are important to consider due to the high amount of interaction of the wake from the rotating proprotors with the airframe. The flight mechanics is based on the 12 degrees of freedom 3D rigid body equations and the aerodynamic system is evaluated using the unsteady vortex lattice method. The aerodynamic model is reduced by two methods, using 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 and comparisons are drawn to prior stability analysis found in literature to assess the capability of the methodology. The coupled system indicates more instability when compared to the classical analysis, which suggests that the low order Taylor series expansion removes important dynamics attributed to the time history. The two model order reduction methods are compared which shows close correlation near to the stability boundary however a greater difference in the more unstable eigenvalues. This indicates that the time averaged model is more applicable to only lightly unstable situations.