Including Steady State Information in Reduced Order Modelling for Tiltrotor Aircraft Stability Analysis

This paper builds on the work previously carried out by the authors in exploring flight mechanics and aerodynamics coupled stability analysis of tiltrotor aircraft. The intention of the stability analysis via the coupled model, is to include the full dynamics of the wake in the stability analysis rather than reducing the aerodynamics to a set of first order stability derivatives as found in classical stability analysis. It is important to use the full dynamics in the stability analysis for tiltrotor aircraft due to the significant wake interaction with the airframe. In an effort to reduce the cost of computations and therefore be most applicable to early design stages of aircraft, the Unsteady Vortex Lattice Method (UVLM) is used as a lower fidelity aerodynamic solver and Reduced Order Models (ROMs) are used to limit the system size in the stability analysis. However, the main results of this research are independent of the solver used. This work specifically focuses on including steady state information in the ROM of the aerodynamics, which is coupled with the flight mechanics based on standard 3D rigid body equations of motion. The time averaged aerodynamic response is reduced using the Eigensystem Realisation Algorithm (ERA) as well as an adjusted ERA method that increases the importance of the steady state behaviour in the reduction. The stability derivatives of the aerodynamics were expected to show convergence to the steady step up response at lower ROM sizes when the steady state information is included in the ROM; however, the results obtained show little improvement in the convergence over the standard ERA method. This has been attributed to the aerodynamic simulation results; where the responses do not decay to the steady state, the ROM has sought to model this behaviour causing the stability derivatives to converge to a different value.