Six-degrees-of-freedom (6DOF) simulations to outline the flight path of missiles and predict their rigid body behavior, are normally conducted in the time domain using flight-mechanic equations of motion with nonlinear quasi-steady aerodynamic coefficients, gravity effects and a realistic control system. Dynamic aeroelastic simulations are normally performed for stability and loads analysis at a single height-velocity combination, using linear frequency-domain equations of motion where the structure is represented in modal coordinates and the unsteady aerodynamic coefficient matrices are a function of frequency. Aeroelastic effects are sometimes introduced in 6DOF simulations thru static aeroelastic corrections or introduction of modal frequencies. A 6DOF simulation with integrated dynamic aeroelastic effects would seamlessly predict elastic vibrations and aero-servo-elastic interactions with change of flight conditions. It will enable study of parameter variations and Monte Carlo analysis.
To be used in in state-space time-domain formulation, the unsteady aerodynamics, commonly represented in linear analyses by frequency domain complex functions, are approximated via Rational Function Approximation (RFA). A methodology for the implementation of quasi-steady aerodynamic coefficients from Wind Tunnel or CFD calculation in the RFA matrices, for matching the quasi-steady rigid body dynamics, is presented. RFA is canonically calculated for each flight condition independently. This work introduces a revised RFA procedure that leads to state definitions that are consistent across changing flight conditions and yield continuous state-space equations over the entire flight envelope. Then, a simplified Simulink™ simulation is set to access a pre-calculated unsteady aerodynamic database, retrieve time-dependent state-space matrices and demonstrate an integrated simulation run.
