Generation of a reduced order model of an unmanned combat air vehicle using indicial response functions

Within the NATO STO AVT-251 Task Group a generic Unmanned Combat Air Vehicle (UCAV) planform is redesigned based on requirements derived from parts of the flight envelope of the defined mission. Because of the lambda-shape planform and associated flow phenomena including shocks and vortices, the aerodynamic design process relies heavily on high-fidelity CFD predictions. This enables accurate prediction of flight dynamics and any potential issues early in the design process. The flight dynamics model would be based on a reduced order model (ROM) derived from CFD calculations. This work investigates the creation of a ROM using nonlinear indicial response functions. The response functions are obtained using a grid motion approach that separates the effects of angle of attack and pitch rate. This approach is followed using three different CFD codes from various organizations: ENSOLV at the Netherlands Aerospace Centre, USM3D at NASA Langley Research Center, and Kestrel at the United States Air Force Academy. The ROM predictions were tested with a manoeuver resembling a low speed pull-up. The predictions are found to be sensitive to the quality of the steady state solutions from which step response calculations were started, as well as the convergence of the step response calculations themselves. Nevertheless, the indicial response method can provide accurate predictions including transient effects, and as such is a powerful method for building ROMs.