Modeling the Nonlinear Dynamic Roll of Generic Fighter Using Delayed Detached-Eddy Simulation

Abstract

Recent research at the U.S. Air Force Academy Modeling and Simulation Research Center has focused on developing nonlinear lower-order aerodynamic loads models from unsteady computational fluid dynamics (CFD) simulations. The long-term objective is to develop a high-fidelity computational tool capable of identifying aircraft configurations susceptible to handling quality instabilities before flight testing. In this paper, this approach is validated for a generic fighter having a chined fuselage-delta-wing configuration and known aerodynamic instability occurring well within the flight envelope. Previous static experiments and CFD simulations have shown that asymmetric vortex bursting occurs at angles of attack greater than 23 deg and low angles of sideslip, resulting in significant nonlinearities in lateral stability. The dynamic effects on this instability are investigated using delayed detached-eddy simulation and prescribed motion-constant frequency maneuvers ranging from 1.43 Hz to 17.1 Hz. Results show that the aerodynamic response is highly dependent on frequency. At lower frequencies, there is a significant convective time lag, resulting in a shift of the static nonlinearity to greater sideslip angles. As the frequency was increased, the severity of the nonlinearity was reduced such that, at 17.1 Hz, the aerodynamic response is nearly linear. Aerodynamic loads resulting from a chirpmaneuver are then used as input to a lower-ordermodel employing radial basis functions. The resulting aerodynamic loads model is then used to predict the vastly different aerodynamic response of each constant frequency maneuver with good success.