High Angle of Attack Model Identification with Compressibility Effects

This work presents an aerodynamic model capable of representing lift, drag and pitch moment coefficients at high angles of attack (AOA), accounting for compressibility effects and aerodynamic hysteresis during stall. The modeling was peformed using flight test data collected during accelerated stalls of a fighter trainer jet, which consisted in wind-up turn (WUT) maneuvers from Mach 0.40 up to 0.73. A nonlinear model structure, based on Kirchhoff’s theory of separated flow, was used to deal with high AOA effects. However, as the aircraft reaches high speeds, the aerodynamic coefficients become a function of Mach number. In order to deal with compressibility effects, the flight data were partitioned based on Mach number intervals, so that a different set of coefficients is applicable to each range of Mach. The values obtained for each coefficient as a function of Mach were mostly compatible with those expected from theory. During the validation tests, using a different set of data, it was clear that a single set of coefficients for a single Mach number was not able to reproduce the flight data in the entire envelope of Mach and AOA tested.