TABLE OF CONTENTS The U.S./German experimental aircraft X-31A is a highly control augmented fighter with enhanced maneuverability; see Fig. 12.43. Post-stall maneuvering is enabled by applying advanced technologies such as high angle-of-attack aerodynamics and flight control system-integrated thrust vectoring.58 In Chapter 9, Secs. XIV and XVI.B, we addressed the challenges encountered in estimating the aerodynamic derivatives of the basic airframe. To recapture the essentials once again, it was demonstrated that 1) highly correlated control surface deflections, thrust vectors vanes, and states due to integrated flight control laws resulted in poor estimates with large standard deviations and large scatter, 2) owing to data collinearity, it was possible to estimate only combined derivatives from pilot input maneuvers, 3) single surface excitation provided significantly improved estimates, and 4) from a variety of algorithms in the time domain (artificially stabilized output error method, filter error method, equation decoupling, regression analysis, and extended and unscented Kalman filter techniques), the least squares and filter error methods accounting for process noise were found to be preferable.
A detailed description of X-31A system identification is beyond the scope of this book. A thorough treatment of various aspects is found in Refs. 47 and 59. Principally, the results of X-31A system identification have been used to validate and in several cases update the wind-tunnel predicted database. We provide here two typical results. In Fig. 12.44a we find the estimates of the dihedral effect obtained from pilot input...
