X-31A system identification using single-surface excitation at high angles of attack

Abstract

Parameter identification of the X-31A experimental aircraft was conducted throughout the envelope expansion using mainly pilot input maneuvers. The quality of the identification results, especially in the high-angle-of-attack regime, suffered from high correlations between the aircraft controls and states as well as from insufficient sideslip excitation. These problems are caused by the high feedback gains used in the flight control system to control the basically unstable airplane. Therefore, in 1994-1995 a dedicated flight test program using single surface excitation for parameter identification at high angles of attack was conducted. The results prove that identification of highly augmented aircraft benefits considerably from this technique. This article presents the realization of the single-surface excitation using the X-31A flutter test box. Also, the flight test program, the applied evaluation methods, and the identification models are discussed. Selected identification results from the single-surface excitation tests are presented and compared to those obtained from the pilot input maneuvers.