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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. Nomenclature b-reference span C D , C L = coefficients of drag and lift C/, C m , C n = coefficients of roll, pitch, and yaw moment C x , C F , C z = coefficients of longitudinal, lateral, and vertical force c = reference chord F = engine gross thrust /?, q, r = roll, pitch, and yaw rates q = dynamic pressure S = reference area V = true airspeed *thac = distance from thrust impact point to aerodynamic center a = angle of attack j8 = angle of sideslip 6 fl , 8 r = aileron (differential trailing-edge flap) and rudder deflections S C an, 8 e = canard and elevator (symmetric trailing-edge flap) deflections = thrust vector deflections in pitch and yaw cr, K Superscript * = trim value Introduction T HE X-31A poststall experimental aircraft was developed within the enhanced fighter maneuverability (EFM) program to demonstrate the tactical advantage of a fighter aircraft operating in the poststall region. 1 ' 2 Two fighter-type X-31A airReceived June 9, 1995; presented as Paper 95-3436 at the AIAA

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