X-31 VECTOR System Identification - Approach and Results

Abstract

During the X-31 VECTOR (Vectoring, Extremely short take-off and landing, Control and Tailless Operation Research) Program, the X-31A advanced into new regions of its flight envelope. Accomplishing the ESTOL (Extremely Short Take-Off and Landing) maneuver, the aircraft approached at unconventionally high angles of attack and derotated automatically to a conventional landing pitch attitude immediately before main wheels touchdown to warrant sufficient tail clearance. For the initial ESTOL control law design, a detailed and sound knowledge of the aerodynamic characteristics in the gear down configuration up to 25 degrees angle of attack was imperative. To establish a dependable database, system identification based on specially tailored flight test maneuvers was used to validate the wind tunnel data. The aircraft configuration changes required for the VECTOR Program – such as nose boom relocation and nose cone reshaping to accommodate a flush air data system – were judicious motivation to identify the now altered stability and control derivatives of the X-31A throughout the entire flight envelope. The paper describes the system identification test approach which utilizes single surface excitations as an integral part of an automatic input signal generation embedded within the flight control laws, and the consequently required safety logic and cockpit interface are outlined. The applied identification procedure is introduced briefly and exemplary time histories from the flight test evaluation are given. Typical identification results conclude this paper.