Aircraft Spin Recovery Using a Sliding-Mode Controller

S PIN is an autorotational state of aircraft at high angles of attack characterized by large rotation rate in yaw when compared with rates in roll and pitch. In a typical spin, an aircraft rotates about its center of gravity and an axis perpendicular to Earth descending vertically at high speed following a downward corkscrew path [1]. It is one of themost dangerous phenomena encountered bymany of the modern fighter aircraft required to fly in high-angle-of-attack flight regimes. Improper functioning of aerodynamic control surfaces during spin makes it difficult for the pilot to control the aircraft, leading to many a problem, such as spatial disorientation and uncontrolled motion, resulting in fatal accidents and subsequent loss of aircraft. Spin states for many high-angle-of-attack aircraft models have been computed by using bifurcation analysis and continuation technique methodology. Bifurcation analysis results provide the onset points of bifurcations or the critical values of control surface deflections that may inadvertently or voluntarily land an aircraft into spin. Standard control inputs using proper deflection of rudder to control yaw rate with simultaneous application of elevator to reduce the angle of attack have been recommended for spin recovery [1]. Design of new-generation fighter aircraft with new configurations, however, cannot rely on standard piloting strategies. Instead, a uniform approach to design aircraft-model-based recovery strategies is called for. An introduction to spin problem and needs to design nonlinear controllers to recover aircraft from spin has been reported recently in [2]. Identifying spin and level-trim states from a bifurcation analysis of a nonlinear aircraft model, Raghavendra et al. [2] developed a nonlinear dynamic inversion (NDI)-techniquebased spin-recovery controller. NDI-technique-based methods have emerged as popular control design techniques in aircraft flight dynamics. Controllers based on the NDI techniques, however, come with certain associated disadvantages, such as separation of dynamics based on timescales, resulting in complicated control architecture, lack of robustness due to external uncertainties and unmodeled plant dynamics, and inability to achieve control saturation limits [3]. In this Note, we present a sliding-mode (SM) controller based on variable-structure control technique for spin recovery of aircraft. Variable-structure-technique-based controllers have been found to be robust in the presence of system uncertainties and external disturbances, and, usually result in simpler control algorithms [4]. Controller presented in this Note uses results from a bifurcation analysis of the high-angle-of-attack research vehicle (HARV) model of F-18 available in literature [2]. This Note is organized as follows. In Sec. II, a brief description of the aircraft model with reference states for SM controller design is presented. In Sec. III, SM control design technique is explained. Results and discussions are presented in Sec. IV and conclusions follow thereafter in Sec. V.