A directional control system for UCAV automatic takeoff roll

Purpose - The purpose of this paper is to develop a directional and roll control system for unmanned combat air vehicle (UCAV) automatic takeoff roll, with the objective of keeping the UCAV along the runway centerline and keeping the wings level, especially when there is a crosswind. Design/methodology/ approach - The nonlinear model of the UCAV during takeoff roll is established. The model is linearized about the lateral-directional equilibrium point at different forward speeds. The approximate directional model and roll model are extracted using time-scale decomposition technique. Then the directional control law and roll control law are developed using gain scheduling approach. Nose wheel steering, differential brake and rudder are used as the primary directional control device at low, medium and high speeds, respectively, according to both the qualitative and quantitative analysis of their control effectiveness at different speeds. A priority matrix is developed to determine the secondary control device which is used if the primary control device fails, thus the directional control system can have a certain degree of fault tolerance. Findings - This work developed the directional control law and roll control law by using gain scheduling approach. Experimental results verified that the developed directional and roll control system has high robustness and satisfactory fault tolerance: it can guarantee a safe takeoff under a 50 ft/sec crosswind, even if one directional control device fails, which satisfies the relevant criteria in MIL-HDBK-1797. Practical implications - The directional and roll control system developed can be easily applied to practice and can steer the UCAV during takeoff roll safely, which will considerably increase the autonomy of the UCAV. Originality/value - The paper shows how time-scale decomposition technique is employed to extract the approximate directional model and roll model, which simplifies model analysis and control law design. A fault-tolerant directional control system is designed to improve safety during takeoff.