A flight control system design for highly unstable unmanned combat aerial vehicles

This study proposes a flight control system for tailless unmanned combat aerial vehicles (UCAVs) and applies it to a representative example: the UCAV1303, an unstable blended wing body (BWB) aircraft. The UCAV1303 has no tail wing and a large sweepback angle, as a result of which it shows highly nonlinear aerodynamic characteristics such as wing rock and the pitch break phenomenon. In particular, in the latter, the pitching moment of an aircraft increases with the angle of attack, causing it to pitch up rapidly and then stall. In this study, an L₁ adaptive controller is designed for the UCAV1303 to accommodate and be robust to the pitch break phenomenon, which is used to model uncertain aerodynamics. Furthermore, a moving wing fence is proposed for realizing good stability and performance at a high angle of attack. It delays flow separation and aerodynamic stalling, thereby improving the effectiveness of the wing and other control surfaces at a high angle of attack. Under normal or level flight conditions, during which the angle of attack is low, it may negatively affect the aircraft performance because it increases the radar cross section and parasitic drag. A series of flight tests were performed to validate the proposed controller and moving wing fence. The former is robust to model uncertain aerodynamics, and the latter prevented the pitch break phenomenon at a high angle of attack and afforded an adequate margin between the initial and the pitch break regions.