Longitudinal Control of Agile Fixed-Wing UAV Using Backstepping

Abstract

In this paper, a nonlinear backstepping based longitudinal control is applied to an agile fixed-wing unmanned aerial vehicle (UAV)to achieve transitioning between level flight at low angle of attack (cruise)and level flight at high angle of attack (harrier level)and the performance is compared with a gain scheduled state feedback control. In all flight regimes, the elevator control law is computed using backstepping control strategy designed for error dynamics in pitch and pitch rate. During transition from cruise to harrier flight, the propeller rotations per minute (RPM)is calculated using an additional Lyapunov based nonlinear control which stabilizes the error dynamics in flight path angle to zero. During cruise and harrier level, the respective trim value of RPM is applied and during transition from harrier to cruise the trim value corresponding to cruise is applied. Simulations of the control implemented on a longitudinal model of an agile fixed-wing UAV which includes propwash, wing downwash, flat plate post-stall aerodynamics and actuator constraints (first order dynamics, saturation and rate limit)to achieve the flight objective, are done using Matlab 9.4®. Compared to gain scheduled state feedback control, the advantages of the nonlinear control strategy are: 1)less altitude loss and oscillations in velocity and pitch angle during harrier to cruise transition, 2)reduction of around 60 % in maximum pitch rate during both transitions and 3)less chattering during both transitions. However, significant increase in computational complexity is a disadvantage. Overall, a preliminary assessment can be made that the nonlinear control strategy is better than the gain scheduled state feedback control for achieving the specific flight regime and could be advantageous in high angle of attack (AOA)maneuvers like perching, transition to hover and aggressive maneuvers.