# A Lateral-Directional Control Method for High Aspect Ratio Full-Wing UAV and Flight Tests
## 1 Introduction  
High aspect ratio full-wings have a higher lift-drag ratio at low speed and simpler structure than the conventional configuration, so they are favorable for solar-powered UAVs. Meanwhile, the flat upper surface of high aspect ratio full-wings makes this configuration have a larger and more complete solar-cell laying area. The most famous high aspect ratio full-wing UAVs are Pathfinder and Helios [1], solar-powered UAVs designed by NASA. The high aspect ratio full-wing UAV researched in this paper consists of a central wing, two outer wings, and two vertical stabilizers, as is shown in Figure 1.  
Limited by solar radiation energy, all solar-powered UAVs have high aspect ratio wings and ultra-low structures in order to improve aerodynamic efficiency and reduce structural weight. However, this makes wings prone to torsional deformation, and the aileron's control efficiency is very low, as well as aileron reversal may occur. Meanwhile, in order to reduce the weight, the high aspect ratio full-wing UAV usually does not have rudders. To solve the lateral-dihigh aspect ratio full-wing UAVs, such as Pathfinder [2], use the differential thrust of propellers instead of ailerons and rudders. Different from the aileron directly generating the rolling moment, the differential thrust of propellers will first produce sideslip and then rely on the lateral stability to produce the roll angle, so the stabilities of UAVs become essential.  
Due to the low flight speed and poor yaw static stability of the full-wing UAV, it is sensitive to wind field disturbances and speed changes during its flight. According to the cascade control theory, the lateral-directional control of full-wing UAVs can be divided into the lateral-directional attitude control and the lateral-directional trajectory control. For the former, [PERSON] [3] and [PERSON] [4] proposed the theory of active disturbance rejection control, which is a control theory with a strong robust ability and easy engineering realization [5], and it is very suitable for flight control design [6; 7; 8]. Nonlinear dynamic inversion control (NDI) is widely used in flight control research [9; 10; 11]. The work in [12] showed that NDI has a good control effect, but it needs a precise mathematical model of the controlled drone. For the latter, the vector field trajectory tracking method, proposed by [PERSON] [13], is one of the most popular trajectory tracking methods presently [14; 15; 16]. Compared with other trajectory tracking methods, such as the line-of-sight method [17], the nonlinear guidance method [18], and the LQRmethod [19], [PERSON]'s [20] research showed that the vector field method has the best wind resistance, which is a good choice to control high aspect ratio full-wing UAVs.  
Based on the above analyses, this paper will focus on the lateral attitude control and trajectory control for a high aspect ratio full-wing UAV. First, a lateral-direction mathematical model of a full-wing configuration UAV with twin propellers is established, and its lateral-direction flying qualities are analyzed. Then, based on active disturbance rejection control (ADRC) and NDI theories, a full-wing UAV lateral attitude control law is designed, which uses the differential thrust of propellers as the control output. Third, the vector field trajectory tracking method is improved to meet the requirements of linear tracking control for this UAV. Finally, the control effects of the designed controller are verified with simulations and flight tests.