The longitudinal altitude and airspeed coordinated control is critical for unmanned fixed-wing aircrafts, especially during the taxiing landing process However, due to the strongly coupled relation of kinetic energy and gravitational potential energy, the precision longitudinal control is difficult. The traditional longitudinal control method based on PID (Proportional Integral-Differential) usually adopts different control parameters at different flight stages. But the regulation of the control structure is complex, and some fluctuation may exist during the transition process, which will affect the control performance. The TECS (Total Energy Control System) method adopts the decoupling strategy that the throttle controls the total energy and the pitch angle controls the energy balance between kinetic and potential. However, the parameters-adjusting process is relatively complex and the adaptability to model nonlinearity is imperfect. Based on the INDI (Incremental Nonlinear Dynamic Inversion) method and the trim relation of the aircrafts, a longitudinal altitude/airspeed decoupling control method is proposed in this paper. Firstly, the process of control law design is systematically described. The core points of states balance and transition are described for the design of the method. Secondly, the feasibility and basic control effects of the control method are verified by simulations. During the simulation, a variable coefficient PI pseudo is proposed, to accommodate the requirements of accurate gliding and smooth grounding. Finally, to further verify the effectiveness and superiority of the proposed method, comparisons between the proposed method and the TECS method is carried out. From the simulation results, the in the sense of precise altitude control during auto landing process.

The tail-sitter unmanned aerial vehicles have the advantages of multi-rotors and fixed-wing aircrafts, such as vertical takeoff and landing, long endurance and high-speed cruise. These make the tail-sitter unmanned aerial vehicle capable for special tasks in complex environments. In this article, we present the modeling and the control system design for a quadrotor tail-sitter unmanned aerial vehicle whose main structure consists of a traditional quadrotor with four wings fixed on the four rotor arms. The key point of the control system is the transition process between hover flight mode and level flight mode. However, the normal Euler angle representation cannot tackle both of the hover and level flight modes because of the singularity when pitch angle tends to $\pm {90}^{°}$ . The dual-Euler method using two Euler-angle representations in two body-fixed coordinate frames is presented to couple with this problem, which gives continuous attitude representation throughout the whole flight envelope. The control system is divided into hover and level controllers to adapt to the two different flight modes. The nonlinear dynamic inverse method is employed to realize fuselage rotation and attitude stabilization. In guidance control, the vector field method is used in level flight guidance logic, and the quadrotor guidance method is used in hover flight mode. The framework of the whole system is established by MATLAB and Simulink, and the effectiveness of the guidance and control algorithms are verified by simulation. Finally, the flight test of the prototype shows the feasibility of the whole system.

With high aerodynamic efficiency and low observable design for flying wing configuration, the control ability decreases and the flying risk increases, especially during the takeoff and landing processes for unmanned aerial vehicle (UAV). To solve the nonlinear characteristic problem of the flying wing configuration at a high angle of attack (AoA) for a low aspect ratio flying wing UAV, a controllable region and stability analysis method is proposed in the paper. Firstly, the aerodynamic characteristics and static stability of a flying wing UAV are analyzed. Secondly, according to the characteristics of aerodynamic data, an efficient longitudinal trim state calculation method under the low‐speed range is proposed. The trim state contour map can be used effectively with the proposed trim state calculation method, which can further analyze the constraint relationship between different trim states and the controllable region of the UAV. Thirdly, the stability of different trim states is analyzed through the phase portrait between AoA and airspeed. With the analysis result, longitudinal attitude and AoA decoupling control laws are designed based on the incremental nonlinear dynamic inversion (INDI) method. The effectiveness and adaptability of the designed control methods are verified through simulation at high AoA.

This paper presents a segmented guidance method for multi-UAVs (unmanned aerial vehicles) cooperative attack with spatio-temporal constraints, and successfully implemented in the flight tests. First, the uniform distribution of UAVs around the target is completed through cooperative path planning and tracking in the mid-course phase. The path planning method for cooperative arrival is proposed based on Dubins curve, and the guidance strategy for circular formation flight enables the UAVs to accurately track the reference points which are evenly distributed on the circle. Second, the terminal guidance law in 3D space with impact time cooperation is proposed during the cooperative attack stage, which would further eliminate the accumulated deviation. Finally, simulations and field flight tests for coordinated attack are carried out, which show the effectiveness of the proposed method.