PSD INDI and wake gradient based control of high aspect ratio UAVs' close formation flight

To enhance the cruising performance of a swarm and to ensure its flight safety, the stability and the control of the Close Formation Flight (CFF) are essential elements. This paper proposed a methodology to improve the accuracy of the dynamic characteristics' analysis and reduce the flight control tracking error for CFF using wake vortex information. Firstly, a criterion for trajectory stability during formation flight is derived based on the wake gradient, and the patterns of modal characteristics changing with the relative position between the leader and the follower under wake interference are analyzed. Then, a novel Partial States Determined Incremental Nonlinear Dynamic Inversion (PSD INDI) control method is proposed for the attitude control loop, which the damping derivatives and the wake gradient of the UAVs are integrated into the Extended State Observer (ESO) to estimate the required angular acceleration, the state estimation error is reduced, the controller performance being thus improved. Next, in the trajectory control loop, a method using distributed propellers and rudders synchronous control is proposed to generate direct lateral force as the feedforward for L1 control; this way, the cross-track error caused by external interference is eliminated. Finally, considering a swarm of tailless high aspect ratio UAVs, featured as low longitudinal and directional stability and sensitive to wake disturbances, a hardware-in-the-loop (HIL) simulation test of CFF is carried out. While encountering sudden changes in wake gradients near the optimal cruising position, the attitude and trajectory disturbances, as well as the throttle demand, are smaller, which prove the robustness, the accuracy, and the feasibility of the proposed flight dynamics’ analysis and control method.