Minimum-Time Circling Flight of a Triarm Coaxial Rotor UAV

Abstract

This paper investigates characteristics of minimum-time trajectories for circling flight of a Triarm Coaxial Rotor (TCR) Unmanned Aerial Vehicle (UAV). Optimal open-loop solutions for a circling TCR UAV are presented including scenarios with failed rotors. The optimal control problem is solved using pseudospectral (PS) optimal control theory. With dual rotors on each of the three arms, even if one of the rotors stops working, the TCR UAV can maintain stable flight using the remaining five rotors. Furthermore, depending on the motor power, if two counter-rotating rotors mounted on different arms are stopped, the UAV can fly using the remaining four rotors. Numerical results demonstrate that this PS-control approach has the ability to obtain optimal circling flight trajectories consisting of three phases. With the majority of the flight being steady-state circling, the other phases consist of transient flight between circling and hovering at the boundary points. It is shown that although the optimal controls vary with each failed rotor scenario, the state response remains the same. This preliminary assessment is indicative of a simplified approach to adaptive flight control for improved mission robustness.