Design of Flight Control System for Heavy-lift Tethered Multicopter Using Iterative Feedback Tuning Method

Electric vertical takeoff and landing (eVTOL) vehicles can be employed to rescue individuals stranded in elevated locations inaccessible to conventional fire ladders. Conventional eVTOL vehicles face challenges in the performance of long-endurance and repetitive missions due to limitations in onboard battery capacity. Thus, integrating a tethered flight system to the vehicle is necessary. In this study, a preliminary design of a tethered heavy-lift multicopter for human rescue missions was implemented according to an existing eVTOL vehicle with proven flight performance. A multibody dynamic simulation model was developed, in which the structural properties of the electric power cable for a tethered system were considered. The command-following performance was evaluated under various flight conditions. Iterative feedback tuning was performed to determine the optimal control gains for the multicopter control system without the tethered flight power cable. The results were applied to the multicopter dynamic and control model, in which a flexible multibody dynamics model of the electric power cable was incorporated. Then, the command-following performances of the control system with and without the electric power cable for a tethered flight were compared. The overall command-following performance was lower in the former scenario than in the latter one. The moments induced by the bending rigidity of the power cable at lower altitudes acted as larger disturbances on the multicopter, particularly at higher altitudes. Therefore, for the effective utilization of tethered heavy-lift multicopters in search and rescue missions involving repetitive landings in confined spaces, a robust control system resilient to disturbances induced by the electric power cable should be applied.