Multicopter With Series Connected Propeller Drives

This study considered a tethered multicopter, powered from ground by electrical cables integrated in the tether. For a low tether mass and diameter, but high-efficient and simple propeller drives, a higher tether voltage than the rated voltage of the propeller drives was considered. To avoid additional power converters to step-down the tether voltage, the propeller drives were proposed to be connected partly in series. A control method was developed to stabilize the voltages and simultaneously achieve the requested propeller forces and moments. The feasibility of the proposed concept and the effectiveness of the developed control method were demonstrated by experiments. The proof-of-principle demonstrator was a tethered octocopter with 48 V tether voltage, wherein two or four voltage levels were created by a series connection. The demonstrator flew stably for several minutes until a test flight was completed successfully. During flight, the voltage control errors were always less than 0.75 V. A possible application of such a system with a scaled-up voltage is crosswind kite power, where the kite is equipped with small onboard turbines and the electricity is transmitted to the ground via electrical cables. During launching and landing, the turbines are used as propellers, i.e., the kite is a tethered multicopter and hovers from ground into a launching position, or vice versa.