Multi-rotors in descent– Capturing the true dynamics of a descending drone*

When descending, rotorcraft inherently enter their own downwash, leading to a highly unsteady flow field which can cause a loss of thrust and reduced stability. Our work aims to understand the interaction of multiple rotors in vertical descent by investigating a multi-rotor craft in a low speed, vertical wind tunnel. Previous experiments have shown that the instabilities are caused by rotor tip vortices convected through the rotor disk at specific descent velocities. Furthermore, these results showed considerable deviations between a fixed mounted and a rotationally free mounted multi-copter in a vertical flow, suggesting that a restricted system is not an accurate representative of the actual descent dynamics.

The presented material focuses on the investigation of a fully unrestricted, free flying drone in a vertically rising flow and its response to the disturbances. Supported by thrust measurements and PIV analysis, the study provides further insight into the nature of the instabilities. The resulting data is compared to previous experiments in order to validate the experimental technique and help us understand the effects of restricting a multi-rotor.