Experimental Investigation of the Aerodynamic Interaction Between Overlapping Propellers in Tandem for eVTOL Airplane-Mode Flight Conditions

The latest trends of Urban Air Mobility (UAM) pushed the aeronautical community towards the eVTOL concept, i.e. electrical vertical take-off and landing aircraft. Electrical power, tilt-wing configuration and multiple propellers in tandem configuration (i.e. with the propellers placed one after the other) are the key features of such concept. In particular, the presence of multiple propellers working at close range introduces a new challenge, i.e. the investigation of the aerodynamic interaction between front propeller slipstreams and rear propellers. The present work aims to investigate the main physical aspects of this phenomenon in a typical eVTOL configuration in airplane mode. A dedicated wind tunnel testing campaign was performed to investigate deeply the interaction between two co-rotating tandem propellers at fixed axial distance and variable lateral separation. The tests included both thrust and torque measurements of the propellers and Particle Image Velocimetry (PIV) surveys. Load measurements showed a significant loss in the rear propeller performance as a function of the overlapping ratio between the propellers. Furthermore a dedicated spectral analysis of wind tunnel thrust signals outlined high amplitude fluctuations in partial overlapping configurations. The collected experimental results represent a reference database for the validation of numerical codes implemented during the design phase of such vehicles. In particular, some experimental data were compared with results obtained by the mid-fidelity aerodynamic solver DUST relying upon Vortex Particle Method (VPM) in order to enhance the comprehension of the phenomenon. The analysis of the numerical results allowed to access the flow behaviour involving the front propeller slipstream and the rear propeller disk, which is responsible of the massive losses experienced by the rear propeller.