An Experimental and Numerical Study of the Aerodynamic Interaction Between Tandem Overlapping Propellers

The latest trends of Urban Air Mobility pushed the aeronautical industrial sector towards the eVTOL concept, i.e. electrical vertical take-off and landing. 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: the investigation of the rotor-rotor aerodynamic interaction between front propeller slipstreams and rear propellers. This topic is rather new, thus a lack of experimental literature is noticed. The present work aims to partially fill the gap through an extensive experimental activity which investigates the main physical aspects of the phenomenon in a typical eVTOL configuration. A dedicated wind tunnel testing campaign is performed to investigate deeply the interaction between two co-rotating tandem propellers at fixed axial distance and variable lateral separation. The performance of the tandem propellers were compared with an isolated configuration both in terms of thrust and torque measurements and Particle Image Velocimetry (PIV) surveys. The experimental results are the first step in the creation of a reference database for the validation of numerical codes implemented during the design phase of such vehicles. 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. In parallel a numerical activity was performed using a mid-fidelity aerodynamic solver 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.