Aerodynamic interaction between tandem propellers in eVTOL transition flight configurations

The present study investigates the aerodynamic interactions in a tandem propellers configuration typical of a tilt-wing eVTOL aircraft during the transition manoeuvre. Particular focus is on how the relative position and the propeller's tilting angle influence the aerodynamic performance. A systematic series of wind tunnel tests, including thrust and torque measurements with Particle Image Velocimetry (PIV) surveys, were performed on two co-rotating propellers models with fixed axial distance while the propellers tilting angle and the lateral separation distance were changed. The comprehensive wind tunnel campaign explored all the phases of the transition from take-off to cruise, thus highlighting possible detrimental effects on the multi-rotor system due to aerodynamic interactional mechanisms occurring due to front propeller impingement on rear propeller disks. To achieve detailed insights into the physical comprehension of the complex interactional effects produced on the rear propeller disk, the activity was completed by a numerical investigation performed through the mid-fidelity solver DUST, based on Vortex Particle Method for the wake modelling. From the perspective of a preliminary design phase of eVTOL concepts, this work showed that particular attention must be paid to the transition flight regime since both the tilting angle of attack of propellers and the free-stream velocity heavily affects the propulsive system behaviour. As a general outcome, higher vertical distances between propellers guarantee to reduce the performance losses on the rear propeller despite the presence of mutual interference phenomena.