Aerodynamic interaction between tandem overlapping propellers in eVTOL airplane mode flight condition

The architecture of new electric aircraft concepts (eVTOLs) for urban air mobility is typically characterised by multi-propellers in tandem configurations with different degrees of rotor disks overlapping. Consequently, the aerodynamic interaction between front propellers slipstream and rear propellers represents one of the key-phenomena which influenced the performance and design of these novel aircraft configurations. A wind tunnel campaign was performed to investigate the aerodynamic interaction between two propellers in tandem with particular focus to airplane mode flight condition of eVTOLs. A systematic series of tests, including thrust and torque measurements and stereoscopic Particle Image Velocimetry (PIV) surveys, were performed on two co-rotating propellers models. The axial distance was fixed during tests while the lateral separation distance was changed to evaluate the effects of aerodynamic interaction on propellers performance and flow field due to different propeller disks overlapping. Load measurements pointed out a remarkable loss of rear propeller thrust occurring when the degree of overlapping between rotors disks is increased. On the other hand, spectral analysis of measured loads signals showed a higher amount of thrust fluctuations amplitude occurring when the degree of overlapping between propeller disks is partial. Stereo PIV results provided insights on the effects of aerodynamic interaction on rear propeller inflow and wake flow physics. Moreover, numerical simulations performed using a mid-fidelity aerodynamic solver based on vortex particle method (VPM) provided enhanced insights to comprehend the interacting flow mechanisms between front propeller slipstream and rear propeller blades responsible for the detrimental effects observed on rear propeller performance.