Experimental investigation of the vortex ring state for side-by-side propellers in descent flight

The present paper describes an experimental wind tunnel campaign aimed at investigating the aerodynamic interactions between two side-by-side (SBS) propeller models operating in Vortex Ring State (VRS) conditions. Load measurements and Particle Image Velocimetry (PIV) were employed in this comprehensive test campaign. Several configurations were considered, varying the tip-to-tip distance between the SBS propellers (i.e., with aligned propeller disks) in both co-rotating and counter-rotating configurations. The test results enabled characterization of the mutual aerodynamic influence between the two SBS propellers, providing insight into the loss of performance that occurs when the propellers enter this specific descent flight condition. Moreover, PIV measurements provided a detailed characterization of the velocity flow field and the dimensions of the vortical structures associated with VRS occurrence across the different configurations tested.