Experimental Investigation of Boundary Layer Ingesting Propellers’ Aerodynamics

This study presents an experimental investigation into the aerodynamic characteristics of Boundary Layer Ingestion (BLI) propellers. The custom-designed test rig, intended to enhance propeller research capabilities at the University of Bristol, was used to test an isolated 4-bladed propeller (15.5 × 12 inches) in boundary layer flow within the Boundary Layer Wind Tunnel facility. Thrust and torque measurements were recorded for two inflow speeds, 12 m/s and 6 m/s, at four tip clearance heights and rotational speeds ranging from 2000 to 5500 RPM in 250 RPM intervals, corresponding to an advance ratio (J) from 0.18 to 0.91. Additionally, thrust and torque measurements were taken at one tip clearance height, with yaw angles varying from 0 to 38 degrees at the same inflow speeds and RPM range. The research focuses on two main aspects: determining how aerodynamic performance changes with increasing submersion depth into the boundary layer (BL) and increasing yaw angle relative to the free stream. When the propeller is fully inside the turbulent flow (the smallest s/D), the thrust coefficient and propeller efficiency increase by approximately 12\% and 10\%, respectively, compared to when the propeller is outside the turbulent flow (the largest s/D). This trend, when compared to the boundary layer velocity and Turbulent Kinetic Energy (TKE) profiles, can be attributed to lower inflow speeds and higher kinetic energy in the smallest s/D. Additionally, as the yaw angle increases, the efficiency decreases, and the effective advance ratio at which maximum efficiency occurs also decreases. The findings from this study contribute to a better understanding of the aerodynamic characteristics of BLI propellers, a key technology for future Urban Air Mobility solutions and new civil aircraft configurations, such as Distributed Electric Propulsion.