Aerodynamic and Aeroacoustic Characteristics of an Isolated Propeller at Positive and Negative Thrust

Regenerative propellers offer many potential benefits such as improved maneuverability, reduced landing run, and decreased community noise, besides the potential to reduce energy consumption by recovering energy during descent and landing. Since the blade loading in regenerative mode will be opposite to that in the conventional propulsive mode, the aerodynamic and aeroacoustic performance of the propeller will be markedly different in both modes of operation. This paper analyzes the performance of an isolated propeller at positive and negative thrust using multi-fidelity numerical approach and validation experiments to identify the most relevant flow phenomena and resulting tonal-noise mechanisms. The results show that the low-fidelity BEM model does not perform well in energy-harvesting conditions mainly due to polar data limitations near the stall conditions at negative angles of attack. The study of tonal noise sources reveals that at a given advance ratio, the loading noise depends upon the relative level of thrust and torque noise in the upstream direction of the propeller, whereas it is lower or similar to the propulsive case in the downstream direction.