A Preliminary Noise and Flow Field Study of a Propeller Tilted Negatively to a Free-Stream

This paper evaluates the aerodynamic and aeroacoustic performance of propellers when operating in reverse non-axial inflow and edgewise flight conditions. By carrying out aeroacoustic and aerodynamic wind tunnel experiments, the far-field noise characteristics of the propeller in both operating conditions are measured and subsequently related to the flow field of the propeller. This revealed the underlying noise sources for the propeller operating in reverse non-axial flight conditions. The thrust and torque results show that a moderate negative tilt angle has limited impact on the aerodynamic performance of the propeller. However, initial analysis of the far-field noise results shows evident differences in the sound pressure level spectra when the propeller operates at a negative tilt angle compared with the edgewise flight condition. In particular, haystacking can be observed at particular observer locations for the negatively tilted case. Across all of the inflow velocities tested, there are significant increases in the overall sound pressure levels when the propeller is in reverse non-axial inflow. The magnitude and directivity of the blade passing frequency tone is also observed to shift when the propeller is oriented at a negative tilt angle. Preliminary flow field results from particle image velocimetry verifies that the present experimental set-up is able to capture key flow structures of an isolated propeller in the phase-locked configuration, and it is utilised to examine the temporal evolution of the vortical structure for the negatively tilted propeller. In the negative tilting configuration, the tip vortices generated by the upstream blade are drawn closer to the downstream blade and ingested, offering some explanation for increased noise in this flight mode.