Blade Tip Geometry Impact on the Rotor Acoustic Signatures

As rotor-based vehicles have spread, the noise generated by rotors has become of great concern. Therefore, it is desired to attenuate the acoustic signature of rotors without compromising the aerodynamic performance and efficiency. This thesis explores rotor noise with a particular focus on the role of blade tip geometry in acoustic signature, aiming to examine the feasibility of noise reduction without compromising aerodynamic efficiency.

Our extensive study on rotor noise suggests that aeroacoustic noise generated at the tip of the rotor blade is a major contributor to the rotor noise, particularly in the broadband range. An experimental approach was applied because broadband noise is often challenging to forecast in models and noise predictions. Three propeller tip geometries were studied in the newly established anechoic chamber at the Technion – Israel Institute of Technology, to provide insights into propellers’ acoustic signatures. A multi-arm spiral phased microphone array was designed and integrated with time-frequency beamforming for acoustic measurements. To complement the aeroacoustic study the aerodynamic forces and moments were acquired.

The results have highlighted the significance of broadband noise, often perceived as an annoying noise component. We have confirmed that blade tips are the main sources of broadband noise, while tonal noise remains largely consistent across different configurations. It was shown that tip geometry modifications primarily manifested in acoustic signature changes to noise magnitude, allowing a significant reduction in broadband noise. The experiments on three propeller designs revealed that certain tips consistently exhibited lower noise levels, while others were noisier. Importantly, these changes have shown minimal effects on aerodynamic performance, showing that optimizing tip geometry can lead to substantial propeller noise reduction without notably compromising aerodynamic performance.