Experimental aeroacoustics of rotors with aeroelastic effects under edgewise forward flight conditions

Rotor is the primary noise contributor of eVTOL aircraft. The investigations on rotor noise problems assume the rotor blades to be rigid and neglect their aeroelastic effects. However, the rotor blades are usually designed to be thin and slender for better aerodynamic efficiency, making them highly susceptible to aeroelastic deformations, especially during edgewise forward flight. In this work, we investigate the aerodynamic and aeroacoustic performance of a 229 mm diameter baseline rotor with a thin-root variant of significantly lower flapwise stiffness. The measurements are conducted in an anechoic wind tunnel under edgewise forward flight conditions. First, deformation measurements show that the thin-root rotor experiences a significantly larger time-varying deformation. Then, acoustic measurements present that the thin-root rotor produces a higher blade-passing frequency (BPF) tone and a lower broadband noise level compared with the baseline. Next, the results from a wavelet-based beamforming method present that the advancing blade produces the primary noise source, with broadband noise reduction attributed to weakened blade-vortex interaction (BVI). Finally, phase-locked stereo PIV measurements reveal that the reduced tip vortex intensity on the advancing blade results in the BVI noise reduction. These findings enhance our understanding of the impact of rotor blade deformation on eVTOL rotor acoustics.