Aeroacoustic Implications of Installed Propeller Interactional Aerodynamics and Transient Propeller Motions

Abstract

The emergence of advanced air mobility and sustainable aviation concepts have revived the interest in propeller-driven aircraft. A number of electric vertical take-off and landing (eVTOL) aircraft have been developed to cater to the demands of urban air mobility (UAM) and significant advancements have been made in unmanned aerial vehicles (UAV) equipped with vertical take-off and landing capabilities. However, the community acceptance of these new aircraft configurations highly depends on having a low noise footprint as they will operate in dense urban environments. Propeller noise is considered the major source of noise in these aircraft with the introduction of electric propulsion and it can significantly increase with the effects of installation and transient propeller motions. This study aims to comprehend the complex aerodynamic interactions within such aircraft that result from propeller installation and contribute to the generation of high noise levels. To understand the physics of propeller installation, a wingtip-mounted propeller was analyzed at several angles of attack using computational fluid dynamics (CFD) based on Reynolds-averaged Navier-Stokes (RANS) equations and computational aeroacoustics based on the Ffowcs Williams - Hawkings equation. The aeroacoustic implications of the propeller axis inclination and the propeller-wing aerodynamic interaction were studied in-depth. The propeller-wing interaction leads to a significant increase in propeller noise (~20 to 30 dB increase along the rotational axis) and causes the wing to generate a loading noise in the same order of magnitude as the propeller noise. To extrapolate the understanding of installation effects to a full aircraft, the aeroacoustic characteristics of a quadrotor biplane tailsitter were analyzed in both hover and forward flight focusing on the rotor-rotor and rotor-airframe aerodynamic interaction. The rotor-rotor interaction was found to be a significant source of loading noise in hover but having the fuselage as a physical barrier between the rotors largely reduces its effect. The airframe loading noise and rotor broadband noise are equally dominant as the rotor tonal noise when the aircraft is in forward flight. Moreover, the study evaluated the effectiveness of rotor synchrophasing in reducing the aircraft noise footprint and it showed promising results in hover, causing a reduction of aircraft noise by more than 10 dB.

Furthermore, an efficient computational aeroacoustics framework was developed to facilitate the computations, ensuring optimal utilization of the computational resources. The CPU and GPU parallelization and other optimization techniques were able to achieve a 98% reduction in computation time for an isolated propeller case. This enabled the rapid aeroacoustic computations of periodic and non-periodic problems. This was used to analyze the aeroacoustics of an isolated propeller undergoing a transition from hover to forward flight. The aerodynamic and acoustic results of the unsteady case were compared with quasi-steady cases performed at intermediate tilt angles. The quasi-steady CFD simulations predicted the unsteady transition aeroacoustics with reasonable accuracy. A tilting quasi-steady approach was proposed to better capture the aerodynamics and acoustics of the unsteady transition.