Investigation of Unsteady Rotor Loading on the Performance and Aeroacoustics of an eVTOL Vehicle

Abstract

With burgeoning interest within the aerospace industry towards the development of Electric Vertical Take-off and Landing (eVTOL) vehicles as part of Urban Air Mobility (UAM) networks of the future, the topic of rotor aeroacoustics has become an important area of research. Any future eVTOL aircraft must have minimal noise emissions if regulatory requirements are to be met, with noise likely to dictate the acceptance of their presence as a new facet within urban transportation infrastructure by the wider community. The noise from eVTOL rotors is primarily driven by loading noise generation mechanisms, which can be both deterministic and nondeterministic in nature. The present research seeks to characterise rotor noise in the context of eVTOL aircraft, across a broad range of operating conditions which are representative of a prospective flight envelope. A particular focus of the present work is dedicated towards understanding the impacts of unsteady loading noise and turbulence ingestion noise contributions across the different flight conditions experienced by eVTOL rotors.

Using small-scale fixed pitch rotors, a comprehensive set of experimental test campaigns were conducted in an anechoic wind tunnel facility at representative tip Mach number operating conditions, with the links between the aerodynamic performance, the flow-field characteristics and the far-field noise emissions explored in depth. A novel experimental methodology was developed, which, through synchronous load cell, microphone and hotwire anemometry measurements, permitted both time- and phase-averaged analysis of the rotor noise, aerodynamic loading and flow field characteristics across different flight conditions. The relationships between the rotor’s aerodynamic and aeroacoustic performance was considered with respect to the rotor tip Mach number and the freestream velocity conditions. After a thorough evaluation of the coupling between the rotor aerodynamics and noise in axial cruise flight conditions, significant efforts were dedicated towards characterising the complex transitional aerodynamics of tilting rotors in both forward and negative tilting orientations with respect to the freestream. The key noise generation mechanisms were identified in different phases of transition, with blade-vortex and blade-wake interactions identified as key noise sources in these flight conditions. To evaluate the changing noise and aerodynamic characteristics which might be expected of rotors operating in environmental turbulence, a detailed investigation of the impacts of large-scale turbulence ingestion was conducted for a rotor operating in axial flight conditions. Unique insights into complex forms of haystacking which result due to repeated chopping of large-scale turbulent eddies were revealed experimentally which differ to smaller-scale turbulence ingestion cases which are widely investigated in the broader literature. The results in the present work have identified many of the key noise generation mechanisms which can be expected of eVTOL rotors during various stages of their flight envelope and emphasised the need to test rotors at tip Mach number conditions which are representative of full-scale vehicles. The vast set of simultaneously sampled data presented here forms a significant contribution to the wider scientific community, with scope for broad utility both in industrial applications as well as in facilitating the development and validation of computational and analytical modelling of rotor aerodynamics and aeroacoustics.

Date of Award	17 Jun 2025
Original language	English
Awarding Institution	University of Bristol
Supervisor	Nick Zang (Supervisor) & Mahdi Azarpeyvand (Supervisor)