A groundbreaking study conducted by researchers at the University of Bristol, in collaboration with the University of Salford, has identified the source of an annoying noise produced by a specific type of electric aircraft engine known as boundary layer ingesting (BLI) engines. These engines are poised to play a crucial role in the aviation industry’s transition to electric and hybrid technologies, making this research particularly timely and significant. As the aerospace sector increasingly explores more sustainable forms of flight, understanding the acoustics generated by these engines becomes essential for public acceptance and the overall success of urban air mobility initiatives.
The research reveals that the annoying sounds produced by BLI engines arise from intricate interactions between the turbulent flow of air over an aircraft’s surface and the mechanics of the engines themselves. In their investigation, published in a recent edition of Nature npj Acoustics, the scientists delve into the nuances of aerodynamic sound generation, dissecting how various airflow patterns affect the perception of noise in aviation settings. Through this study, they highlight the complexities of noise generation and its impact on both flight safety and passenger comfort, thereby addressing a fundamental barrier to the widespread adoption of electric aircraft.
Central to this study is the concept of "haystacking," a term in the field of acoustics that describes the scattering of tonal sound fields due to turbulent airflow. This phenomenon leads to the dispersion of specific tonal sounds over a broader spectrum of frequencies, resulting in a sound that is not only loud but also unpleasant. The researchers conducted a fluid-mechanics-based assessment that informs the understanding of how two distinct types of broadband noise patterns—namely duct haystacking and fan haystacking—emerges when an aircraft operates under different thrust conditions.
During cruise conditions, characterized by low thrust, the fan’s suction is weaker, allowing the boundary layer flow around the aircraft to remain relatively undisturbed. This results in a lasered focus on the interaction between the turbulent flow and the acoustic properties of the duct, where duct haystacking becomes the primary contributor to perceived noise. In contrast, during take-off, high thrust results in significant changes to the airflow due to the strong suction from the fan. This phenomenon introduces high-momentum turbulent structures into the airflow, leading to fan haystacking that is particularly pronounced as the rotating blades slice through this chaotic flow.
Lead researcher, Dr. Feroz Ahmed, emphasizes the study’s implications beyond mere noise measurement. He notes that the two forms of haystacking can make future aircraft not only loud but also subjectively irritating. This raises pressing questions about the acoustic design of future aircraft and how engineers can optimize performance to create aircraft that fulfill both speed and noise reduction criteria. The findings provide a valuable framework for engineers aiming to develop quieter aircraft, ultimately enhancing the passenger experience and supporting urban mobility initiatives.
The research was conducted using a high-fidelity wind tunnel setup that closely mirrors real-world flight conditions. By employing cutting-edge instrumentation, including hot-wire anemometry, pressure sensors, and advanced microphones, the researchers gathered unprecedented levels of data on both airflow and acoustic signatures across various flight regimes. This exhaustive analysis enabled them to establish a direct link between the aerodynamic mechanisms at play and the actual sounds that passengers and communities nearby would experience.
The implications of this study extend to the entire aviation industry, especially for companies and systems working toward creating quieter electric vertical take-off and landing (eVTOL) aircraft. As urban air mobility solutions gain traction, the critical need to address the noise pollution caused by these new technologies rises. This research offers actionable insights that could shape design considerations for future electric aircraft models, including prominent examples like the Airbus ZEROe and the ONERA NOVA.
As the aerospace industry evolves, adhering to ambitious noise reduction goals — such as the European Union’s FlightPath 2050 objective of cutting aircraft noise by 65% — becomes imperative. The research provides designers with the tools necessary to innovate within the realm of aeronautical acoustics, fostering advancements that can lead to perceptually quieter engines. Such progress is not merely about compliance with regulations; it directly ties into the public’s acceptance of new aviation technologies.
Moving forward, the research team plans to refine and develop aerodynamic and acoustic controls that would minimize the effects of both fan and duct haystacking. Furthermore, they aim to broaden their analysis to include other propulsion concepts that involve turbulent flow ingestion. This will allow them to create a comprehensive understanding of the acoustic landscape associated with future aircraft designs.
In summary, this revolutionary work challenges conventional practices in aircraft noise management and provides a fresh perspective on how engineers can harness knowledge of airflow and acoustics to create future aircraft that not only maintain high-performance standards but also integrate seamlessly into urban environments. Understanding the noise produced by BLI engines could therefore set the stage for innovations that reinvigorate public trust and acceptance of electric flight technology.
Subject of Research:
Article Title: Aeroacoustics and psychoacoustics characterization of a boundary layer ingesting ducted fan
News Publication Date: 15-May-2025
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Image Credits: Credit: Dr Feroz Ahmed
Keywords
Engineering
