10:30
Experimental Aeroacoustic Measurements – Laboratory
10:30
20 mins
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Understanding and Controlling Laminar Separation Bubbles for Quieter Drone Propellers
SIMON WATKINS
Abstract: Small multi rotor drones and emerging Advanced Air Mobility vehicles rely on propellers that operate at low Reynolds numbers, where laminar boundary layer separation and the formation of laminar separation bubbles are common. These flow phenomena reduce aerodynamic efficiency and contribute significantly to increased broadband and tonal noise, limiting endurance, payload capability, and community acceptance in noise sensitive environments. Despite the importance of this regime, experimental data on low Reynolds number propellers and effective flow control strategies remains scarce.
This work presents an experimental investigation into the aerodynamic and acoustic performance of a propeller operating at low Reynolds numbers, with a focus on boundary layer augmentation through surface tripping. The primary research questions address how trip type and placement influence blade surface flow behaviour, propeller efficiency, and noise generation, and whether spanwise specific tripping can offer further performance benefits. A 9×6 propeller operating at a tip Reynolds number range of 4.3×10⁴ to 4.38×10⁴ was tested in a wind tunnel over wind tunnel speeds from 0 to 40 km/h. Aerodynamic forces and moments were measured using a calibrated force and moment balance, while acoustic measurements were obtained using a Brüel and Kjær sound level meter. Surface oil flow visualisation was employed to directly observe boundary layer behaviour, separation, reattachment, and vortex shedding associated with laminar separation bubbles. Tests were conducted in both static conditions and in a controlled inflow corresponding to a max of 40 km/h.
The results show that the chordwise location of boundary layer tripping plays a critical role in performance enhancement. When trips are positioned upstream of the natural laminar separation location, propulsive efficiency improves due to increased thrust generation with nominally similar torque levels. Oil flow visualisations confirm that appropriate tripping suppresses or weakens laminar separation bubbles and associated unsteady vortex shedding. Spanwise variations in Reynolds number were found to produce distinct flow responses along the blade, motivating the investigation of spanwise specific tripping strategies. This approach demonstrates potential efficiency gains beyond those achieved using uniform tripping methods (trips all the way to the tip of the propeller).
Overall, this study provides new experimental insight into low Reynolds number propeller aerodynamics and acoustics, introduces the concept of spanwise specific boundary layer tripping, and contributes data that can inform quieter and more efficient propeller designs for drones and future AAM applications.
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10:50
20 mins
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Potential annoyance from air-taxi manoeuvres in vertiport residents
Julia V. Lippold
Abstract: As urban transportation evolves to bypass traffic, reduce environmental impact, and improve connectivity, the rise of air-based mobility options like electric air-taxis offers a promising alternative to traditional ground transportation. However, the introduction of these air-taxis poses challenges in addressing potential noise-induced annoyance and disturbance in both urban and rural areas, underscoring the importance of balancing the benefits of UAM with residents’ needs and noise pollution limits.
To investigate the potential impact of air-taxi noise, we conducted a laboratory study with 48 participants (gender balanced, aged 19-70 years), testing a total of 144 different scenarios, compiled of the following factors: 4 numbers of movements during the scenario (between 1 and 7 air-taxis), 9 different observer points, consisting of 3 under the trajectory along the center line, 5 on sidelines and 1 in opposite direction to the flight path, urban versus rural background noise context, and 2 flight maneuvers (departure and approach). The scenarios were lasting 23.5 min and are based on auralized air-taxi sounds of a multi-rotor-driven air-taxi with four fixed plus two tilting rotors and real recorded background noises.
The subjects were exposed to six different scenarios, each experiencing at least one scenario with all flyover events, 6 different observer positions, and an equal number of approaches and departures. Background noises were tested as a between-subject factor. Scenario sounds were played through headphones in a laboratory designed to resemble a cozy living room. Subjects provided ratings on a numerical 11-point scale to assess the level of annoyance and disturbance caused by the presented target and background noises, while imaging themselves sitting in a living room, similar to the laboratory setting.
Multiple linear mixed-effects models using random intercepts to account for interindividual baseline differences, revealed that annoyance significantly increased with the number of flyovers. Likewise, annoyance increased closer to the vertiport. Annoyance was, furthermore, rated higher for take-offs compared to approaches whereas there was no significant difference between rural background noise scenarios compared to urban scenarios.
Our results emphasize the need of establishing data-driven regulations regarding flight numbers and flight paths, especially in residential areas, already before the launch of air-taxis.
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11:10
20 mins
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Acoustic characterization of drone response under various atmospheric conditions using an onboard microphone cage
Peter Hartford, Riccardo Zamponi, Christophe Schram
Abstract: As the use of unmanned aircraft systems becomes more frequent in urban areas, perceived noise will play a decisive role in public acceptance of new designs . Drone emission noise is primarily characterized by tonal components generated by the propellers, while electric motors and atmospheric turbulence mainly contribute to broadband noise . Rapid fluctuations in propeller rotational speed, caused by flight controller corrections in response to atmospheric conditions, can introduce additional sources of annoyance. These effects motivate the need for improved methods to localize noise emission sources under real-flight conditions.
The conventional approach for field characterization relies on fly-over measurements using ground-based microphone arrays . However, this method presents several limitations, including propagation effects related to source directivity, intrinsic source modulation, and the need for corrections associated with the drone’s position and orientation.
To address these limitations, we propose a novel measurement approach based on an onboard microphone cage attached to the drone to directly capture and localize noise generation sources. In this measurement campaign, a baseline characterization will first be conducted in an indoor environment, at the same location as in Zamponi et al. , to identify and localize nominal noise sources. Subsequently, a field characterization will be performed under varying atmospheric conditions, with a synchronized meteorological mast to characterize the inflow conditions. This approach will enable a direct comparison between the acoustic fields generated in quiescent and turbulent environments, fueling further discussion regarding strategies and design modifications aimed at reducing drone noise emissions.
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