This study investigates the acoustic characteristics of a distributed propeller-wing system in a tractor configuration, focusing on the potential for noise reduction using the synchrophasing technique. The experimental setup features three propellers with nacelles mounted side-by-side on the leading edge of a wing, resulting in aerodynamic interference between adjacent propellers due to a tip clearance of 5% of the radius. The study investigates the acoustic impact of varying relative blade-phase angles (synchrophasing) and further explores the implications of changing the angle of attack. The findings show that synchrophasing influences noise emissions at the locations investigated, potentially amplifying or reducing noise relative to a random configuration. Moreover, these effects persist when investigating higher angles of attack. @en

The integration of distributed electric propeller systems on aircraft wings presents complex aerodynamic interactions that are not yet fully understood. This study investigates the slipstream deformation in a distributed propeller configuration and compares it with a single propeller setup, visualised by experimental measurements of total pressure in the wake. Furthermore, we investigate the effects of relative blade phase angle on the resulting deformation. Overall, we identify several phenomena in distributed propeller-wing aerodynamic interaction that warrant attention in future research.@en

An experimental aeroacoustic study on the influence of the collective blade pitch angle in the noise emissions by an isolated propeller under different turbulent inflow conditions is presented. Acoustic and aerodynamic measurements are conducted in an anechoic, open-jet wind tunnel facility. Different inflow turbulence characteristics are achieved by employing square-mesh, square-bar turbulence grids positioned ahead of an additional 2:1 contraction at the wind tunnel's exit. It is found that the ingestion of grid-generated turbulence does not significantly impact the thrust produced by the propeller for any of the tested collective blade pitch angles. On the other hand, turbulence ingestion greatly increases noise production in both broadband and tonal components. The grouping of broadband noise around the Blade Passing Frequency (BPF) and its harmonics ("haystacking'') does not prove to be a phenomenon of particular relevance in grid-generated turbulence ingestion. A directivity analysis shows that an increase in inflow turbulence intensity is responsible for increased noise emissions downstream of the propeller.@en

This preliminary study evaluates the expected psychoacoustics annoyance of an array of distributed propellers operating at different synchrophasing angles between the propeller blades. The study, conducted in a hybrid test section wind tunnel, focused on a three-propeller array in a tractor configuration. The noise emissions were measured using a phased microphone array. Both conventional noise metrics and state-of-the-art psychoacoustic metrics were calculated, enabling a thorough analysis of the noise characteristics produced by these configurations and their expected noise perception. The study found that the relative blade-phase angle significantly influences the levels of the conventional noise metrics and psychoacoustic metrics. These results highlight the potential of controlling relative blade-phase angles to reduce noise annoyance. They also emphasize the importance of conducting psychoacoustic analysis in design configurations like the one examined here.@en

A low-order numerical tool for estimating noise emissions from distributed propeller configurations is presented. The paper describes the tool’s computational framework, which uses Hanson’s near-field theory to calculate the thickness and loading noise components. The formulation assumes steady blade loading, but an unsteady case can be handled numerically by redefining the pressure distribution over the blade at each new time step. Two represent ativecases are analyzed to validate the tool: an isolated propeller operating in uniform flow and an array of three propellers in a side-by-side configuration under aerodynamic inter ference caused by adjacent propellers. The results obtained from the low-fidelity tool are compared to high-fidelity data to evaluate the accuracy and differences in predicting the noise of a distributed propeller system. The low-fidelity tool provides accurate results for both cases, with less than a1.5 dB difference up to the fifth blade-passage frequency (BPF) when comparing tonal noise predictions at an observer located 10 diameters away and at the propeller plane. When analysing the source directivity at the first BPF, there is a difference of approximately 0.5 dB at the propeller plane. However, this difference increases to 6 dB as the observer moves toward the inflow direction. This difference is due to the dominance of broadband noise near the propelleraxis. The paper concludes with a noise analysis of the distributed propeller system, examining the relative importance of aerodynamic interference in the noise emitted by a propeller. In this case, the unsteady blade loading generated a tonal component of 40 dB at the first BPF in the propeller axis, while it had an insignificant effect at the rotor plane.

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