Noise Patterns of Overlapping Propellers in Forward Flight

The development of distributed electric propulsion (DEP) systems is gaining increasing attention as a core means to achieve greener and more sustainable aviation goals. This innovation is essential for advancing Urban Air Mobility (UAM), which utilizes multiple propellers to efficiently distribute thrust. Nevertheless, a significant challenge inherent in the implementation of DEP configurations is the production of elevated noise levels. This noise is a major concern not only for its environmental implications but also due to its direct impact on the public acceptance of these emerging air transportation technologies ^[1,2,3].

To mitigate these acoustic challenges, the aerodynamic interaction between propellers has been widely investigated, with special efforts dedicated to configurations such as tandem, side-by-side, and coaxial propellers. Understanding the mechanisms behind noise generation in these multi-propeller arrangements is crucial during both hover and forward flight. Concurrently, there is a growing demand across both industry and academia for higher payload capacities, which requires increasing the number of propellers. To maximize space efficiency in these increasingly complex systems, one engineering solution involves arranging the propellers in overlapping configurations^[4,5].

Overlapping configurations present a more complex aeroacoustic problem compared to traditional layouts, primarily because the wake shed from one propeller can directly impinge on others. Preliminary studies have shown that these interactions significantly contribute to both tonal and broadband noise. For instance, partial overlap can lead to a substantial reduction in the rear propeller’s aerodynamic performance due to the influence of the front propeller’s slipstream (Figure 1), while simultaneously increasing acoustic emissions. Overlapping propellers generally experience reduced thrust, increased thrust fluctuations, and intensified tonal noise, sometimes by up to 10 dB at low observation angles. A key characteristic of these setups is that one propeller interacts asymmetrically with the incoming wake of the other.

Despite these crucial findings, there is a distinct lack of experimental activities documented in the existing literature aimed at gaining knowledge about the aeroacoustic interactions between overlapping propeller configurations specifically under forward flight conditions. Previous research has often focused on hover flight conditions or solely on aerodynamic performance. Therefore, this experimental campaign focuses on the noise characteristics of partially overlapping propellers at very close axial distances. Specifically, it aims to identify the common and distinctive features of the aeroacoustic interaction compared to non-overlapping configurations. All results from these measurements can be found in Turhan et al. 2024 ^[6], which offers a detailed description of the test conditions and wind tunnel setup, and their original dataset can be found here.

Figure 1. General flow development of overlapping propellers.

Model Geometry

The experimental arrangement consisted of two identical propulsion units equipped with five-bladed constant-pitch propellers, representing a simplified dual-propeller configuration for aeroacoustic assessment. Each propeller had a diameter of 9 in. (228.6 mm) with a pitch-to-diameter ratio of $P/D$=1, and was manufactured from carbon-fibre/epoxy composites by Mejzlik (https://www.mejzlik.eu/). The corresponding blade chord distribution ($c/R$) and pitch angle variation are shown in Figure 2.

Figure 2. Distribution of the blade chord length c/R and the pitch angle for the 2-bladed 9 X 9 in² propeller used in this study.

Figure 3 presents the propulsion system arrangements examined in this study, covering both non-overlapped and overlapped configurations. The axial spacing between the two propellers is denoted by $l$, while s represents the lateral separation. Test cases were conducted for $s/D$> 1 to represent laterally spaced propellers, and for $s/D$< 1 to investigate overlap conditions. The specific lateral spacing ratios evaluated were $s/D$= 1.05 for the non-overlapped case and $s/D$=1.00, 0.74, and 0.39 for the overlapped cases. The reference spacing of $s/D$=1.05 was selected based on findings from Turhan et al. 2024^[6], which showed that changes in separation up to $s/D$=1.01 produced negligible aerodynamic differences. Using $s/D$=1.05 therefore provided an effective baseline, ensuring the propeller operated in an isolated manner without mutual interference. The axial separation was kept constant at $l/D$=0.06 for all configurations.

Figure 3. Propulsion system configuration.

Measurement Location and Techniques

Figure 4 presents the schematic of the far-field microphone arrangement and the overall experimental setup. The microphone array used for far-field noise measurements was positioned 1.75 m from the centre of the overlapped propellers, while the forward propeller was located 0.5 m downstream of the wind-tunnel nozzle exit. Acoustic data were acquired using G.R.A.S. 40PL microphones (1/4 inch), which provide a frequency response from 10 Hz to 20 kHz and a dynamic range of 142 dB, with an accuracy of ±1 dB. The array consisted of 22 microphones distributed in 5° increments over polar angles ranging from 35° to 140°. Signals were sampled at 216 kHz for a duration of 32 s. Power spectral densities were computed using the pwelch method in MATLAB (Welch, 1967), yielding a frequency resolution of 1 Hz.

Figure 4. Schematic representation of the experimental setup, and Image of the test rig.

Aeroacoustic measurements were performed over free-stream velocities between 8 m/s and 20 m/s in steps of 2 m/s. At a constant rotational speed of 8000 rpm, these conditions corresponded to an advance-ratio envelope of J=0.26–0.66, which falls within the range identified as aerodynamically relevant for this configuration.

The advance ratio is a nondimensional parameter defined as J = U_∞ / (nD), where U_∞ is the free-stream velocity, n is the propeller rotational speed in revolutions per second, and D is the propeller diameter.

Experimental Facility

The experiments were conducted in the aeroacoustic wind tunnel at the University of Bristol. This facility is a temperature-controlled, closed-circuit, open-jet wind tunnel designed for aeroacoustic testing. It provides a free-stream velocity range between 4 m/s and 40 m/s, with turbulence intensity as low as 0.2% in an anechoic environment.

The wind tunnel features a nozzle with a width of 0.5 m and a height of 0.775 m, operating with a contraction ratio of 8.4. The test chamber is acoustically treated, offering an approximate cut-off frequency of 160 Hz in compliance with ISO 3745 standards. To minimize acoustic reflections, all exposed surfaces, including the model support struts, anechoic chamber walls, and contraction nozzle are covered with foam wedges.

Flow Conditions

Inlet velocity, U_∞ = 8, 10, 12, 14, 16, 18, and 20 m/s.
Free-stream turbulence $\approx$ 0.2 %.
Propeller rotational speed = 8000 rpm.
Advance ratio(J) = 0.262, 0.328, 0.393, 0.459, 0.524, 0.590, 0.656.
Blade Passing frequency of two bladed propeller = 666.6 Hz.
Total number of microphones = 22.

Available Datasets

Each file contains three columns of data. The first column is the microphone number, ranging from 1 to 22, corresponding to microphone positions from 35° to 140° in 5° increments. The second column is the frequency, normalised by the Blade Passing Frequency, and the third column provides the sound pressure level (SPL).

s/D	U∞ = 8 m/s	U∞ = 10 m/s	U∞ = 12 m/s	U∞ = 14 m/s
0.39	SPL_sD_0.39_U∞_8ms	SPL_sD_0.39_U∞_10ms	SPL_sD_0.39_U∞_12ms	SPL_sD_0.39_U∞_14ms
0.74	SPL_sD_0.74_U∞_8ms	SPL_sD_0.74_U∞_10ms	SPL_sD_0.74_U∞_12ms	SPL_sD_0.74_U∞_14ms
1.00	SPL_sD_1_U∞_8ms	SPL_sD_1_U∞_10ms	SPL_sD_1_U∞_12ms	SPL_sD_1_U∞_14ms
1.05	SPL_sD_1.05_U∞_8ms	SPL_sD_1.05_U∞_10ms	SPL_sD_1.05_U∞_12ms	SPL_sD_1.05_U∞_14ms

s/D	U∞ = 16 m/s	U∞ = 18 m/s	U∞ = 20 m/s
0.39	SPL_sD_0.39_U∞_16ms	SPL_sD_0.39_U∞_18ms	SPL_sD_0.39_U∞_20ms
0.74	SPL_sD_0.74_U∞_16ms	SPL_sD_0.74_U∞_18ms	SPL_sD_0.74_U∞_20ms
1.00	SPL_sD_1_U∞_16ms	SPL_sD_1_U∞_18ms	SPL_sD_1_U∞_20ms
1.05	SPL_sD_1.05_U∞_16ms	SPL_sD_1.05_U∞_18ms	SPL_sD_1.05_U∞_20ms

Sample plots

To help you analyze the dataset, a MATLAB script has been provided. This script can be used to automatically create plots of SPL, and the sample plots using the matlab code is given in figure 5.

Figure 5. Sample plots of the SPL from the dataset, s/D = 1; velocity = 12 m/s at microphone position = 90°. Here the frequency (f) is normalised with the Blade Passing Frequency (BPF)

Open Access

This metadata is provided under the Creative Commons Attribution-NonCommercial 4.0 International License https://creativecommons.org/licenses/by-nc/4.0/). This license allows for unrestricted use, distribution, and reproduction in any medium, provided that proper credit is given to the original author(s) and the source. Also provide a link to the license, and indicate if any changes were made. Furthermore, this license does not allow the use of this material for commercial purposes.

Acknowledgments

The NWTF acknowledge the support for the metadata work from the EPSRC Network Grant, EP/X011836/1. The original dataset was funded by the European Union’s Horizon 2020 Research and Innovation Programme under Grant Agreement No. 882842 (SilentProp project).

Citation

If the user wants to cite the data presented here, then please cite both the NWTF metadata and the corresponding paper

References

1. Sahoo, S., Zhao, X. and Kyprianidis, K., 2020. A review of concepts, benefits, and challenges for future electrical propulsion-based aircraft. Aerospace, 7(4), p.44. DOI

2. Greenwood, E., Brentner, K.S., Rau, R.F. and Ted Gan, Z.F., 2022. Challenges and opportunities for low noise electric aircraft. International Journal of Aeroacoustics, 21(5-7), pp.315-381. DOI

3. Floreano, D. and Wood, R.J., 2015. Science, technology and the future of small autonomous drones. nature, 521(7553), pp.460-466.DOI

4. Huston, R.J., 1963. Wind-tunnel measurements of performance, blade motions, and blade air loads for tandem-rotor configurations with and without overlap (No. NASA-TN-D-1971). PDF

5. Turhan, B.B., Jawahar, H.K., Bowen, L., Rezgui, D. and Azarpeyvand, M., 2024. Turbulent flow impact on the acoustic and aerodynamic performance of overlapping propellers. In 30th AIAA/CEAS Aeroacoustics Conference (2024) (p. 3422). DOI

6. Turhan, B.B., Jawahar, H.K., Rezgui, D. and Azarpeyvand, M., 2025. Characterizing the noise patterns of overlapping propellers in forward flight. The Journal of the Acoustical Society of America, 157(4), pp.2790-2801. DOI