Thrust and Noise Experimental Assessment on Counter-Rotating Coaxial Rotors

Round 1

Reviewer 1 Report (New Reviewer)

1. The title of the paper needs to be corrected. Of the characteristics, the paper considers thrust, spectrograms, 1/3-octave spectrum of sound pressure levels and the A-weighted equivalent overall noise level.

2. Fig. 1 is not related to the problem considered in the article. It is proposed to delete it, leaving only the description in the text.

3. From the data presented in the article, it is difficult to understand whether the room in which this study was performed was intended for acoustic measurements. If the room is not intended for acoustic measurements, then it is necessary to estimate the measurement error, but it is better to present the noise data in the article in the form of energy characteristics of the sound field, which do not depend on the experimental setup.

4. In Fig. 9 and onwards, it is necessary to indicate the frequencies, not the numbers of 1/3 octave bands.

5. It is recommended to add 5-7 publications in the last 5 years to the list of references.

Author Response

1. The title of the paper needs to be corrected. Of the characteristics, the paper considers thrust, spectrograms, 1/3-octave spectrum of sound pressure levels and the A-weighted equivalent overall noise level.

RESPONSE: Thank you for your suggestion. We updated the title: “Thrust and Noise Experimental Assessment on Counter-rotating Coaxial Rotors”

2. Fig. 1 is not related to the problem considered in the article. It is proposed to delete it, leaving only the description in the text.

RESPONSE: Thank you for your suggestion. We deleted Figure 1.

3. From the data presented in the article, it is difficult to understand whether the room in which this study was performed was intended for acoustic measurements. If the room is not intended for acoustic measurements, then it is necessary to estimate the measurement error, but it is better to present the noise data in the article in the form of energy characteristics of the sound field, which do not depend on the experimental setup.

RESPONSE: Thank you for your comment. The testing room was indeed not directly intended for acoustic measurements. The possibility of providing the results in energetic terms will be exploited in the follow-ups of the work to provide a better understanding of the physic of the problem. In the first place, the paper aims at providing a preliminary sensitivity analysis of the variation of the performances (thrust and noise) concerning the distance between the propellers. Given the general aim of the test, a comparative approach has been followed.

4. In Fig. 9 and onwards, it is necessary to indicate the frequencies, not the numbers of 1/3 octave bands.

RESPONSE: Thank you for your suggestion. We specified the frequencies instead of the 1/3 octave bands on the y-axis of all the Figures.

5. It is recommended to add 5-7 publications in the last 5 years to the list of references.

RESPONSE: Thank you for your suggestion. We added 7 recent bibliographic items in the references:

1. Manetti, E. CFD Analysis, Experimental Validation and Optimization of an Octocopter Drone with Counter-Rotating Propellers. Aerotec. Missili Spaz. 102, 17–27 (2023).
2. Algarotti, D. An Experimental and Numerical Study of the Aerodynamic Interaction Between Tandem Overlapping Propellers. Aerotec. Missili Spaz. 102, 77–89 (2023). https://doi.org/10.1007/s42496-022-00138-1
3. Xue, Dongwen, et al. "Multidisciplinary Optimization Design of Low-Noise Propellers." Aerospace 10.3 (2023): 254.
4. Kotwicz Herniczek, Mark T., et al. "Evaluation of acoustic frequency methods for the prediction of propeller noise." AIAA Journal 57.6 (2019): 2465-2478.
5. Hambrey, Jason, et al. "Comparison of three popular methods for the prediction of high speed propeller noise." 23rd aiaa/ceas aeroacoustics conference. 2017.
6. Hambrey, Jason, et al. "Acoustic Prediction of High Speed Propeller Noise Using URANS and a Ffowcs Williams-Hawkings Solver." 35th AIAA Applied Aerodynamics Conference. 2017.
7. Capone, Alessandro, Fabio Di Felice, and Francisco Alves Pereira. "On the flow field induced by two counter-rotating propellers at varying load conditions." Ocean Engineering 221 (2021): 108322.

[Added in rows 44-55 and row 196]

Author Response File: Author Response.docx

Reviewer 2 Report (New Reviewer)

The authors considered the effect of the gap between two co-axial rotors on the thrust and noise. Tests were conducted on both single isolated propellers as well as on the counter-rotating system. Also, a comparison with a numerical model is performed.  By varying the distances between the propellers, a sensitivity analysis is performed with the aim of identifying the optimum configuration considering both noise and thrust performances. The authors showed that the distance between the two propellers is a sensitive parameter concerning both the thrust and the noise.

 The work provides valuable data. However, I believe the work can be improved by considering these two points:

1.      The noise spectra of the single and co-axial propeller do not show the conventional tonal noise. Any explanation?

 2.      It is well known that synchronization between the two co-axial propellers makes a big difference on the noise and can eliminate the tones. This issue needs to be discussed.

Author Response

1. The noise spectra of the single and co-axial propeller do not show the conventional tonal noise. Any explanation?

RESPONSE: Thank you for your interesting question. The spectra of both the single propeller and coaxial configuration do not show any tonal aliquot because the spectrogram was set in a 1/3 octave band rather than a narrowband. To extract tonal components, such as BPF, a narrowband analysis should have been carried but, at least in this first phase of our work, the main focus was oriented on understanding general parameters such as the overall level. Another factor that could have concurred with masking the tonal aliquot could have been the fact that the results are expressed with A-weighting that naturally provides some attenuation at least in the frequency range in which the BPFis supposed to fall given the angular velocity and the number of blades.

2. It is well known that synchronization between the two co-axial propellers makes a big difference on the noise and can eliminate the tones. This issue needs to be discussed.

RESPONSE: Thank you for your comment. We strongly agree that the synchronization between the propeller does represent an essential point when dealing with tonal noise. At this point, our control system was not suited to operate in synch mode so, unfortunately, it was not possible to perform the suggested analysis.

Author Response File: Author Response.docx

This manuscript is a resubmission of an earlier submission. The following is a list of the peer review reports and author responses from that submission.

Round 1

Reviewer 1 Report

The paper presents a nice report on an experimental study of counter-rotating propellers. The applied methods seem adequate and the reviewer is sure that the authors did a profound study with an interesting background. However, the authors aim to publish the report in a scientific journal, and the provided manuscript must be significantly improved to be of archival quality. The description of the test setup is far too detailed and not aiming to support the presented results. The actual analysis has not enough depth. It is recommended to perform a more detailed spectral analysis, including blade passing noise, background noise, harmonics, etc. Furthermore, spectra should be either in Hz or in Engine Order. Generally, the graphs are not presented nice, Fig 15-18 should be reduced to spectral content instead of temporal signals and the image quality of fig19 is poor. Overall, these shortcomings prevent publication in the current form and major rework is necessary. The reviewer leaves it to the editors to decide if this is acceptable for a major revision round. In this case the reviewer is happy to assess the quality again, if the manuscript is improved significantly

Reviewer 2 Report

Dear authors,

the reviewers main concerns are as follows, more details can be found in the attached document:

• Commercialism should be avoided at all costs. To the reviewer’s opinion, this also includes promotion of the companies work and field of research as done in the abstract and the introduction.
• The submitted manuscript strikes the reviewer as being a prelininary work/report rather than a well-prepared manuscript of archival nature. Multiple figures and plots provide little to no additional information (e.g. Fig. 3 – 13).
• The paper entirely lacks a detailed outline of the rotor blade design (shape, sweep, vortex design etc.), preventing to relate the gathered aerodynamic and acoustic data to a specific geometry.
• Section 1 provides an extended overview on possible vertical take-off concepts. However, the aim of the submitted paper is the aeroacoustic analysis of a rather rudimentary counter rotating rotor configuration, requiring a different motivation than generally highlighting the full concept. Moreover, methods and approaches used within the study can also be motivated in the introductory section.
• Section 2 might be reduced by a significant margin through focussing on the relevant information and combining Fig. 3 – 12 to a single Figure, outlining the experimental test. Everything else might be of use for internal documentation but has only little value for the scientific community.
• Entire details for aeroacoustic measurements including data processing are missing.
• Setup for acoustic measurements (sensor details, positioning, spectrala analysis parameters, sampling rate etc.) is missing entirely.
• Distinct tones are extracted from a time series which is simply not possible for a complex aeroacoustic signature of a propeller. Moreover, non-steady conditions are analysed and labelled as well as interpreted as steady-state. Moreover, the entire analysis seems to be based on these time series of questionable representative character.
• A numerical study is mentioned but outlined nowhere let alone results of such an analysis shown or discussed.

The significant lack of details for the experimental testing and data processing as well as shown data series of highly questionable quality in combination with apparent deficits in the spectral analysis lead the reviewer to suggest a rejection on the submitted manuscript.

Comments for author File: Comments.pdf