Small-Scale Rotor Aeroacoustics for Drone Propulsion: A Review of Noise Sources and Control Strategies

Round 1

Reviewer 1 Report

In this paper, the authors present a review on the present state-of-the-art on the reduction for noise emission in small-scale drones. The topic is interesting and deserves full attention nowadays. The authors are limiting the scope of their research to the use of passive flow control as the more accurate strategies for this type of propellers. The presentation of the relevant bibliography in the review is appropriate and major challenges (in physical and phenomenological terms) are identified and discussed. However, the description of the major findings up to date would require a more comprehensive summary. In addition, the relevant implications in terms of practicability and feasibility (in the context of typical industrial applications) is not treated convincingly. Excessive academic points-of-view are addressed, with a weak view of actual normative and standards regulations and presentation of general guidelines for efficient and silent drones is not promoted in the paper. This should be included in the paper to enrich the contents and the target of the investigation, On the other hand, methods to address the trailing edge noise in the case of broadband noise are not discussed.

Author Response

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Author Response File: Author Response.pdf

Reviewer 2 Report

The paper presents a review of noise mitigations strategies for small-scale UAVs. The authors provide a decent introduction to the problem and explain the major physics appropriately as well. They then explain the common active and passive noise control strategies studied today.

However, the methods presented here are more general strategies that are applicable to all scales of rotorcraft. There doesn’t seem to be too much description of small-scale UAVs. The methods described in this paper are all widely known and have been reviewed extensively for rotorcrafts. I would recommend major revisions so that the authors can include more small-scale rotorcraft strategies. They have also given only very short explanations on the unique challenges that small UAV aeroacoustics face which would be more helpful given the title and aim of the paper.

Some other questions:

Line 73 – Please provide evidence for the statement that the global drone market is ~127 billion in 2022. The year(2020) for the statement have already passed so this statement is invalid or new data is needed.

Line 109 – Why is the list of universities restricted to the EU? Since this is a review paper focused on general drone aeroacoustics I would expect the researchers to be familiar with global research happening on this subject. There are a lot of universities across the world (US-Penn State, Umaryland, Stanford University, NASA Ames etc.) that are also doing a lot of work on aeroacoustics.

Line 231 – Please provide a citation for this statement – “Due to the typical sizes of the control systems and of the actuators, these technologies are not suitable for small-scale propellers employed by MAVs.”

Author Response

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Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

The authors only seem to have addressed the minor points and not added anything substantial to the review and not addressed the major points mentioned in the review report at all. 

I suggest rejection or major revisions since it seems the authors are still only reviewing commonly used noise mitigations strategies and do not address the specific sources and strategies small drone aeroacoustics use. 

Author Response

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Author Response File: Author Response.pdf

Round 3

Reviewer 2 Report

I still do not see any substantial inclusion of methods specific to small scale rotorcraft. The review still is showing the common methods applicable to all scales of rotorcraft. 

Since it seems the authors are not addressing the major points and have not provided details about gaps in small scale UAV noise mitigation strategies and avenues for future research (which is the purpose of a review paper) I am recommending rejection of this manuscripts

Author Response

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Author Response File: Author Response.pdf