The Influence of Rotor Overlapping Azimuth on Compound Coaxial Helicopter Performance Based on Unsteady CFD Simulation
Round 1
Reviewer 1 Report
This paper presented numerical investigations on influence of rotor configurations on coaxial helicopter performance. The two rotor blades were stagger-positioned in the simulations. It is a piece of work interesting to helicopter design and operations. The study generated some useful data. However, the paper is not well written. It is hard to follow. This reviewer had to guess at many places. It is suggested that the authors ask someone who is good in both language and techniques to read through the paper to improve the paper quality.
Majors:
1) There are a number of errors in language.
2) Many places are quite confusing. It was difficult to understand what is compared, e.g. differences between two rotors or two states.
3) More information on the grid is required, e.g., grid size, sensitivity study.
4) It is not understood why the authors talked about the pressure of the lower surface of the wing only.
Minors:
1) The figures are too small and the legends are hard to read.
2) It is needed to more clearly quantify the phase shift (e.g., 45, 90, or 180 degrees?).
3) It is too heavy to list all the corrections/questions. A number of suggestions/questions are marked on the scanned copy for the authors’ reference.
Comments for author File: 
Comments.pdf
Author Response
Dear Editor and reviewers,
We have provided a point-by-point response to the reviewer's comments, please see the attachment.
Best regards!
Yours sincerely
Author Response File: 
Author Response.docx
Reviewer 2 Report
The work presents CFD simulations of a coaxial helicopter rotor, focusing on the influence of overlapping azimuth. This is a good contribution to the work on coaxial rotors.
The abstract gives the main details of the paper, but includes some terminology that is not explain. The introduction gives a good overview of the benefits of a coaxial rotor system, and previous work done on the problem. The section on previous work mainly lists the previous work, and the authors should try to instead use the previous work to point out research gaps and motivate their current contribution.
The results section gives a comprehensive and thorough analysis of the overlapping azimuth. The description is good, but the figures are too small and low resolution, making them difficult to read.
Some additional comments that must be corrected:
· Given the low amount of research on CFD simulations of coaxial rotors, it’s worth also citing work on UAVs. Please also cite the following papers:
Giljarhus, K.E.T.; Porcarelli, A.; Apeland, J. Investigation of Rotor Efficiency with Varying Rotor Pitch Angle for a Coaxial Drone. Drones 2022, 6, 91. https://doi.org/10.3390/drones6040091
Jinghui, D.; Feng, F.; Huang, S.; Yongfeng, L. Aerodynamic characteristics of rigid coaxial rotor by wind tunnel test and numerical calculation. Chin. J. Aeronaut. 2019, 32, 568–576.
Kim, Y.T.; Park, C.H.; Kim, H.Y. Three-Dimensional CFD Investigation of Performance and Interference Effect of Coaxial Propellers. In Proceedings of the IEEE 10th International Conference on Mechanical and Aerospace Engineering (ICMAE), Brussels, Belgium, 22–25 July 2019; pp. 376–383.
Yoon, S.; Chan, W.M.; Pulliam, T.H. Computations of torque-balanced coaxial rotor flows. In Proceedings of the 55th AIAA Aerospace Sciences Meeting, Grapevine, TX, USA, 9–13 January 2017; p. 52.
· All figures must be improved. They are too low resolution (consider using vector graphics, i.e. store as PDF), and labels and legends are too small
· The manuscript should be proofread to improve the English, as there are many grammar errors.
· See additional comments in annotated PDF.
Comments for author File: Comments.pdf
Author Response
Dear Editor and reviewers,
We have provided a point-by-point response to the reviewer's comments, please see the attachment.
Best regards!
Yours sincerely
Author Response File: Author Response.docx
Reviewer 3 Report
Dear Authors,
Coaxial helicopter rotors is an important topic for several reasons and any scientific study to explore the interaction of the rotors, effect on stability and thrust of the vehicle is valuable. The manuscript addresses the rotor overlapping azimuth effect on vehicle performance using unsteady CFD simulation. The problem statement looks to be clear and the work presented has value. However, there are some improvements needed to be accepted as below.
1) Include 'unsteady' in the title
2) Briefly describe state 1 and state 2. It is out of context so does not make sense to the reader
3) Figures must be enlarged.
4) References must be cleaned up.
5) Please include flow physics based images to show the interaction of the rotors
6) Include the 3D geometry of the rotor system and 2D airfoils in a separate image.
7) Need a good grid dependency study paragraph.
8) Describe which solver was used. Commercial or in-house?
9) A separate section describing the flow physics is necessary to explain the benefit of this rotor system.
Author Response
Dear Editor and reviewers,
We have provided a point-by-point response to the reviewer's comments, please see the attachment.
Best regards!
Yours sincerely
Author Response File: Author Response.docx
Round 2
Reviewer 1 Report
2nd round Review comments on paper
“The Influence of rotor overlapping azimuth on compound coaxial helicopter performance based on CFD simulation”
for publication in Applied Sciences
This paper presented numerical investigations on influence of rotor configurations on coaxial helicopter performance. The two rotor blades were stagger-positioned in the simulations. It is a piece of work interesting to helicopter design and operations. The study generated some interesting data. Compared to the previous version, it is much improved. However, there are still some major mistakes that must be corrected/addressed before accepting for publication.
Majors:
1) Unanswered question from previous review: More information on the grid is required, e.g., grid size, sensitivity study. How many control cells? Did you perform grid sensitivity study using coarse and fine meshes? What is the spacing at the wall? …
2) Uncorrected requirement from the previous review: Figure 1 is wrong. The loop “Grid block =1,N” must be within the loop “sub-iteration process”.
3) Page 14, line 305: This reviewer does not understand the sentence “As can be seen 305 from Figure 16, the total rotor power load in state 1 is essentially identical with the 306 power load of the upper or lower rotor.” Why is it identical not nearly double?
Minors:
1) Still a number of suggestions/questions for grammatical corrections as listed in the WORD file.
Comments for author File: Comments.pdf
Author Response
Dear Editor and reviewers,
We have provided a point-by-point response to the reviewer's comments, please see the attachment.
Best regards!
Yours sincerely
Author Response File: Author Response.docx
Reviewer 3 Report
Dear Authors,
All my questions have been answered and the scientific rigor of the manuscript has been improved. It is now acceptable for publication.
Author Response
Dear Editor and reviewers,
We have provided a point-by-point response to the reviewer's comments, please see the attachment.
Best regards!
Yours sincerely
Author Response File: Author Response.docx
