Pitch-Regulated Control Strategy for Coaxial Drone with Variable Rotor Space Ratio

Round 1

Reviewer 1 Report

The structure that the authors have followed when writing this article is valid and fits perfectly with a research standard. I would like to highlight the effort that the authors have made to highlight the results of their research by providing graphs and figures and even photographs of the facilities. I would like to provide the following comments on the article. Minor comments: - The phrase located on lines 113 and 114 is repeated on lines 96 and 97. Please correct this duplicate phrase. - Figure 14: it is not clearly visible where the specimen under test is located. - On line 352 the data of 1000r/min is included. It should say 1000rpm/min. - Same as the previous paragraph for line 357. - Figure 12: there is an error in one of the figures. It says "sapcing" where it should say "spacing". - Figure 15: The size of the photographs does not allow you to clearly see where the system under test is located. Major comments: The general approach of the article aims to understand the behavior of a rotor system composed of two propellers that rotate in opposite directions, calculating an indicator called pitch. The authors emphasize the aerodynamic performance of this set and the resolution of the problem by optimizing some parameters. The academic expression (1) must be referenced, indicating where it comes from, to help readers understand. In lines 198 and 199, a "nadir" is identified. Please indicate where in the graph. Identify the origin of the mathematical expression (8), and include it in the text appropriately referenced. Chapter 4 of the article refers to optimization. In this chapter, a broad definition and contextualization of the control problem is made, indicating that there is a significant improvement with respect to conventional control systems. The article seems to suggest that there is an automatic control strategy, although in reality the adjustment is manual, or so it seems. The authors should explain the entire chapter on control in more detail with some mathematical support to support the optimization theory. Even Figure 12 does not fully explain the optimization process, as it ends with the indication "Complete Optimization". In lines 312 to 314, percentage figures related to the optimization process are provided, but in reality, no specific objectives have been provided for the system performance. In lines 319 and 320, something related to pneumatic actuation is mentioned. It should be explained in detail what this statement is referring to. In summary, more attention should be paid to a good description of the parameter optimization process, as it is not clearly described.

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Author Response

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Author Response File: Author Response.pdf

Reviewer 2 Report

The authors have studied the change of propeller performance on different vertical spacing while implementing an actuated pitch control. 

In Page 2, sentence 92, consider writing as rpm or rev/min instead of r/min.

In Page 3, it is unclear which CFD tool was used. Please clarify and add details about the CFD platform. 

Also, the authors have briefly mentioned, RANS and SST, and later, it says the transient simulations are conducted using LES. So, was the hybrid model (e.g. DES, SBES, SAS-SST) used for simulations? 

Clarify those details and describe them appropriately.

In Page 6, it says about using Fluent mesh, and also talks about k-w and LES, which means the simulation was conducted in ANSYS. 

Did the authors study mesh independently on the propeller? If not why?

In page 5, Table1, the Pitch ratio sounds like, the ratio of the pitch angle with any other relevant parameter. It should be vertical spacing ratio Kd = 0.2. From equation 7, it is clear it is a spacing ratio, not a pitch ratio. Consider amending it throughout the manuscript.

In Page 6, from the observation of Figure 2 and 3, it appears the blade has a twist angle, often referred to as Beta. Can you please clarify, whether there is any variation on twist while changing the pitch angle? it would be better to add change in blade profile at the different pitch, which has been studied here.  

In Page 8, Figure 6 is rather visually complicated. It is better to present in 2-dimensional view, and perhaps normalize the thrust fluctuations by dividing them with average thrust. It will then be a clear visual.

Figure 6. what is the vertical spacing of the propeller used here, the same for figure 7; add those details here.

In page 9, Figure 8, add a range of contours to Figure 8. Also, consider increasing the upper colour range for the colour scheme. Currently, the red colour seems excessive. 

In page 9, it is better to represent as "co-axial rotors or co-axial propellers" rather than coxilcopter rotors. Consider changing them.

Page 10, figure 9 describes different pitches, but the results are for different propeller spacings. 

Page 15, Conclusions, talks about a 3.02% thrust increase and a 14.15% Cp decrease. However, the authors didn't mention maintaining the torque balance between the two propellers (upper and lower).  Please clarify the pitch angles of both propellers are changed simultaneously to maintain the balance, otherwise the study wouldn't be valid.

Author Response

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Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Thank you very much for taking into account my comments to your paper.