A Review of Flapping Mechanisms for Avian-Inspired Flapping-Wing Air Vehicles

Round 1

Reviewer 1 Report

In this work, the flapping mechanisms in recently developed biometric flapping-wing air vehicles are reviewed. Flapping mechanisms were classified and analyzed according to wing motion and aerodynamic performance improvement strategy. The results obtained in this this work is interesting , and will provide guidance in the initial design step of FWAV flapping mechanisms., therefore, I suggest to accept to publish this article in aerospace.

Author Response

Thank you for your kind efforts on my manuscript.

Reviewer 2 Report

This paper provides an extensive review on the flapping mechanisms of avian-inspired flapping wing aerial vehicles. The followings are my comment:

 

1.      Line 52: "90% of FWAVs are insect-inspired FWAVs". How was this 90% determined? I suggest changing this to something less definitive.

 

2.      The manuscript classified the mechanisms into linkage mechanism and compliant mechanism. As a matter of fact, both types are driven by linkages. The difference is on the joints, one category uses revolute joint, the other uses compliant joint. Perhaps, it is better to name them “revolute joint mechanism” and “compliant joint mechanism”.

 

3.      Instead of calling the mechanisms "multi-motion", it is better to call them "multi-degree-of-freedom".

 

4.      DelFly-IV was mentioned, as far as I am aware, no prototype is being called DelFly-IV. The notable DelFly prototypes are DelFly-II, DelFly Explorer, and DelFly Nimble.

 

5.      In the strategies to increase lift-to-drag ratio, clap-and-fling was mentioned, and DelFly is being used as example. Please check on how the lift of DelFly is being defined. The prototypes almost fly like they are hovering due to the high incident angle flight, “lift” can be confused with “thrust”. Since this paper is focusing on avian-inspired prototype, using DelFly as an example is not appropriate. Throughout the paper, please check the lift and thrust definition of the cited references to ensure fair comparison, especially in Table 3.

 

6.      Given the title and focus of the paper, the authors must draw a clear line between insect-inspired and avian-inspired. In my opinion, avian-inspired prototypes are supposedly designed to achieve forward flight at the very least. One can argue that hummingbird is an avian species, but hummingbirds fly more like insects (large flapping angle, high flapping frequency, hovering). A more stringent classification would involve mimicking avian wing shape, and avian flapping angle and frequency. In the discussion of flapping mechanism, the following reference are found to be non-avian-inspired: [44-45], [47], [50], [53], [60], [62-67], [72-73], [75], and [89]. Reference [91] is mimicking hummingbird hovering motion, I think hummingbird is more like insects in hovering mode. The authors should consider removing these references and significantly shorten the discussion on single-axis mechanism, which is more relevant to insect-inspired prototypes anyway.

 

7.      It is inappropriate to refer to the same mechanism twice. It could give the wrong impression that a certain type of mechanism is being utilized more than others. The following identical mechanisms are cited twice: [51] and [52], [77] and [78].

 

8.      The following references are based on simulation or mathematical models, no physical prototypes were built: [39], [59], [60], [65], [71], [83], and [85]. I believe a review paper like this should be referring to physical prototypes that are at least successfully tested in bench tests. I suggest either removing these references or emphasize that the works are purely simulation.

 

9.      Citation on reference [60] in Table 1 is highly inappropriate. It is cited as a slide-crank mechanism prototype, but no mechanism was presented in the paper.

 

10.  Citation on reference [6] and [67] in Table 1 are not appropriate. They are cited as flexible frame mechanism prototypes, but the frames are rigid. The flapping motions are driven by pully systems, but I don’t think they count as flexible frames.

 

11.  Citation on reference [82] in Table 2 is not appropriate. The description on the mechanism is very vague, it mentioned only one actuator for the spread and folding motion, and one servomotor for flapping. It is hard to tell if the wing folding mechanism is passive or not.

 

12.  Some DOI links don’t work.

 

13.  Some IEEE papers have DOIs, but not included in the reference list. Please be consistent with the reference format.

Author Response

Please see the attached file.

Author Response File: Author Response.docx

Reviewer 3 Report

This paper introduces the avian-inspired flapping-wing air vehicles, overviews the classification of flapping mechanism and identifies the future development direction. As a review paper, the length of this paper should be longer. Many literature evaluations are too general. For example, in Section 3.1.2, many literature are just listed here without sufficient comparison or evaluation from the authors. More objective evaluation of the advantage and disadvantage for each motion or mechanism should be conducted, and the applicable situation should be explained for each mechanism. In addition, there is a lack of recent literature in the past five years. It is suggested to add more recent publication for analysis and discussion.

 

Author Response

>> First of all, we would like to thank you for taking your valuable time to review this manuscript. We have carefully revised our manuscript according to your comments. Detailed answers to your comments are given below. In the revised manuscript, the modifications are highlighted with the blue-colored text.

We have added a brief analysis of the advantages and disadvantages of each categorized mechanism and the applicable situation in section 3.1.2. Additionally, we introduced in section 3.2 which mechanism was used, taking into consideration the flight specifications and strategies based on operational purposes. To emphasize this point, we added the following sentence in section 3.2: 'This section can be a reference for selecting a flapping mechanism according to operational purposes.' (line: 342 in rev. manuscript)

• Added: As mentioned earlier, when using rigid mechanism, it is easy to implement the de-sired wing-tip trajectory. However, when using compliant mechanism, it is difficult to create the desired trajectory. It is implemented passively, so there is a disadvantage that the trajectory changes depending on the operating environment. (line: 259 in rev. manuscript)
• Added: When performing torsional motion through a mechanism, there is a disadvantage of increased complexity and weight of the driving unit. On the other hand, when creating structural torsion of the wing itself, there is a disadvantage of increased complexity and weight of the wing structure. Therefore, for small-span flight vehicles with high frequency operation, it is advantageous to implement torsional motion through a mechanism due to the additional torque required by the increased wing weight when using a wing mechanism that creates structural torsion of the wing. (line: 282 in rev. manuscript)
• Added: Similar to folding mechanism, the rigid mechanism is easy to implement the intended trajectory, but the compliant mechanism has the disadvantage of being difficult to implement the intended trajectory and may not be able to implement wrist flexing motion depending on the flight speed or operating environment. Both wrist flexing and folding mechanisms have the disadvantage of making the wing structure more complex and heavier. In particular, the rigid linkage mechanism requires additional links and joints compared to the mechanism that only performs the one axis wing motion. The rigid linkage mechanism is suitable for FWAVs with a large span that can generate a large lift force to compensate for the increased weight, while for small FWAVs, the compliant mechanism is relatively lightweight and simple, making it suitable. (line: 302 in rev. manuscript)

When classifying mechanisms into large categories, we mostly referred to older papers due to the absence of recent novel mechanisms. As suggested in your comment, we have added some recently published mechanisms that could be useful to reference in the paper.

• Added: Dewangan et al. [69] proposed a new concept of flexing mechanism using rigid linkages. Although it is still in the concept stage, it has the advantage of being able to increase lift while quickly folding using a crank slotted lever mechanism. (line: 299 in rev. manuscript)
• Added: Li et al. [88] proposed a bat-type flapping-folding mechanism. The flapping motion is implemented based on the rigid linkage mechanism, and the locking system fully spreads the wings during the downstroke, and controls the folding by retracting and expanding the wings during the upstroke. There is no prototype of the mechanism, but aerodynamic analysis has demonstrated an increase in average lift. (line: 374 in rev. manuscript)
• Added: Carollo et al. [97] used 3D printing technology to prototype a flapping mechanism. The mechanism is characterized by being composed of one component. It has the ad-vantage of being light and easy to manufacture and replace. Although only a concept has been proposed, the technology applied to the mechanism in the paper can be ap-plied to a flapping mechanism mimicking a small bird. (line: 441 in rev. manuscript)

Reviewer 4 Report

See the reviewer's comments file.

Comments for author File: Comments.pdf

English of the whole manuscript must be revised.

Author Response

1. Since it is a review paper, the authors must improve the manuscript by discussing different flapping mechanisms.

>> First of all, we would like to thank you for taking your valuable time to review this manuscript. We have carefully revised our manuscript according to your comments. Detailed answers to your comments are given below. In the revised manuscript, the modifications are highlighted with blue-colored text.

As you commented, we have added explanation of what are the pros and cons of the mechanism and for what purpose it can be used. In addition, examples of several mechanisms have been added to present examples of various mechanisms and explanations have been added for application.

• Added: As mentioned earlier, when using rigid mechanism, it is easy to implement the de-sired wing-tip trajectory. However, when using compliant mechanism, it is difficult to create the desired trajectory. It is implemented passively, so there is a disadvantage that the trajectory changes depending on the operating environment. (line: 259 in rev. manuscript)
• Added: When performing torsional motion through a mechanism, there is a disadvantage of increased complexity and weight of the driving unit. On the other hand, when creating structural torsion of the wing itself, there is a disadvantage of increased complexity and weight of the wing structure. Therefore, for small-span flight vehicles with high frequency operation, it is advantageous to implement torsional motion through a mechanism due to the additional torque required by the increased wing weight when using a wing mechanism that creates structural torsion of the wing. (line: 282 in rev. manuscript)
• Added: Similar to folding mechanism, the rigid mechanism is easy to implement the intended trajectory, but the compliant mechanism has the disadvantage of being difficult to implement the intended trajectory and may not be able to implement wrist flexing motion depending on the flight speed or operating environment. Both wrist flexing and folding mechanisms have the disadvantage of making the wing structure more complex and heavier. In particular, the rigid linkage mechanism requires additional links and joints compared to the mechanism that only performs the one axis wing motion. The rigid linkage mechanism is suitable for FWAVs with a large span that can generate a large lift force to compensate for the increased weight, while for small FWAVs, the compliant mechanism is relatively lightweight and simple, making it suitable. (line: 302 in rev. manuscript)

 

2. Remove the words like “We, Our”, etc. and rephrase them with the proper words.

>> Thank you for the comment. Below are the line numbers with sentences where "We" or "Our" are used.

We: Line 13, 14, 16, 67, 69, 70, 76, 133, 463

Our: Doesn’t exist

Line 13:

• Original: In this review, we first introduce the flight characteristics of birds and bats, and explain how current studies have used these flight characteristics in the development of FWAVs.
• Revised: In this review, the flight characteristics of birds and bats are introduced, and the incorporation of these flight characteristics into the development of FWAVs is elucidated, and the utilization of these flight characteristics in the development of FWAVs is explained.

Line 14:

• Original: We then classify and analyze the flapping mechanisms according to wing motion and aerodynamic performance improvement strategy.
• Revised: Next, the classification and analysis of flapping mechanisms are conducted based on wing motion and the strategy for improving aerodynamic performance.

Line 16:

• Original: Finally, we explain the current research gap and propose possible future directions for further research.
• Revised: Lastly, the current research gap is elucidated, and potential future directions for further research are proposed. This review can serve as a guide during the early development stage of FWAVs.

Line 67:

• Original: In this paper, we review the mechanisms for avian-inspired FWAVs. We first introduce avian flight characteristics and explain how current studies have mimicked these flight characteristics.
• Revised: In this paper, the mechanisms for avian-inspired FWAVs are reviewed. The avian flight characteristics are first introduced, followed by an explanation of how current studies have successfully mimicked these flight characteristics.

Line 69:

• Original: We then classify and analyze various flapping mechanisms according to wing motion and aerodynamic performance improvement strategy.
• Revised: Next, various flapping mechanisms are classified and analyzed according to wing motion and the strategy for improving aerodynamic performance.

Line 70:

• Original: Finally, we ex-plain the current research gap and future directions for FWAV studies.
• Revised: Finally, the current research gap in FWAV studies is explained, and future directions for research in this field are proposed.

Line 76:

• Original: By discussing which wing motions effectively generate aerodynamic force and which flight strategies they adopt, we can understand strategies applicable to FWAVs.
• Revised: By discussing which wing motions effectively generate aerodynamic force and which flight strategies they adopt, applicable strategies for FWAVs can be understood.

Line 133:

• Original: In this section, we analyze how flapping mechanisms mimic the wing motions of flying animals, and discuss the aerodynamic advantages obtained from the flapping motions.
• Revised: The following section provides an analysis of how flapping mechanisms emulate the wing motions of flying animals, and the aerodynamic advantages derived from the flapping motions are discussed.

Line 463:

• Original: Here, we performed a review of mechanisms for realizing FWAV wing motion.
• Revised: Here, a review was conducted on mechanisms utilized to achieve wing motion in FWAV.

 

3. Figure 1 shows the Reynolds number and Reduced Frequency. Explain the Reynolds number.

>> We agree that we need to explain what Reynolds Number and Reduced Frequency is.

 

• Added: The Reynolds numbers is a dimensionless quantity which represents the ratio of inertial forces to viscous forces of a fluid. The reduced frequency is a dimensionless number representing the ratio of forward speed and flapping frequency.” (line: 28 in rev. manuscript)

 

4. Discuss the Active and Passive controlled systems.

>> Thanks for pointing out Active and Passive systems.

First, a passive controlled system means a system in which two or more different motions do not occur independently but affect each other and occur simultaneously. For example, twisting motion occurs automatically during wing flapping due to the flexibility of the wing and the effect of the aerodynamic force acting on the wing. In this case, the twisting motion is passively caused by the flapping motion. The passive controlled system has the advantage of being able to create additional motions without an actuator and using them to increase the efficiency of aerodynamic force.

Next, an active controlled system refers to a system in which two or more different motions occur independently. At this time, the degree of freedom of the system is the number of motions to be controlled, and each motion requires a motor to control it. Therefore, as the degree of freedom of the system increases, the number of motors also increases. In the example presented above, the twisting motion occurs passively to the flapping motion, but if we want to control the twisting motion separately from the flapping motion while adjusting it to suit the flight situation, we must additionally mount a motor that can control the twisting motion. In this case, there is an advantage in that each action can be changed as desired by the user.

 

5. Discuss the Artificial Intelligence of a Flapping Wing system.

>> Thank you for the comment. However, this review paper focuses on the flapping mechanism and classifies flapping aircraft based on their flapping motions. The discussion of artificial intelligence in the flapping system is slightly off-topic for this review paper. The reason for citing reference [103] in line 343 (in original manuscript) is to introduce the flapping motion of Delfly and the mechanism that drives this motion. The mentioned book in reference [103] introduces artificial intelligence as a tool for autonomous flight and vision-based navigation. Therefore, it seems that the discussion of artificial intelligence can be disregarded at least within the scope of this paper.

 

6. Provide the Nomenclature.

>> Thank you for your comment. The Nomenclature has been added in the revised manuscript.

 

Round 2

Reviewer 2 Report

The authors have made significant changes to the manuscript according to my suggestions. I agree that the paper can be published in the current form.