An Obstacle Avoidance Trajectory Planning Methodology Based on Energy Minimization (OTPEM) for the Tilt-Wing eVTOL in the Takeoff Phase
Round 1
Reviewer 1 Report
Comments and Suggestions for AuthorsThe paper deals with one of the issues linked to the new incoming urban transport system, the so-called Urban Air Mobility (UAM). In detail, the paper wants to solve the problem related to energy consumption and obstacle-avoidance trajectories by solving a suitable optimization problem.
Although interesting in principle, there are many unconvincing aspects. The following comments are reported in the order they have been remarked by reading the manuscript.
- Acronyms are not always explained before their use, particularly they should be avoided in the abstract and in the title, as well as in the keywords list
- The statement “Electric tilt-rotor flying car(eVTOL) is an efficient, economical and environmentally friendly solution to urban traffic congestion and travel efficiency issues” has not been proved yet, there are many studies that show contrasting results concerning the effective role of eVTOL – and UAM – to be a solution to urban traffic problems.
- References are reported, in most cases, without the year (e.g., page 2, lines 56, 65, 71, 87-88, 94, and so on in the next pages) and with not necessary information (e.g., the initials of the name). Some are re-called twice – apparently – in two different parts of the introduction section (e.g., page 2, line 87-88 and page 3 line 105-106).
- Almost all the references are conference papers, conference proceedings. Only about 10% are published on scientific journals, which is not enough for a scientific manuscript submitted to a scientific journal.
- The optimization model is based on some aerodynamics equations, generally used for fixed wings aircraft. It is not clear: a) to what extent these equations might be applied to a eVTOL as the one proposed in the paper; b) which is the approximation, in case, if these equations are not specifically applicable to the considered drones; c) references must be provided for all the reported formulas, in order to make clear which ones are from the literature and which ones, in case, have been originally proposed.
- Wing angle generally changes during the different flight phases. The adopted wing angle ranges should be clearly linked with the corresponding flight phase.
- One of the goals of the paper is to propose an obstacle-avoidance trajectory planning. However, it must be stated clearly which kind of “obstacle” is here considered – the following/preceding eVTOL? Obstacles due the urban context, which is one of the major issues to be solved for making UAM effective? Other?
- The obstacle avoidance problem is really important for adopting UAM solutions. For this reason, there are many in-progress documents – see for example FAA reports and UE-Eurocontrol reports – that are trying to identify the lower space structure in order to allow UAM flights in a safe, efficient and fluid way. In this perspective, some studies in the literature have developed the concept of 3D Urban Air networks – see for example https://www.mdpi.com/2071-1050/15/18/13551 and the related literature in the reference list – in order to set corridors where UAM flights can be allocated. The paper does not mention this important part of the literature, and the relationship between the existing studies in this context and the proposed obstacle-avoidance trajectory planning method.
Comments on the Quality of English Language
English is generally good, but there are some typos and some unclear sentences that should be reviewed
Author Response
1.Explanations have been provided for abbreviations that appear for the first time in the text.
2.'NASA Electric Vertical Takeoff and Landing (eVTOL) Aircraft Technology for
Public Services - A White Paper' describes the beneficial impacts of eVTOL on cities in terms of urban transport, regional mobility, reducing carbon emissions, and easing the burden on infrastructure.
3.Some references have been updated. For the second question, it's actually two different studies done by the same person.
4.Some references have been updated.
5.The subject of the study is the Airbus Vahana A3, a vertical take-off and landing vehicle with wings, which also has fixed-wing characteristics. For problem c, some formulas have been made references.
6.This paper focuses on trajectory planning for the take-off transition section of a tilting wing evtol, where the wing angle mainly ranges from 0 degrees (angle with the vertical) at the beginning to 90 degrees in the cruise state.The optimised state parameter plots show its variation over time.
7.Additions have been made to address this issue, and a note has been given at 4.2 that the obstacles are taken from two different environments around Purple Mountain.
8.Urban aviation networks do have an impact on trajectory planning, which will be one of the subsequent research directions.
Reviewer 2 Report
Comments and Suggestions for AuthorsThis is an interesting paper and addresses the challenges of high energy consumption and obstacles during the takeoff phase of e-VTOL aircraft. It proposes a trajectory planning method that focuses on minimizing energy while avoiding obstacles. By analyzing dynamics and establishing an energy minimization objective function, decision variables are determined and integrated with the SLSQP algorithm. The authors claimed in the abstract that validation with Airbus Vahana A3 was accomplished. However, it is not properly presented. Hence, the paper needs some adjustments and some additional inputs to be ready for publication.
Here are my comments:
Point 1.
Due to the use of several Roman and Greek symbols, a nomenclature must be included at the end of the paper.
Point 2.
The introduction must be expanded. It would be interesting to include a paragraph and discuss the recently published works from FAA regarding UAM operations. It would also be interesting to expand the discussion about the ongoing attempts towards the optimization of new eVTOL aircraft and optimal trajectories for e-VTOL operations. You may refer to the following publications about optimization of UAM operations and optimal eVTOL aircraft design.
1. Urban Air Mobility (UAM) Concept of Operations 2.0, https://www.faa.gov/sites/faa.gov/files/Urban%20Air%20Mobility%20%28UAM%29%20Concept%20of%20Operations%202.0_0.pdf
2. Rostami M, Bardin J, Neufeld D, Chung J. EVTOL Tilt-Wing Aircraft Design under Uncertainty Using a Multidisciplinary Possibilistic Approach. Aerospace. 2023 Aug 16;10(8):718. doi:10.3390/aerospace10080718
Point 3.
Many of the relations used require references. So, in the revised version you need to add them as most of the relations are taken from other sources, otherwise you have to prove that your relations are validated.
Point 4.
You need to add a 3-view picture of the Airbus e-VTOL aircraft. It is not clear to me how the validation has been done! Is there any comparison between your results and an existing result? I could not find that in your manuscript. Make sure to compare your results with the existing results and talk about the variations and possible reasons for that.
Point 5.
Add a paragraph and discuss the limitations and challenges of the proposed method, such as computational requirements, data limitations, or potential biases.
Comments on the Quality of English LanguageMinor editing of English language required
Author Response
1.I have added the Nomenclature at the end.
2.A brief introduction about UAM has been added at the beginning of the article.
3.The relevant section has been cited.
4.I have added an illustration of Vahana A3. The validation can be seen in figure4.
5.Added at the end of the article
Reviewer 3 Report
Comments and Suggestions for Authors(1) While reading the abstract, it is noted that the main objective of the study is to minimize energy consumption and avoid obstacles. However, the novelty of the study and its contribution to the field are not sufficiently emphasized. Please add a sentence highlighting the novelty of the study and its contribution to the existing literature.
(2) Throughout the article, the explanations of technical terms and abbreviations are missing at their first occurrence.
(3) The comparison of the results with the literature is insufficient. It has not been discussed how the obtained results align with or differ from other studies. Please compare the obtained results with the literature and discuss how these results contribute to or differ from existing literature.
(4) The revisions for some sentences are suggested as below:
“Electric tilt-rotor flying cars (eVTOL) are efficient, economical, and environmentally friendly solutions to urban traffic congestion and travel efficiency issues.”
“Secondly, based on the power discretization during the takeoff phase, the energy minimization objective function is established, and the constraints of performance limitations and spatial obstacles are derived.”
“The optimization needs to take into account the performance of the Vahana A3, including parameters such as battery power, speed, acceleration, and angle of approach.”
“The SLSQP algorithm is used to solve the trajectory optimization problem based on minimizing energy consumption.”
(5) The numbers on the horizontal and vertical axes, as well as the text on the graphs in Figures 3, 4, 5, and 7, are quite small. To improve readability and ensure that the results are clearly understood, I recommend increasing the size of the axis labels and the text on the graphs.
Author Response
1.The contribution of this paper is mentioned at the end of the fourth point of the conclusion, which is mainly to provide some reference for trajectory planning of other tilting wing evtol and to pave the way for 3D obstacle avoidance trajectory planning in the future.
2.First occurrence of abbreviations explained.
3.The comparative analyses were mainly carried out in conjunction with Figure 4.
4.Relevant sentences have been revised.
5.The relevant image has been reformatted and enlarged a bit.
Reviewer 4 Report
Comments and Suggestions for AuthorsThe paper aims to propose a trajectory planning method for the tilt-rotor flying car that minimizes energy consumption and avoids obstacles during the takeoff phase. The subject of the paper is exciting and in line with the aims and scope of the Journal. However, some issues need to be addressed.
1. The abstract should be improved. It should present the background, methodology, results, main conclusions, and contributions of the paper in a structured way. Currently, background, methodology and results overlap, while findings and contributions are not highlighted.
2. Part of the Introduction should be moved to a newly established Literature review section. Additionally, it should be supplemented with the main goal, research questions, used methodology, main results, conclusions, and contributions.
3. The paper lacks a proper literature review. The authors did not present the relevant literature from the field. They did not cover all aspects of the problem dealt with in the paper. The authors should add a section dealing with the related papers, covering all main elements, using the current literature (currently cited papers are outdated), identifying research gaps, etc.
4. The authors should consider merging sections 2 and 3.
5. The authors should check the notation used in the equations to make sure that they explained all symbols.
6. The paper does not have a discussion. The authors did not discuss how the results can be interpreted from the perspective of previous studies. Discussion should clearly and concisely explain the significance of the obtained results to demonstrate the actual contribution of the article to this field of research when compared with the existing and studied literature.
7. The authors did not provide any managerial (practical) or theoretical implications of the paper. Who can use the results of this study and for what?
8. The authors did not provide any limitations of the study.
9. The authors did not provide any future research directions. They should provide at least 3-5 solid future research directions interesting to most Journal readers.
10. Some technical issues should be addressed:
a) The definition of some abbreviations/acronyms is not clear. How is eVTOL an abbreviation/acronym for “Electric tilt-rotor flying car”? Additionally, some abbreviations are not defined the first time they appear in the paper (e.g. „SLSQP“, „OTPEM“, etc.). Define them all.
b) References should be numbered in order of their appearance. Currently, they are not.
c) References in the reference list are not formatted according to the Instructions for authors.
Comments on the Quality of English LanguageSome syntax and style errors should be addressed.
Author Response
1.Added a contribution at the end of the summary.
2.For reasons of time, it is too late to add an overview section.
3.This paper focuses on tilting wing evtol takeoff obstacle avoidance trajectory planning based on energy consumption minimisation, related research is relatively small, at present fails to cover the full range of please understand. Some references have been updated.
4.I apologise that the overview section could not be added in time due to time issues, so the merging of sections 2 and 3 could not be completed.
5.The relevant symbols have been explained and the Nomenclature has been added at the end.
6.The acceleration constraint is guaranteed to be below 0.3g, which ensures the comfort of the passengers; obstacle avoidance ensures the safety of the take-off process; and the objective function is to minimise the energy consumption, which has the practical significance of energy saving.
7.In response to this observation, this paper provides some clarification in the conclusion section, mainly to provide a reference for trajectory planning of tilting wing evtol.
8.A little addition has been made at the end.The sensitivity of the SLSQP to initial values is challenging in determining the initial values.
9.Four additions have been made, including four aspects of evtol 3D obstacle avoidance, landing trajectory planning, dynamic obstacle avoidance, and algorithm optimisation.
10.All three points have been changed accordingly.
Round 2
Reviewer 2 Report
Comments and Suggestions for AuthorsThe authors addressed all my concerns.
Comments on the Quality of English LanguageMinor editing of English language required
Reviewer 4 Report
Comments and Suggestions for AuthorsThe authors have addressed most of the issues identified in the previous review round, thus improving the quality of their paper. Therefore, I suggest an acceptance of the paper in its present form.
