Preliminary Concept of Urban Air Mobility Traffic Rules

Round 1

Reviewer 1 Report

Dear authors, congratulations for your draft papper and the concept of space-time-capsules for UAVs in UAM. Hoewever, you concept can only work if the UAV also properly adhere to the exisiting rules of manned aviation (so-called SERA). In several places of your papper, you must mention this contraints/requirements:

Lines 28 through 35: Please note (and correct) that UAM is not limited to transport of person but it is expected that logistics and monitoring tasks will contribute significantly to the aforementioned traffic growth.

Lines 47 and 48: Besides this argument, also certification of the different types of eVTOL is a key bottleneck. Certification and the control of eVTOL in large numbers are highly interacting matters and thus should at least be mentioned here. In europe for example these are mandated by EASA as the so-called Standardised European Rules of the Air (SERA). See for example: Easy Access Rules for Standardised European Rules of the Air (SERA) - Revision from March 2022 – Available in pdf, online & XML format | EASA (europa.eu) FAA has published and mandates similar rules which can be found in the open domain.

Lines 58 though 60: This is a very nice dynamic approach and nicely extents the existing procedures with static safety and contingency buffers! You could mention and reference this static approach as a starting point for your research.

Figure 1: All mentioned tasks in Figure 1 should be plural since layouts, rules, designs will have to cover a large number of variations.

Line 96: You must mention performance characteristics (i.e. propulsion system especially also in case of degraded performance and/or malfunction) here too!

Line 140: There is no commonly defined “resistance factor” in aerospace sciences. You must please define/describe this resistance factor with commonly known terms like "lift over drag curve (Lilienthal)", "aerodynamic performance number" or something similar! 

Line 162: please include reference for wake decay distances chosen by you

Lines 234 though 237: These assumptions for accuracy errors are not acceptable. GPS info is sometimes as bad as +- 52 m. Therefore you are one order of magnitude too optimistic. And accuracy in vertical axis is highly dependent on properly aligning barometric altitude of manned aviation with GPS/GNSS altitude information of the UAVs!

Lines up to 284: Again, as mentioned before, this will only work safely, if height information is accurately aligned among all air traffic partiicpantes (manned and unmanned). You should mention this hard requirement here again!

Lines 339 and 340: The vertical corridors are highly inefficent for the limeted perfomance of UAV (they need to debleed their forward speed and go into a hover/vertical eV-mode). Spiralling or something similar would be much better. At least you must mention the lack of efficienycy wrt flight performance and endurance.

Figure 16 b: Please delete Chinese language symbols in Fig 16 b 

Line 377: You must mention that this differs from the regulations for vertiports for manned helicopters and also differs from the currently proposed FAA and EASA vertiport design requirements.

Line 474: You must add a little bit more details on the still open technology and procedural questions, like large differences in UAV performance, insufficient accuracy of position data, lack of harmonization and alignment with manned traffic.

Comments for author File: Comments.pdf

Author Response

Dear Reviewer 1, Thank you very much for your helpful comments on my article. Your comments have improved the quality of this manuscript, especially the description of the details, which I was lacking before. I believe that this experience of revising the article will also help me a lot in writing future articles as well. Thank you again!

Author Response File: Author Response.pdf

Reviewer 2 Report

The specific description of comments are set forth in appendix.

Comments for author File: Comments.pdf

Author Response

Dear Reviewer 2, Thank you very much for your helpful comments on my article. 
I am so lucky to have met a reviewer as attentive as you. Your comments have improved the quality of this manuscript, especially the description of the details, which I was lacking before. I believe that this experience of revising the article will also help me a lot in writing future articles as well. Thank you again!

Author Response File: Author Response.pdf

Reviewer 3 Report

The topic is novel, and the problem that the authors hope to solve is significant to the long-term development of UAM, but there are the following shortcomings

(1) The applicable scenarios for this problem are not clearly described in the introduction, and the literature review is too limited. For example, it does not specify what level of UAM requires traffic management and needs such traffic rules. 

(2)Some places in the full text that do not distinguish the content between UTM  and UAM. For example, figure 2 reflects the model of a light and small UAV, which is different from the eVTOL in urban air traffic.

(3) A series of formulas and models in section 4.1 such as designing aircraft movement space capsules are not reflected in the subsequent design of traffic rules.

(4) Fig 16? Is there Chinese in this picture? 

(5) The final experiment cannot provide adequate support for verifying these traffic rules proposed by the authors. It is better to do a simulation. For details to do such kinds of simulation, you can refer to some work done by the "Metropolis" project or the Airbus UTM team.

Author Response

Dear Reviewer 3, Thank you very much for your helpful comments on my article. We are so lucky to have met a reviewer as professional as you. Your comments have improved the quality of this manuscript, especially the description of the details, which we are lacking before. We believe that this experience of revising the article will also help us a lot in writing future articles as well. Thank you again!

Author Response File: Author Response.pdf

Reviewer 4 Report

Thank you for the opportunity to review this paper on what is a very pertinent topic. I found this paper interesting, but also a little strange in places. Some of the suggestions don't appear to be fully thought out or reasoned, which presents a significant issue for what the paper is trying to achieve. While these were simulated - only two aircraft were used and they were not flown autonomously. While I am not convinced yet of the paper's contribution, I do think it has potential. Below are the issues that I think need to be addressed to improve the paper and its contribution. These are listed in the order I came across them, not in order of importance.

1. Line 30 - the term "remote captain pilot systems" is not one I have heard before - I suspect the authors mean "remotely piloted aircraft systems"

2. There are missing spaces before the numbered citations

3. The literature review is quite sparse and does not touch on what are very important related issues. In particular, the entire paper is written from the point of view of UAM aircraft giving way with each other. However, there are going to continue to be drones, helicopters, aeroplanes and other aircraft within urban airspace. Not all airspace over cities is controlled - indeed significant portions over major cities are often classified as Class G airspace - it tends only to be airspace in proximity to major airports where control zones exist over urban areas. In Class G airspace, the current traffic rules rely on visual separation, not detect and avoid. Of course, airspace may change as a result of UAM, but it is also possible to have traffic rules within existing airspace, such as requiring piloted and remotely piloted aircraft to give way, or by requiring them to carry onboard equipment that will work with the detect and avoid systems of UAM. However, you cannot avoid this issue of deconflicting with extant traffic if you are going to write a paper about UAM traffic rules. Perhaps this is due to differences in how airspace is managed around the world, in which case, maybe a geographical context can be applied instead (e.g., maybe the situation is different in China where this research was carried out).

4. The UAM aircraft types that are used for this study are not typical of the designs being developed. Most are fixed-wing VTOL aircraft, using direct lift when taking off and landing, and then moving to aerodynamic lift once moving forward. Can the authors explain why they have dealt with multi-rotor and fixed-wing separately instead of looking at fixed-wing VTOL craft?

5. Section 4.1.1. - why are we calculating braking distance? Braking distance only applies on the ground for fixed-wing aircraft. However, eVTOL aircraft do not brake on the ground or in the air. They can decelerate or change course. Perhaps this is a matter of incorrect wording rather than incorrect content.

6. The suggested distances don't have much room for error. They may theoretically work if nothing fails, but typically within aviation, we build redundancy into the system in case something fails. How will the aircraft be accurately positioned? What systems will be used for positioning? What are their back-up systems if the primary means for positioning fails? These will all influence the safe distances for both x and y-axes. If the paper wants to be purely theoretical, then it at least needs to acknowledge these real-world constraints.

7. It seems to be appropriate to give at least a brief appraisal of the detect and avoid systems that might be used as part of this system. These will be the primary means of collision avoidance. For a review article that addresses common DAA technologies applied to drones, you may like to see: https://doi.org/10.3390/drones6070167

8. Lines 260 - 265 - this seems to contradict the earlier parts of the paragraph - can you please clarify these comments?

9. 4.2.2. again raises the question as to how the aircraft will detect each other and communicate their intentions with each other. How will one aircraft recognise that another aircraft is on an "emergency" flight?

10. 4.2.3. this will also be very difficult for fixed-wing aircraft as you cannot slow down below stall speed, and the relative airflow must always be sufficient to achieve lift. That said, going back to the earlier comment about most eVTOL aircraft being fixed-wing VTOL, they could revert to direct lift, but that seems very inefficient. Or the circle/ellipse could be very large to facilitate turns that are not too tight, but that would introduce other inefficiencies.

11. 4.2.4 - how will this work for fixed-wing aircraft, which cannot fly vertically? Even for multi-copter or fixed-wing VTOL, the use of direct lift straight up and down is very inefficient, so will reduce the flight endurance significantly. It would also seem to leave large spaces unfilled with traffic, potentially making an inefficient system.

12. Figure 16 has got one label in Mandarin.

13. It looks like a fixed-wing VTOL aircraft was used rather than a fixed-wing aircraft. This needs to be clarified early on in the manuscript, rather than waiting for readers to see a picture of the aircraft in figure 20(b)

14. The conclusion is a stub - this should be expanded upon to explain what the contributions of the paper are to the literature. There should also be a limitations and future research section to highlight some of the limitations of what has been proposed and how it has been tested, to then suggest future research to ameliorate those limitations.

Overall, I think the paper as it has been presented is quite limited in its contribution. However, if the above points can be adequately addressed, then it may have a useful contribution to the literature. I wish the authors all the best for revising the manuscript.

Author Response

Dear Reviewer 4, Thank you very much for your helpful comments on my article. We are so lucky to have met a reviewer as professional as you. It can be seen that you have extensive drone flying experience. Your comments have improved the quality of this manuscript, especially the description of the details, which we are lacking before. We believe that this experience of revising the article will also help us a lot in writing future articles as well. Thank you again!

Author Response File: Author Response.pdf

Round 2

Reviewer 4 Report

Thank you for the many changes made to improve the manuscript. I do think that these have made a significant improvement. However, there were a few of my comments that were unaddressed with changes as the authors argued that they were out of scope for the paper. Perhaps they do not warrant lengthy discussion, but upon re-reading the manuscript, I still see these issues as major limiting factors.

Rather than getting all of them revisited, I think the best change could be to introduce an "assumptions" subsection at the beginning of section 3. In that you can outline some of the assumptions like all flights taking place in urban environments, always being within controlled airspace, that there is a suitable DAA system, that this is a first iteration and can be improved to be made more efficient, all flights take off and land at vertiports and so on. That would at least make it clear that these rules are only going to work in some environments. I also would like the authors to add in a "limitations" section after the conclusion, which can then revisit the limitations associated with the assumptions that were made and call for future research to ameliorate those limitations. I agree with the authors that all issues cannot be solved now and making some progress in the area is valuable (which this manuscript does). However, I feel that the authors need to be explicit about the assumptions and associated limitations as this will make the paper a more scientifically sound paper.

Author Response

Dear Reviewer 4, we apologize for not addressing all your comments. In response to your comments, I have made the following three main changes.

① In response to the comments you made in the round 1, I have revised the entire manuscript.

② Introduced an "assumptions" subsection at section 3.

③. Added in a "limitations" section after the conclusion.

Author Response File: Author Response.docx