Innovative Box-Wing Aircraft: Emissions and Climate Change

Round 1

Reviewer 1 Report

1. I would recommend writing abbreviations through a dash (eg AFR - air-to-fuel mass ratio, etc.).
2. In my opinion, the introduction should be an introduction to the whole article and not start right from the subchapter. In my opinion, it is more appropriate to combine the subsections in the introduction into a section on literature review.
3. The title 2.2 should be clarified, as it is not clear what the boundaries of the system are, i.e. whether it refers to a research methodology or an airplane.
4. The title of section 2.4 also needs to be clarified, as the impact of what or for whom is unclear. This same is with 3.2 section
5. It is not clear why section 4.1 Methodology appears in the discussion and recommendations section. In my opinion, methodological issues are already undisputed here, and the results of the applied methodology are discussed.
6. Specify the title of section 4.3, as the challenges for whom or by whom are unclear.

Author Response

Thank you for you advice. Below, our answers.
Regards

1. I would recommend writing abbreviations through a dash (eg AFR - air-to-fuel mass ratio, etc.).

Thank you. We are surprised, as in the literature we did not see other terms here used written as you suggested, with the exception of AFR, which is reported as you recommended. Please, if you know other abbreviations that you commonly see written as you advise, let us know and we will consider editing.

2. In my opinion, the introduction should be an introduction to the whole article and not start right from the subchapter. In my opinion, it is more appropriate to combine the subsections in the introduction into a section on literature review.

Thank you. Previously the Introduction was a single chapter, as you recommend. We did these modifications according to a Reviewer’s preferences. Hence, to follow also your advice we removed the first title ‘Aviation and climate change’. Now, aviation and climate change are introduced as before at the beginning of the Introduction, but without being a subchapter.

3. The title 2.2 should be clarified, as it is not clear what the boundaries of the system are, i.e. whether it refers to a research methodology or an airplane.

The terminology ‘System boundaries’ is a standard of Life Cycle Assessments. It refers to the boundaries of the system under study. We do not recommend any modifications.

4. The title of section 2.4 also needs to be clarified, as the impact of what or for whom is unclear. This same is with 3.2 section.

Thank you. Title of section 2.4 is ‘Impact assessment method’. The section describes the impact assessment method used to estimate the impacts of the emissions. Hence, we do not recommend further modification. Concerning the title of section 3.2, it has been modified according to your suggestion as follows: ‘Impact assessment of Prandtlplane emissions’.

5. It is not clear why section 4.1 Methodology appears in the discussion and recommendations section. In my opinion, methodological issues are already undisputed here, and the results of the applied methodology are discussed.

We agree. We removed the title of section 4.1 and we slightly modified first paragraph of Section 4.

6. Specify the title of section 4.3, as the challenges for whom or by whom are unclear.

Apologies. Title is now reported as follows: ‘Prandtlplane development: Challenges’. Thank you.

Reviewer 2 Report

The paper presents an assessment of the advantage of a long-haul passenger Prandtl box wing airplane adoption on the greenhouse emissions and compares it with a conventional configuration airliner. 

The study is relevant and was carried out within a European Union funded project.

The configuration of the so-called Prandtlplane has been the target of several publications by the authors. Nevertheless, this paper is original in that it concerns the merit of this unconventional airplane configuration regarding its emissions.

The main weakness of the paper is that neither the introduction covers the state of the art on equivalent studies dedicated to other unconventional aircraft configurationsnor their results on the Prandtlplane are compared to any other innovative aircraft configurations.  This is partially offset by using a conventional configuration as a benchmark but it would be more valuable to include other authors' state of the art results in assessing the merit of the box wing configuration relative to other alternative aircraft configurations, even if just the conventional benchmark configuration was compared with similar benchmark assessments existing in the literature.

line 180: Prandtlpane should be Prandtlplane

Not complete references
[7] is [online] but not stated as such
[38] Frediani, A., Cipolla, V., & Oliviero, F. (2015). Design of a prototype of light amphibious PrandtlPlane. 56th AIAA/ASCE/AHS/ASC Structures, 
Structural Dynamics, and Materials Conference. doi:10.2514/6.2015-0700

Author Response

Dear Reviewer,

Thank you. Below our answers.

The main weakness of the paper is that neither the introduction covers the state of the art on equivalent studies dedicated to other unconventional aircraft configurationsnor their results on the Prandtlplane are compared to any other innovative aircraft configurations.  This is partially offset by using a conventional configuration as a benchmark but it would be more valuable to include other authors' state of the art results in assessing the merit of the box wing configuration relative to other alternative aircraft configurations, even if just the conventional benchmark configuration was compared with similar benchmark assessments existing in the literature.

Since the disruptive/innovative aircraft architecture under study are many, the choice of using a common reference conventional aircraft, such as the CeRAS or the NASA Common Research Model (CRM), is the preferred way to assess the potential benefits of an alternative solution. Each research group working on a different architecture can use the same publicly available data and models for their comparisons. In addition, data and model of other innovative aircraft configurations are usually not public or incomplete, therefore making high fidelity analyses not reliable or impossible.

line 180: Prandtlpane should be Prandtlplane

Thank you, text has been amended.

Not complete references
[7] is [online] but not stated as such
[38] Frediani, A., Cipolla, V., & Oliviero, F. (2015). Design of a prototype of light amphibious PrandtlPlane. 56th AIAA/ASCE/AHS/ASC Structures, 
Structural Dynamics, and Materials Conference. doi:10.2514/6.2015-0700

Apologies, references have been updated. If further modification is needed, please let us know.

Reviewer 3 Report

I read the manuscript very carefully and the conclusion is that it can be published. The paper is very well documented, with many bibliographic sources and can brings added value to the field of aviation by contributing to the mitigation of emissions and climate change.

Author Response

Thank you, we appreciate.

Regards

Round 2

Reviewer 1 Report

1. With the shortening of your term, AFR is all right. I meant you did a List of Abbreviation before the introduction. That is where I recommend that a dash and an explanation follow the abbreviation and that the abbreviation you provide appear as follows:
   AFR - air-to-fuel mass ratio
   BC - black carbon
2. I would, however, recommend that you make an introduction to the general article, highlighting the purpose of the article, and move the subsections to a literature review, for example. But if these corrections are the result of comments from other reviewers, they may in principle remain as they are.

Author Response

Dear Reviewer.

Thank you, list of abbreviations have been updated.

With regard t the Introduction, we would like to maintain the actual structure, with aim and scope of the manuscript at the end and and an introduction on climate change and aviation at the beginning.

This manuscript is a resubmission of an earlier submission. The following is a list of the peer review reports and author responses from that submission.

Round 1

Reviewer 1 Report

• In the introduction line 37-38, air transport demand is given by the unit of passengers, however, the academic approach for transport demand is given by passenger kilometers (p-km) unit, and also it would be valuable if you provide statistic for tonne-kilometer (t-km), such data is in IATA or ICAO.
• Aviation emission is given again in the introduction as CO2, and NOx, which sounds good, while, scientific readers have a willingness to see the statistic of megatonne [Mt] unit for GHG production, so this value should be given for a particular year like 2015, which Paris agreement occurred. 
• This bullet point is not obligatory, but if provided will add highly to the value of the work. In accordance with the previous point, Mt emission of aviation can be compared with other transport modes [Road, Rail, Marine] before engine shifts. This comparison can be for different years and for 6 or 9 major regions worldwide in a table or a diagram.  
• The Figures are clear and the references are adequate, but there are several English errors, which can be checked by a native English person.
• The work has novelty and interesting for scientific readers, so I show no objection to its publication.             

Author Response

• In the introduction line 37-38, air transport demand is given by the unit of passengers, however, the academic approach for transport demand is given by passenger kilometers (p-km) unit, and also it would be valuable if you provide statistic for tonne-kilometer (t-km), such data is in IATA or ICAO

  Thank you. The following paragraph has been added to the Introduction:

  “ The demand for air transport continues to grow; the ICAO [ref] forecasts in terms of Revenue Passengers-Kilometre (RPK) are shown in Figure S.1. The predicted Compound Annual Growth Rate (CAGR) until 2035 is equal to 4.3%.”

  Figure S.1 has been added to the Supporting Information.

• Aviation emission is given again in the introduction as CO2, and NOx, which sounds good, while, scientific readers have a willingness to see the statistic of megatonne [Mt] unit for GHG production, so this value should be given for a particular year like 2015, which Paris agreement occurred. 
• This bullet point is not obligatory, but if provided will add highly to the value of the work. In accordance with the previous point, Mt emission of aviation can be compared with other transport modes [Road, Rail, Marine] before engine shifts. This comparison can be for different years and for 6 or 9 major regions worldwide in a table or a diagram.  
• Thank you. We would like to avoid providing data in terms of 'GHG', as this term can be comprehensive of few/many and of some/other gases. Furthermore, a comparison with other sectors is beyond the scope of the Introduction. The following sentence was added in the Introduction: "In 2015, international aviation consumed ~160 Mt of fuel, corresponding to the emission of ~506 Mt of carbon dioxide and ~2.50 Mt of nitrogen oxides [ref]."
• The Figures are clear and the references are adequate, but there are several English errors, which can be checked by a native English person.
• We apologies. Many errors have been corrected.
  

Reviewer 2 Report

Dear Authors,

 

I am happy to report, that I find your paper well-written, well-thought through and general of high quality.

There are some minor issues, I recommend to improve, and one major issue, i must insist you improve.

Minor issues:

• Novelty: please elaborate more clearly on the novelty of your paper (in the introduction). Has this type of study been conducted before, but not on the prandtl-plane (your references to lund et al. suggest that)? Are parts of your investigation based on own models/studies regarding the RF-Effects? Did you expect any of your findings beforehand (other than the reduction in fuel consumption)?
• Use of abbreviations & readability: your paper has a clear engineering focus and design, but even engineers/physicists would have trouble keeping track with all the abbrevations. Please include an abbreviation table at the start and consider writing out some abb. in crucial parts of the paper (e.g. when discussing regions)
• Some tables would benefit from some form of editing (e.g. keep table 2 together) or alternative layout (e.g. better separation of ICAO & GSP sizing in table 3)
• Many diagrams would greatly benefit from some form of notes (what is shown; in particular the "CeRAS Harmonic Point" in figure 9 and the coloured inlet in figure 10). Please consider the journal you chosen and the possible backround of your readers: some things might be self-explainatory for you but certainly not for every interested party.
• I am a bit confused by the section below the main result table (from line 391 onward). This part seems more appropriate for your introduction (what effects are discussed and why) or the discussion of your methodology (section 4). If it is intended to be there, please introduce that section in one form or another that explains to the reader why it is there.
• The prandtl-plane seems like a great alternative (which I am sure is your intention): What are potential problems with it? Costs (there seems to be a lot of additional material)? Mass production? Missing proof-of-concept? The paper would benefit from some form of discussion of this point, even if it is "We do not yet know what challenges..."

Major Issue: Missing interpretation of results

I was inclined to recommend "minor corrections" until i stumpled on the result table 5. Beforehand, you do a great job of summarizing and interpreting results. Here, 4 different impacts are depicted over 5 dimensions and 7 regions, but you do NOT

• indicate which of these results are – in one form or another – noteworthy, interesting and/or out of the ordinary (you do some of this work in the discussion, but first you need to point the finger at it)
• summarize the results
• explain why you decided to show the different results for GWP and GTP. Do you just do it because others depicted it in this way as well or are there insights you want the reader to understand?

You might also consider extracting the most interesting results from the table and transform it into a diagram (similar to figure 8).

Overall, i am confident that these issues can be solved quite easily and your paper will be published.

 

Author Response

• Novelty: please elaborate more clearly on the novelty of your paper (in the introduction). Has this type of study been conducted before, but not on the prandtl-plane (your references to lund et al. suggest that)? Are parts of your investigation based on own models/studies regarding the RF-Effects? Did you expect any of your findings beforehand (other than the reduction in fuel consumption)?
• Thank you. We were the first to estimate the emission from this aircraft configuration. Hence, we were the first too doing a comparison with a conventional aircraft and also over different regions, while ther is no novelty related to the study of RF effects. Fuel reduction was expected, but the emission profile was unknown. Hence, we just modify a sentence of the Introduction as follows: "We estimated for the first time the emissions from the innovative box-wing aircraft and we compared the corresponding impacts to those of a conventional reference aircraft in six source regions in terms of GWP and GTP on two different time horizons (20 and 100 years)." 
• Use of abbreviations & readability: your paper has a clear engineering focus and design, but even engineers/physicists would have trouble keeping track with all the abbrevations. Please include an abbreviation table at the start and consider writing out some abb. in crucial parts of the paper (e.g. when discussing regions)
• Thank you, we followed your advise.
  
• Some tables would benefit from some form of editing (e.g. keep table 2 together) or alternative layout (e.g. better separation of ICAO & GSP sizing in table 3)
• Apologies. Tables were modified accordingly.
  
• Many diagrams would greatly benefit from some form of notes (what is shown; in particular the "CeRAS Harmonic Point" in figure 9 and the coloured inlet in figure 10). Please consider the journal you chosen and the possible backround of your readers: some things might be self-explainatory for you but certainly not for every interested party.

• Thank you. The unnecessary reference to the CeRAS Harmonic Point in Figure 9 has been removed. If by 'coloured inlet' in Figure 10 it is intended the coloured contour map within the pax-range envelope, the explanation of the graph is now provided in the Results section. The caption of Figure 10 has been modified accordingly.

• I am a bit confused by the section below the main result table (from line 391 onward). This part seems more appropriate for your introduction (what effects are discussed and why) or the discussion of your methodology (section 4). If it is intended to be there, please introduce that section in one form or another that explains to the reader why it is there.
• Thank you. A description of the emissions is intended to be in the Introduction, contrary to their radiative forcing, which has to be discussed within the results. Hence, we modify the sentence removing the definition of the aerosols, which was already given in the Introduction.
• The prandtl-plane seems like a great alternative (which I am sure is your intention): What are potential problems with it? Costs (there seems to be a lot of additional material)? Mass production? Missing proof-of-concept? The paper would benefit from some form of discussion of this point, even if it is "We do not yet know what challenges..."

• Thank you. A related section has been introduced in the Results:
  

  “Challenges

  The mass production process of the PrandtlPlane still remains an unexplored field, as the assessment of serial production lines is an issue related to the individual industrial policy. Moreover, this topic is generally not conceived in aeronautical research. However, the macro-components of the aircraft (wings, fuselage, undercarriage, engines, moveable surfaces, fins) are not different from those of a conventional aircraft, as well as the assembly areas are not required to be larger than today. Hence, a preliminary analysis could exclude the major showstoppers related to the mass production of the PrP.

  The aeromechanical characteristics of the PrandtlPlane have been preliminarily evaluated by means of predictive methods consolidated in the literature; subsequently, a radio-controlled flying model in dynamic scale has been built, and the flight qualities of the aircraft have been evaluated as a result of flight test campaigns [38]. The aerodynamics of the concept has again been evaluated by means of high-fidelity numerical aerodynamic analyses [117]; however, it has not yet been possible to perform wind tunnel tests, due to the limited availability and the high cost of transonic wind tunnels. A dynamically scaled flying model has also been built concerning the PARSIFAL PrandtlPlane (Figure S.2). Flight tests on this model did not reveal any critical issues regarding the aeromechanics of the PrandtlPlane configuration and its flight qualities."

Major Issue: Missing interpretation of results

I was inclined to recommend "minor corrections" until i stumpled on the result table 5. Beforehand, you do a great job of summarizing and interpreting results. Here, 4 different impacts are depicted over 5 dimensions and 7 regions, but you do NOT

• indicate which of these results are – in one form or another – noteworthy, interesting and/or out of the ordinary (you do some of this work in the discussion, but first you need to point the finger at it)
• summarize the results
• Thank you. The macro-section Discussion and recommendations has been expanded according to your advise. Please, let us know any detailed point you would recommend to be further highlighted.
  
• explain why you decided to show the different results for GWP and GTP. Do you just do it because others depicted it in this way as well or are there insights you want the reader to understand?
• Climate change can be evaluated by different indexes, i.e.: looked from different corners. Hence, while GWP refers to the radiative forcing, GTP denotes the temperature response. This is what we like the readers keep in mind. We discussed this, concerning also other indexes, in the Introduction section.

You might also consider extracting the most interesting results from the table and transform it into a diagram (similar to figure 8).

We prefer do not add further Figures (10 are already presented; in addition, 3 more have been provided in the Supporting Information).

Reviewer 3 Report

1. Redo annotation. Currently, it is more focused on the promotion of the project, rather than to the article.
2. The introduction provides general information, but lacks the emphasis on what the main purpose of the article is.
3. It is advisable to adjust the title 3.2 as it is clear impact of what is
4. The section on discussions and recommendations is completely unclear. In particular, it re-introduces subsections such as methodology, as well as some non-informative titles of subsections. Second of all, the material in this chapter does not, by all logic, fit in with the previous text (the feeling that there is a big "leap" of thought). It is clear that here for this article material is taken from the project in which the authors of the article participated, but the placement of a large amount of information on a small scale must be clear, consistent, and logical.
5. The conclusions do not meet the methodological requirements of the article

 

Author Response

1. Redo annotation. Currently, it is more focused on the promotion of the project, rather than to the article. We apologies. Abstract lenght is 198 words; the first 12 are related to the project, then aim and scope, methodology and results are discusses. Hence, we do no find a way to reduce the space dedicated to the project, unless we avoid mentioning it.
2. The introduction provides general information, but lacks the emphasis on what the main purpose of the article is. Thank you. We divided the Introduction in subheadings and we slightly modified the text. We hope that aim and scope, as well as the novelty of the manuscript, are now clear.
   
3. It is advisable to adjust the title 3.2 as it is clear impact of what is. Thank you. Section 3.1 presents the results of the accounting/estimation of fuel consumption and emission. Section 3.2 shows the impact of them on climate change, by the use of indexes (GWP and GTP) geographically tuned. Hence, we would leave the title as it is, but we are open to suggestion from the Reviewer.
4. The section on discussions and recommendations is completely unclear. In particular, it re-introduces subsections such as methodology, as well as some non-informative titles of subsections. Second of all, the material in this chapter does not, by all logic, fit in with the previous text (the feeling that there is a big "leap" of thought). It is clear that here for this article material is taken from the project in which the authors of the article participated, but the placement of a large amount of information on a small scale must be clear, consistent, and logical. Thank you. We improved the section, but we want also to provide here a brief explanation to shed light of its non-conventional structure. This manuscript aims to estimate the emission of the Prandtplane and the related Impact (Section 3). Moreover, being LCA-oriented, it aims to discuss the gaps which hinder the application of this methodology; Section 4.1 and 4.2 are indeed typical of LCAs. Finally, section 4.3 was added as advised from another Reviewer.
5. The conclusions do not meet the methodological requirements of the article. Thank you. We checked the instruction for authors, which states: "Conclusions: This section is not mandatory, but can be added to the manuscript if the discussion is unusually long or complex." and also "Indicate the main conclusions or interpretations. ". We don't know if the Reviewer refers to the second statement. If this is the case, the section indicates conclusion and interpretation: "The introduction of the PrandtlPlane is expected to bring a considerable reduction of climate change in all the source regions considered, on both the time-horizons examined. Furthermore, the development of a comprehensive Life Cycle Assessment on aircraft emissions will play a significant role in the definition of future impact reduction and mitigation strategies." If we did not understand the Reviewer advice, we apologies and we would be happy to modify the section if the Reviewer provides additional detail.

Reviewer 4 Report

Review of

 Innovative Box-Wing Aircraft: Emissions and Climate Change

 

The manuscript describes the hypothetical climatic impacts of a hypothetical aircraft/engine configuration based on predicted engine EI values and fuel burned calculations from references 64 and 65, which have not been peer reviewed.  Given the relevance of these two results for the study’s conclusions, the manuscript must validate these results before publication, as well as including an uncertainty analysis linked to the calculated impact of the different species’ dependence on the fuel burned calculations, and their corresponding RF calculations. Until these aspects are addressed, and the style and clarity of the manuscript are thoroughly improved, it is my opinion that the article should be rejected.

 

 

Abstract:

The abstract has inaccurate and unsubstantiated claims. The aircraft design is not “innovative” it is an old design. The hypothetical introduction of the aircraft cannot “bring a considerable reduction of climate change” as aviation only accounts for a small fraction of anthropogenic radiative forcing. The fuel consumption reduction has not been “confirmed”, as a simulation cannot “confirm” an estimate. And finally, mitigation strategies are not appropriately addressed if one considers that the hypothetical introduction of the aircraft in a few decades would not incorporate the engine technology assumed in the article. Because of this, the aims of the study have to be reassessed in a consistent way, following a logical structure, which is definitely lacking in the current manuscript.

 

Introduction:

 

The introduction (and the discussion section) reads more like a tutorial than a scientific journal contribution, with disconnected and unnecessary topics and misleading claims. Why are intermediate stop operations introduced if they are not part of the simulations and have nothing to do with the proposed aircraft configuration?  The same can be said about introducing ETS. It is not true that the study includes a full LCA, and this is made unclear again in the conclusions section.

 

Methods:

 

Ln 218.How can CH4 changes be excluded from cruise? CH4 changes at UTLS are paramount in terms of their chemistry and RF altitude dependence. And if they are “neglected” at LTO you should simply say that CH4 is no considered at all. Or am I missing something here?

 

Ln 228. The reference to the models used to estimate NOx and BC EIs cannot be reference number 6.

 

Table 2, the Time column of the GSP section is wrong.

 

All tables should be labelled in order to be self-explanatory.

 

Ln 333. There is no reference to how the NOx chemistry that feeds the Oslo RTM was calculated.

 

Results:

 

Paragraph 371. The uncertainties here have a very poor justification, as they neglect the CDF uncertainties, and mainly because the CDF estimates have not been validated. This aspect makes the study unpublishable.

 

The conclusion of BC’s contribution simply being “more than 10%” is completely unjustified given that a 34% increment from the ICAO approach may well counterbalance in the short term the corresponding CO2 and NOx reductions. Unless a proper uncertainty analysis of their RF estimates is included in the study the reader is left with no way of assessing if the claimed climatic benefits are scientifically robust. The justification of the potential RF reductions based on Table 4 do not take into account the 34% increase in BC which may imply an increase in climatic impacts in the short term. The RF calculations MUST include this 34% increase, and a proper uncertainty analysis should be included to compare the impact and reliability of non CO2 contributions.

 

Fig. 8. This figure should be produced for each species for global calculations only, so that the reader can compare their separate impact, together with their corresponding uncertainties, and the results should be given as percentage reductions so that the reader can assess the relative savings.

 

 

Ln 386. The word “considerable” is not justified here (see comments for Fig. 8)

 

Table 5. These results should be given as percentage reductions and the BC differences for both the ICAO and the GSP results be given. I suspect the results here correspond to GSP, which is not stated in the manuscript nor in the Figure’s label.

 

Paragraph Ln 391. This long paragraph adds nothing to the results from the study.

 

Discussion:

 

Ln 430. Clarify that no LCA is included in the study.

 

Ln 464. This uncertainty should be compounded with the CDF uncertainty and then linked to each RF estimate and this has to be done for all species not only NOx.

 

Paragraph 497. These uncertainties have to be included in the results, otherwise the authors should not report metrics with high uncertainties, and would be better off sticking to instant RF values and simply comment on their temporal impact differences.

 

Ln 525. This paragraph is irrelevant when one thinks that this hypothetical configuration would take decades to be developed, at which point current ground operation assumptions will be obsolete. This aspect is extremely relevant in terms of the aim of the study, which currently combines past, present and future assumptions with little logical consistency.

 

Fig. 9. No scientific justification for this figure is given, and the 22% comparison to current options disregards the potential improvements in those options by the time the proposed aircraft is developed. The results should be given with a consistent approach, and not in terms of payload, range, passenger kilometre, and RFs. Having multiple inconsistent metrics confuses the reader and results in the length of the manuscript being unnecessarily long and confusing. Same should be said for Fig. 10, for which in one sentence one could simply say that fuel consumption reductions are slightly less for short ranges.

 

Paragraph 560. Please explain how this applies to the study and what consequences it with respect to the proposed aircraft.

 

Paragraph 572. Please explain how this applies to the study and what consequences it with respect to the proposed aircraft.

 

Paragraph 577 The implications of NOx and CO2 trade-offs for the study should be assessed but mentioning potential trade-offs for present aircraft does not give the reader a way to know how PrP’s trad offs are different.

 

Paragraph 592. Please explain how this applies to the study and what consequences it with respect to the proposed aircraft.

 

Conclusions:

 

Again, this aircraft design has been proposed long time ago, please explain how it is “innovative”. The statement of “considerable reductions in climate change impacts” is not justified in terms of absolute anthropogenic impacts nor in terms of a peer reviewed backing of the potential fuel consumption reductions. It is not clear at all if you claim that you did an LCA or if you are expecting other people to do it in the future.

 

 

 

Minor comments

 

 Pg 1  Update the reference of Lee et al. 2010 to the more the Lee et al. (2020) update.

 

Line 136. The RFI definition is badly presented. It should make it clear that it compares one species to carbon dioxide, and not all emissions to carbon dioxide effects

 

Ling 143. The conclusion of equivalence to CO2 emissions at UTLS cannot be correct for well mixed gases.

 

Ln 161. When you say “transport aircraft” you may mean cargo aircraft, as opposed to passenger aircraft. Appropriate nomenclature is the least that can be expected in scientific journals, and the authors must have picked up these terms from the extensive bibliography that they reference.

 

Several references are not directly related to the topics. For example, references 13 and 14 are definitely not focused on contrail cirrus development. Many references (33 to 36 amongst others) do not provide type of publication and are probably not available to non specialised readers.

 

Author Response

The manuscript describes the hypothetical climatic impacts of a hypothetical aircraft/engine configuration based on predicted engine EI values and fuel burned calculations from references 64 and 65, which have not been peer reviewed. Given the relevance of these two results for the study’s conclusions, the manuscript must validate these results before publication

The methods for the calculation of the EI are presented in this paper for the first time, concerning the cases under study. The method based on the interpolation of ICAO data is explained in section 2.3, as the GSP method is related to the software described in ref [66]. In [67] and [68] there are no additional elements concerning the calculation of EI. Data from references 67 and 68 were used as input for these computations.

Abstract:

The abstract has inaccurate and unsubstantiated claims. The aircraft design is not “innovative” it is an old design.

In the abstract "innovative" refers to the aircraft, not to the design. Anyway, it is innovative.

The hypothetical introduction of the aircraft cannot “bring a considerable reduction of climate change” as aviation only accounts for a small fraction of anthropogenic radiative forcing.

Manuscript concerns with the climate change due to aviation. Hence, in lights of the results obtained, "considerable" is the term that we choose to use.

The fuel consumption reduction has not been “confirmed”, as a simulation cannot “confirm” an estimate.

A simulation can confirm or deny everything. Reliability of the confirm depends on:

the reliability of the simulation

the fact that it is a simulation itself confirming something, and not direct observation.

And finally, mitigation strategies are not appropriately addressed if one considers that the hypothetical introduction of the aircraft in a few decades would not incorporate the engine technology assumed in the article.

The term used is not "addressed", but "discussed".

Because of this, the aims of the study have to be reassessed in a consistent way, following a logical structure, which is definitely lacking in the current manuscript.

Manuscript has been improved thanks to your advice and to the comments of other Reviewers.

Introduction:

The introduction (and the discussion section) reads more like a tutorial than a scientific journal contribution, with disconnected and unnecessary topics and misleading claims. Why are intermediate stop operations introduced if they are not part of the simulations and have nothing to do with the proposed aircraft configuration? The same can be said about introducing ETS. 

Reduction of climate change due to aviation is the aim. Introduction provides a description of the available strategies, in order to frame our strategy.

It is not true that the study includes a full LCA, and this is made unclear again in the conclusions section.

It was never stated.

Methods:

Ln 218.How can CH4 changes be excluded from cruise? CH4 changes at UTLS are paramount in terms of their chemistry and RF altitude dependence.

Please, see the reference at that line

And if they are “neglected” at LTO you should simply say that CH4 is no considered at all. Or am I missing something here?

No, we shouldn't, as it would be different from what we stated:

"No CH₄ was assumed to be generated during the cruise phase [9], while the fraction produced in the LTO phase has been neglected."

Ln 228. The reference to the models used to estimate NOx and BC EIs cannot be reference number 6.

Apologies. The error has been corrected.

Table 2, the Time column of the GSP section is wrong.

Thank you. It has been amended.

All tables should be labelled in order to be self-explanatory.

Thank you, we checked all the captions. If the Reviewer would provide additional detail on specific captions, we would proceed with the improvement suggested.

Ln 333. There is no reference to how the NOx chemistry that feeds the Oslo RTM was calculated.

Well, this is because the NOx chemistry is related to the Oslo RTM, which was used to define the metrics, which have been here applied to carry out the impact assessment. Hence, being references to the metrics and also to the Oslo RTM provided, further deepening is available to the interested reader by following the references provided.

Results:

Paragraph 371. The uncertainties here have a very poor justification, as they neglect the CDF uncertainties, and mainly because the CDF estimates have not been validated. This aspect makes the study unpublishable.

Dear Reviewer,

we kindly ask detailed explanation of your concern, to be able to improve the manuscript. Thank you

The conclusion of BC’s contribution simply being “more than 10%” is completely unjustified given that a 34% increment from the ICAO approach may well counterbalance in the short term the corresponding CO2 and NOx reductions. Unless a proper uncertainty analysis of their RF estimates is included in the study the reader is left with no way of assessing if the claimed climatic benefits are scientifically robust. The justification of the potential RF reductions based on Table 4 do not take into account the 34% increase in BC which may imply an increase in climatic impacts in the short term. The RF calculations MUST include this 34% increase, and a proper uncertainty analysis should be included to compare the impact and reliability of non CO2 contributions.

Thank you. "More than 10%" has been replaced by "Increased emision of BC". With regard to the rest, please see values of Table 5 to understand that the criticism is wrong.

Fig. 8. This figure should be produced for each species for global calculations only, so that the reader can compare their separate impact, together with their corresponding uncertainties, and the results should be given as percentage reductions so that the reader can assess the relative savings.

Thank you. We consider Figure 8, together with Table 5, suitable to well explain the Results.

Ln 386. The word “considerable” is not justified here (see comments for Fig. 8)

Thank you. We consider the word appropriate (see comments related to the Abstract advice)

Table 5. These results should be given as percentage reductions and the BC differences for both the ICAO and the GSP results be given. I suspect the results here correspond to GSP, which is not stated in the manuscript nor in the Figure’s label.

We ask the Reviewer to better read the caption.

Paragraph Ln 391. This long paragraph adds nothing to the results from the study.

Thank you. The paragraph has been slightly amended. What remains, it is part of the Results or given to the reader for interpreting the Results.

Discussion:

Ln 430. Clarify that no LCA is included in the study.

It was never stated. We adopt the LCA methodology to carry out the assessment (which is deeper than an impact assessment), and we discuss the gaps to be filled in order to be able to carry out an LCA.

Ln 464. This uncertainty should be compounded with the CDF uncertainty and then linked to each RF estimate and this has to be done for all species not only NOx. Paragraph 497. These uncertainties have to be included in the results, otherwise the authors should not report metrics with high uncertainties, and would be better off sticking to instant RF values and simply comment on their temporal impact differences. Ln 525. This paragraph is irrelevant when one thinks that this hypothetical configuration would take decades to be developed, at which point current ground operation assumptions will be obsolete. This aspect is extremely relevant in terms of the aim of the study, which currently combines past, present and future assumptions with little logical consistency.

We disagree with the Reviewer.

Fig. 9. No scientific justification for this figure is given, and the 22% comparison to current options disregards the potential improvements in those options by the time the proposed aircraft is developed. The results should be given with a consistent approach, and not in terms of payload, range, passenger kilometre, and RFs. Having multiple inconsistent metrics confuses the reader and results in the length of the manuscript being unnecessarily long and confusing. Same should be said for Fig. 10, for which in one sentence one could simply say that fuel consumption reductions are slightly less for short ranges.

The comparison with the conventional competitor is, in our opinion, consistent. Evolutionary improvements on monoplane configurations can also be applied at the same stage to box-wing aircraft (e.g. laminar wings, full composite structures, more electric aircraft, high by-pass ratio turbofan, hybrid propulsion, etc.); the opposite, however, is not true. Considering an A320-like aircraft appears to be the best strategy for comparison, taking the development background of the PrandtlPlane in the PARSIFAL project into account.

It is unclear why 'The results should be given with a consistent approach, and not in terms of payload, range, passenger kilometre', as this appears to be the consistent approach in the technical literature.

The explanation of the graph in figure 9 has been provided.

Paragraph 560. Please explain how this applies to the study and what consequences it with respect to the proposed aircraft.

We proposed a strategy to reduce the climate change due to aviation. We discuss our strategy together with those available.

Paragraph 572. Please explain how this applies to the study and what consequences it with respect to the proposed aircraft.

There is no impact reduction and mitigation if there is no adequate metric to define the impact.

Paragraph 577 The implications of NOx and CO2 trade-offs for the study should be assessed but mentioning potential trade-offs for present aircraft does not give the reader a way to know how PrP’s trad offs are different.

We do not understand the point. The aim of the paragraph is discuss the trade off CO2-NOx, as the PrP could have decreased CO2 emission and increased NOx emission.

Paragraph 592. Please explain how this applies to the study and what consequences it with respect to the proposed aircraft.

The results of this investigation highlight once more the relevance of including non-CO2 emission in climate protocols. In this paragraph, the feasibility of this option is discussed.

Conclusions:

Again, this aircraft design has been proposed long time ago, please explain how it is “innovative”. The statement of “considerable reductions in climate change impacts” is not justified in terms of absolute anthropogenic impacts nor in terms of a peer reviewed backing of the potential fuel consumption reductions. It is not clear at all if you claim that you did an LCA or if you are expecting other people to do it in the future.

All the points here raised by the Reviewer has been discussed in our previous answers.

Minor comments

Pg 1 Update the reference of Lee et al. 2010 to the more the Lee et al. (2020) update.

Thank you, reference has been updated.

Line 136. The RFI definition is badly presented. It should make it clear that it compares one species to carbon dioxide, and not all emissions to carbon dioxide effects

The definition is right; we do not undestand the Reviewer.

Ling 143. The conclusion of equivalence to CO2 emissions at UTLS cannot be correct for well mixed gases.

We do not understand the Reviewer. At line 143 we reported literature findings.

Ln 161. When you say “transport aircraft” you may mean cargo aircraft, as opposed to passenger aircraft. Appropriate nomenclature is the least that can be expected in scientific journals, and the authors must have picked up these terms from the extensive bibliography that they reference.

Text has been amended.

Several references are not directly related to the topics. For example, references 13 and 14 are definitely not focused on contrail cirrus development. Many references (33 to 36 amongst others) do not provide type of publication and are probably not available to non specialised readers.

We updated the Reference list, maintaining the references we used as source of information.

Reviewer 5 Report

The authors present a method to evaluate aircraft design based on global warming potential collected from a large array of sources. The method is applied to one novel aircraft, the prandtl plane, and a comparison baseline aircraft. This is done for several different economic regions.

The connection between this paper and the Prandtl plane is too weak. Either more technical data from ref 60 needs to be brought in to allow reviewers to evaluate them, OR the authors need to be very explicit with the fact that the PrP is still a conceptual design and that the engineering data has to be assumed to be correct.

The choise of reference aircraft is poor. The CERAS reference AC appears to be selected becaus it fits into the same parking space at the airport. -While an important consideration, it is not all coverning. The suggestion is to keep the CERAS, but also add an aircraft with the same PAX, one with the same design range and one contemporary existing aircraft with the same PAX. With a larger comparison base, the merits of the PrP should be clearer.

Structure:
If journal settning guidelines allow, the paper would benefit form having more subheadings in the introduction. "Operational measures, technological advances, regolatory..."

Explain the meaning of "Aerodynamic efficiency", is it glideslope?

Errata:
Row 243: "Equation X" ?
Row 452: "[53]._"   superflous underscore

Author Response

The connection between this paper and the Prandtl plane is too weak. Either more technical data from ref 60 needs to be brought in to allow reviewers to evaluate them, OR the authors need to be very explicit with the fact that the PrP is still a conceptual design and that the engineering data has to be assumed to be correct.

Thank you. A Table and a Figure have been added to the Supporting information. The following paragraph has been added to the Results section:

"The main characteristics of the PrP and CERAS are reported in Table S.1. Numerical CFD simulations have been used to assess the aerodynamics of the two reference configurations; details about these computations are provided in [ref]. Figure S.3 shows the trend of the aerodynamic efficiency, expressed as lift-to-drag ratio E, with respect to the trim lift L is reported for both the configurations; the PARSIFAL PrandtlPlane exhibits higher aerodynamic efficiency in each flight condition taken into account. These data have been used to properly assess the mission simulation outcomes."

The choise of reference aircraft is poor. The CERAS reference AC appears to be selected becaus it fits into the same parking space at the airport. -While an important consideration, it is not all coverning. The suggestion is to keep the CERAS, but also add an aircraft with the same PAX, one with the same design range and one contemporary existing aircraft with the same PAX. With a larger comparison base, the merits of the PrP should be clearer.

Conversely, the choice of the CeRAS reference is appropriate; if the development background of the PARSIFAL project is examined in depth, it becomes clear that the designed PrandtlPlane operates in the same market as the CeRAS, but increases its payload capacity. To do this, the aerodynamic superiority of the PrandtlPlane concept is exploited, together with its increased lifting capacities. In this way, the expected significant increase in traffic demand over the next 15 years in exactly the CeRAS market sector can be met. At the same time, the superiority of the box-wing concept is used to limit the wingspan and avoid causing problems of airport saturation, while increasing the ability to carry more passengers than the selected benchmark. Again, a conventional monoplane is not able to meet this specification.

Using pax-km as a comparison makes the comparison consistent, with no room for ambiguity.

Structure:
If journal settning guidelines allow, the paper would benefit form having more subheadings in the introduction. "Operational measures, technological advances, regolatory..."

Thank you, we amended the Introduction accordingly.

Explain the meaning of "Aerodynamic efficiency", is it glideslope?

Aerodynamic efficiency is defined as the lift to drag ratio (E=L/D).

Errata:
Row 243: "Equation X" ? Row 452: "[53]._" superflous underscore

Apologies and Thank you. Text has been amended.

Round 2

Reviewer 3 Report

1. In my opinion, it is better to leave the structure of the article as follows: introduction, and now move the subsections in the introduction logically after the introduction to a separate chapter entitled "Literature Review" or Analysis of the Current Situation (specify the question to be discussed) or something like this.
2. In my opinion, the title 2.1 should be named / specified, i.e. indicating whose or what area.
3. It is recommended to make the information in Figure 2 different. Possible ways: according to the present figure, split into two parts: the 1st part would be a table with technical parameters, and the second part would be a picture OR The information in the table present it as text before / after the figure. Because at the moment it seems like the table is for itself and the picture - itself (as this information is shared over 2 pages).
4. The conclusions lack further (clearer) directions of research on the issue under consideration.

Reviewer 4 Report

revised version shows minor changes when most reviewers asked for major improvement

Reviewer 5 Report

Equation 8:    "...=a ? ? t[CH4]..."    should the  ? ? be there?