Recent Advances in Boundary Layer Ingestion Technology of Evolving Powertrain Systems

Round 1

Reviewer 1 Report

Dear Authors,

Thank you for your excellent contribution.

I found the paper interesting and considered it a tremendous amount of work with the benefit of the aerospace community.

I would like to give some of my thoughts on how to improve your paper.

1. Line 79 - it would be interesting to see this concept. Can you support this line with a photo of the aeroplane in question?
2. Line 81 - surname repeated twice.
3. There is several misleading referencing in the text. Unfortunately, it is a standard error in scientific writing today when everything, namely everything, is tried to be referenced. To the degree that the well-known formulas or laws have unexpected reference sources, however not related to the original author. How can you say that:
   - a Panel established the components of drag in 1979, or
   - the Breguet equation was used by some guys in 2020, and they defined the components of the formula
   I believe no one today references Newton's laws or Einstein's formula. Breguet equation developed in the dawn of aviation by the French aviator is the only Breguet equation and can be referenced by his work only. Moreover, later you mentioned the Breguet-Coffin equation without any details.
4. Chapter 2 presents a number of "performance metrics". That's perfect but looks incomplete. Later you show several aeroplane designs. How about putting some numbers specific to those designs in the formulas from chapter 2 and making a quantitative comparison?
5. Lines 79 and 159, So, when did actually it start? The 1930s or 1940s?
6. Figure 5. I consider presenting results in the form of bars for both platforms. Now the dashed line looks like a mean or trend of WoS data.
7. Line 213 - Why do you use the word "open"? Do you think there are other criteria in the classified reports in the industry?
8. I would suggest one more feature in your paper. When you tell the story about the development of the new designs, it would be good to mention the year. I know there is a reference number in square brackets. However, it requires going to the end of the text and checking the date. Also, graphics with the timeline and schematic indication of the starting (and ending) year (date) of the project, as well as the horizontal bars for the specific technologies, would be interesting to see.

Best regards

Your reviewer

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 2 Report

General Comment

In this review paper, the authors present a comprehensive overview of the developments carried out in the scope of Boundary Layer Ingestion (BLI). BLI is one of the solutions proposed to mitigate the environmental footprint. I found the paper interesting and covering an active research topic. However, there are some issues that should be addressed before the paper is ready for publication.

Comments and Questions

On page 2, where it reads “low specific thrust levels” shouldn’t it be “low specific fuel consumption levels”?

What about engine failure scenarios? From my point of view a paragraph discussing this issue should be included to improve the quality of the paper.

From the stability point of view, add an engine to the back might not be advantageous. A comment on this regard should be included.

The first sentence of sub-section 1.3.2. seems to be incomplete.

What is the area S considered for equations 13 and 14?

How will be the performance of a BLI engine placed at the fuselage rear when the aircraft is in stall conditions?

The reported lower engine efficiency of a BLI engine at some conditions may lead not only higher CO2 emissions, but also other pollutants (e.g., NOx, soot, particulate matter) that contribute to air quality degradation and global warming. Is there any research covering this issue?

Embedding an engine in the fuselage may induce discomforting vibrations to the passengers, besides noise. A discussion on this regard should be included.

Several of the concepts presented in section 4 are also propulsive fuselage concepts (section 5). What is the main different?

What do you mean by power off-take?

Is there any study on the environmental impact, e.g. using LCA (Life Cycle Assessment) tools, of this type of propulsion systems?

What about robustness and reliability of the BLI system? From my point of view this should be sought in future developments of this technology given the uncertainties in the inlet flow conditions.

From my point of view, the conclusion section would benefit from the introduction of a new subsection highlighting the main challenges that BLI technology still faces. Furthermore, the potential pollutant and noise emissions identified in the literature should also be summarized in the conclusion text besides the already existing tables.

There are several typos in the text:

• On page 13, line 427, where it reads “it presented” it should read “it is presented”.
• On page 17, line 617, where it reads “was identified” it should read “were identified”.
• On page 18, lines 666 and 667, you should revise the following sentence “the with conventional powertrain architecture”.
• On page 22, line 821, where it reads “used the block fuel” it should read “using the block fuel”.
• On page 22: line 828, where it reads “an single-aisle” it should read “a single-aisle”; line 835, where it reads “a 150 PAX” it should read “an 150 PAX”; line 838, where it reads “was places” it should read “was placed”.
• On page 32, line 1329, where it reads “an BLI” it should read “a BLI”.
• On page 33: line 1342, where it reads “to cover the majority of the airports covered” I would suggest to change the word “covered”; line 1369 where it reads “seed” it should read “speed”; line 1370, where it reads “A overall” it should read “An overall”; line 1381, where it reads “a initial” it should read “an initial”.
• On page 34, line 1434, where it reads “reductioned” it should read “reduction”.
• On page 35, line 1467, where it reads “Smirth” it should read “Smith”.
• On page 36, line 1499, where it reads “patial” it should read “partial”.

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

The authors have addressed my concerns.