Review Reports

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

Dear Authors,

please refer to my comments in the attached file.

Best regards.

Comments for author File: Comments.pdf

Author Response

Overall comments

The manuscript presents a study for the improvement of an existing UCAV, namely the MQ-9A Reaper. Its goal is to reduce fuel consumption, increase aerodynamic efficiency, and improve weapon-carrying capability. This is addressed with a new engine, larger wing and tail, and stability analysis for the external payload.

While the manuscript is well written and structured, the results are comprehensive, and their discussion quite complete, it still has many major flaws in its present form that should be addressed prior to publication, as detailed next. In addition, other minor concerns and typos or editing recommendations are also included for completeness.

First, thank you very much for taking the time to read, review, and provide feedback for our MDPI submission. For most of us, this is our first submission into an academic paper, so to receive some positive feedback was very reassuring and motivating.

As you read through the changes, please be aware the Editor-in-Chief requested us to remove as many references to weapons or warfare as possible. As such, the JSM is now referred to as a releasable store, given its ‘AGM-184’ designation, and we use interdiction instead of strike.

Major concerns

• There is a severe lack of review of literature concerning aircraft design in Section 1. It is suggested to add three new paragraphs in between lines 42 and 43, each paragraph addressing the following:
• Conceptual and preliminary design tools, often used in the early design stages that tackle the interaction of multiple disciplines at a very high-level, using Multidisciplinary Design Optimization techniques and typically low-fidelity analysis tools considering the complete aircraft (e.g.: https://arc.aiaa.org/doi/abs/10.2514/6.2021-2411, https://ieeexplore.ieee.org/document/9836228 , https://www.mdpi.com/2226-4310/12/6/556, among many others).
• Discard applicability of MDO here since this is a re-design, often referred to as Mid-Life Update (MLU), where detailed design is sought. This typically iterates existing detailed designs to improve performance or meet new mission requirements (e.g.: https://www.mdpi.com/2226-4310/9/7/349, https://link.springer.com/article/10.1007/s13272-015-0179-7, https://apps.dtic.mil/sti/html/tr/ADA368636/, among many others)
• Go over other Reaper upgrade studies (e.g.: https://www.jstor.org/stable/27022004)

We included the recommended three new paragraphs to bolster the literature search for Section 1. Additionally, we improved the referencing of literature in the JSM (now AGM-184 store) integration section, as we identified that this was most lacking.

• In Section 5.2, the engine analysis should be extended to evaluate the impact of the mass flow assumed (15 kg/s). Consider adding a sensitivity study by also evaluating a slightly lower and higher mass flow cases.

Your feedback on conducting a sensitivity study on the effect of mass flow rate is definitely a key point of concern. As we no longer have access to the computational resources to re-run the studies, we have tried to address this branch of thought by running a qualitative hypothetical scenario, as indicated in the red under Chapter 5.2. We have noted the need for the suggested sensitivity study as a limitation and recommended it for future work.

• The discussion of the impact of lower Reynolds number and lower Mach number in the WT testing must be improved (lines 561-568). The former has a very significant impact on the flow transition and separation/re-attachment, while the latter prevents possible compressibility/shock wave effects from being properly captured.

We improved the discussion of the lower Re and Mach numbers in the WT Section, significantly outlining the expected differences ahead of the extensive post-experimentation scaling effort. We hope that provides the important perspective you sought.

• The quality of the list of references should be improved as there are too many textbooks, non-scientific, and URLs. While the first major concern relative to the literature review in the introduction section will improve

The references have been improved. Some URLs remain as aircraft design looks to the industry for what is commercially viable, and industry often only provides an online source rather than an academic reference. The MDPI proofing usually leads to further refinement.

Minor concerns

We incorporated all the improvements as suggested within the Minor concerns, and most of the formatting suggestions. 

• line 57, figure 1: add comparison to original mission profile to highlight higher cruise altitude, thus justifying the change from turboprop to turbofan engine

The comparison is given in the text just above Figure 1, hopefully, that suffices. Thank you

• line 81: add [reference] to Fusion 360 model

We added the software website details and the public-facing URL used at UNSW, where students obtain the Fusion 360 software and the video tutorials. Thank you

• line 113-115: discuss sizes of engine alternatives

Corrected thank you

• line 131: discuss the higher cost of a custom engine variant

Discussed thank you

• line 402: discuss the considerable increase in tail size (150%)

Discussed thank you

• line 451: discuss why the plane of symmetry was not explored to reduce computational cost

Discussed thank you

• line 453: even if not conclusive, show the results of the mesh convergence study in terms of evaluated CL, CD and Cm values, as well as mesh size and computational cost

We have added a brief summary of the mesh independence study, with a supporting Table 13 added below it. These have been added directly under the Chapter 5 heading.

• line 457: describe the solver parameters used in ANSYS (e.g., equations, turbulence model, iterative solution technique, convergence tolerances, …)

Addressing your kind suggestions, we have also included details of the turbulence model, convergence criteria, and typical iterations. This was a key detail we missed, and we appreciate the feedback.

• line 512, figure 11: add streamlines

Added thank you

• line 535: clarify which variant MQ-9 refers to, original 9A or modified 9X?

Corrected thank you

• line 572, 577: detail the instrumentation and data sampling rate, filtering, and post-processing

Detailed thank you

• line 687, figure A1: also add figure for MQ-9A Reaper for comparison

Added thank you

Typos, editing and format

• line 7: add “for reduced fuel consumption”

Corrected thank you

• line 27: remove reference to figure “(Figure A1)”. To be added in a later section.

Corrected thank you

• Lines 33-43: move this whole paragraph to in between line 26 and 27, and remove section 1.1 Background title

Corrected thank you

• Line 43: remove section 1.2 Objectives title

Deleted thank you

• Line 55: altitude of 45,000ft not consistent with remaining document (50,000ft)

Corrected thank you

• Line 63: remove (summarized in Table A1). To be included later.

Corrected thank you

• Line 72: add some text to introduce table 1.

Added thank you

• Line 77: rephrase to “5.4% relative to the estimates in Tab.1.”

Corrected thank you

• Line 84: define “Centre of Gravity (CoG)”

Defined thank you

• Line 92: include figure A6 here in main text, not in appendix.

Moved thank you

• Lines 98-102: put data specs of the two aircraft variants in tabular form (merge in table A.1)

A new dedicated table has been created to show the changes. Thank you

• Lines 117-119: put data specs of the two engine variants in tabular form (also engine mass from line 222), including data source or estimative indication.

The data has been added to the comparison table.

• Line 142, 147-150 : add these data to table A.1

The data has been added to the comparison table.

• Line 171: include figure A5 here in main text, not in appendix.

Included as Figure 3, thank you

• Line 214: lower case “with”

Corrected thank you

• Line 265: insert table A.1 after this line, as a summary of the previous sections. Also add to it geometry, mass properties and operating conditions (as mentioned in line 269).

Inserted with recommended additions, thank you

• Lines 277-293: convert to table

Converted, thank you

• Line 372: add a new section 3.4.7 Summary of stability derivatives, including a summary table of longitudinal and lateral derivatives

Added thank you

• Line 392, figure 5: use different line types (solid, dashed, dotted)

Sorry, we had to leave this figure with coloured legends as the graduate student has proceeded on leave. We hope the MDPI proof team will accept colour as this is an open access and online publication, and not grey scale.

• Line 417, table 7: convert table to a numbered list instead

Converted thank you

• Line 423, table 8: caption “Key longitudinal stability…”

Corrected thank you

• Line 447: consider simplifying section title to “5. Computational Fluid Dynamics” only

Simplified thank you

• Line 563: lower case number in “Reynolds number”. Applies to whole document.

Corrected throughout, thank you

• Line 607: lower case number in “Mach number”. Applies to the whole document.

Corrected throughout, thank you

• Lines 604-617: summarize data in tabular form of CLalpha and CLmax for CFD, WT raw and WT corrected, in absolute values and percentage differences among them. Also add CDmin to this table (info in lines 621-634)

The table has been added, but it seemed more logical to present it near the summary and not to remove the discussion of how scaling took place or why differences are expected. We also provided the AoA for the required cruise coefficient of lift rather than the differing lift coefficients at a set AoA, as this is the design point and thus a measure of the still significant uncertainty. We hope that meets your intent here. Thank you

• Line 635: start paragraph with reference to new table (created according to point above)

We think we did what you suggested here, thank you

Reviewer 2 Report

Comments and Suggestions for Authors

My congratulations to the Authors for such interesting article. There are some things that need to be addressed, but it will be explained below. The paper is a good example of proper description of airframe design and analysis procedure, including all necessary parameters,  methods used, and could be used in didactics. Additional value is a comparison of different sources of data, ways of calculating the coefficients including available software AVL, X-Plane, ANSYS, also including the experiment with its flaws (small model leading to a lot of work to translate the results to real scale). All the problems were described sufficiently with a proper explanation how to deal with them and providing the sources of information, formulae etc. Yet still the paper has a comprehensible size.

In chapter 5 there is no mention which turbulence model was used in calculations. 

In the  same chapter, the easiest way to check if there is a separation beginning on the model is to check the x component of shear stress surface distribution. The shear stress x component, if calculated in an airframe coordinate system, changes its sign around any reverse flow area, on an upper surface of the wing, on the fuselage, and so on. Also, the oil flow kind of path lines helps a lot in analyzing the surface flow. Authors can upgrade their article in that manner, but it is not necessary.

below some minor mistakes I’ve found:

line 6 and further "we replace" "we update" "we integrate" -> please use the passive voice form, for example "was integrated", "was updated". 
line 19 "we propose enhancements to ..." -> "enhancements to ... were proposed"
line 21 "armed ISR counterpart" - ISR should be in abbreviation list.
line 28 "increased wingspan to 79’" -> in scientific article a ft symbol should be used instead of ' which is non-standard in science and ambiguous. Also translation to the SI units, like: "79 ft (24.08 m)", would be useful.
line 55 "This mission consists of a persistent high-altitude mission" -> style. "This mission consists of a persistent high-altitude flight part" 
"at 45,000’" and further occurences -> as in line 28. You are using three symbols: ', ft, feet - for the same unit. Choose one.
line 65 "we developed", line 68 "We performed" - use passive voice.
line 127 "JSMs" -> "JSMs (Joint Strike Missiles)"
line 200 "CD,0(no stores" -> "CD,0(no stores)"
line 212 "==" -> "="
line 212 " With"-> ", with"
line 241 so where is that comparison between inlet and far field? In the table 2 it is only a comparison between sea level and cruise altitude.
line 322 "[38?]" -> either find the proper description of publication or another adequate source. At least you could cite an instruction for AVL, if there's any, or webpage address with date of visit.

Author Response

My congratulations to the Authors for such an interesting article. There are some things that need to be addressed, but they will be explained below. The paper is a good example of a proper description of airframe design and analysis procedure, including all necessary parameters, methods used, and could be used in didactics. Additional value is a comparison of different sources of data, ways of calculating the coefficients, including available software AVL, X-Plane, ANSYS, and also including the experiment with its flaws (a small model leading to a lot of work to translate the results to real scale). All the problems were described sufficiently with a proper explanation how to deal with them and providing the sources of information, formulae etc. Yet still the paper has a comprehensible size.

First, thank you very much for taking the time to read, review, and provide feedback for our MDPI submission. For most of us, this is our first submission into an academic paper, so to receive some positive feedback was very reassuring and motivating.

As you read through the changes, please be aware the Editor-in-Chief requested us to remove as many references to weapons or warfare as possible. As such, the JSM is now referred to as a releasable store, given its ‘AGM-184’ designation, and we use interdiction instead of strike.

In chapter 5, there is no mention of which turbulence model was used in calculations.

Thank you for your comment regarding the turbulence model used in Chapter 5. This is definitely a key piece of detail that we missed and have now addressed, as highlighted in the updated version. Supporting the turbulence model used, we have also added some details emphasising the CFD package and convergence criteria used.

In the same chapter, the easiest way to check if there is a separation beginning on the model is to check the x-component of shear stress surface distribution. The shear stress x-component, if calculated in an airframe coordinate system, changes its sign around any reverse flow area, on the upper surface of the wing, on the fuselage, and so on. Also, the oil flow kind of path lines helps a lot in analyzing the surface flow. Authors can upgrade their article in that manner, but it is not necessary.

We appreciate your comment on using the x component of Shear Stress and path lines to clearly identify flow separation regions, and we will definitely consider that for future work. Unfortunately, we could not re-run the CFD computation as we have graduated and no longer have ready access to UNSW resources.

Minor mistakes

• line 6 and further "we replace" "we update" "we integrate" ->please use the passive voice form, for example "was integrated", "was updated".

Corrected throughout, thank you

• line 19 "we propose enhancements to ..." -> "enhancements to ...were proposed"

Corrected throughout, thank you

• line 21 "armed ISR counterpart" - ISR should be in the abbreviation list.

Corrected, thank you

• line 28 "increased wingspan to 79’" -> in a scientific article, a ft symbol should be used instead of ', which is non-standard in science and ambiguous. Also, translation to the SI units, like: "79ft (24.08 m)", would be useful.

Corrected throughout, thank you

• line 55 "This mission consists of a persistent high-altitude mission" -> style. "This mission consists of a persistent high-altitude flight part"

Corrected, thank you

• "at 45,000’" and further occurrences -> as in line 28. You are using three symbols: ', ft, feet - for the same unit. Choose one.

Corrected throughout, thank you

• line 65 "we developed",

Corrected throughout, thank you

• line 68 "We performed" - use passive voice.

Corrected throughout, thank you

• line 127 "JSMs" -> "JSMs (Joint Strike Missiles)"

Now listed as AGM-184 store (stores), thank you

• line 200 "CD,0(no stores" -> "CD,0(no stores)"

Corrected, thank you

• line 212 "==" -> "="

Corrected, thank you

• line 212 " With"-> ", with"

Corrected, thank you

• line 241, so where is that comparison between the inlet and far field?

In the Table 2, it is only a comparison between sea level and cruise altitude.

Reference removed, sorry, this analysis was not conducted.

• line 322 "[38?]" -> either find the proper description of publication or another adequate source. At least you could cite an instruction for AVL, if there's any, or a webpage address with the date of visit.

This was a cross-reference error, now corrected, thank you

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

Dear authors,

thank you for carefully addressing my concerns.

Best regards.