Aerodynamic Design Optimization of a Morphing Leading Edge and Trailing Edge Airfoil–Application on the UAS-S45
Round 1
Reviewer 1 Report
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This paper focuses on the aerodynamic design optimization of a morphing wing concept applied to the UAS-S45. Thank you for this highly relevant research. I have read it with pleasure and believe the general approach and modeling have been done correctly. The writing can be improved for clarity.
My main comments are as following:
- Page 1, Line 37-39, “various solutions have been….”. Are we only looking at the solutions for UAS? Many more solutions are being implemented for general aviation, e.g., advanced propulsions, sustainable fuels, etc. I suggest revising this part for clarification.
- Page 3, Line 82-Line 105, these two paragraphs list various projects on morphing concepts. It would be more helpful to summarize these projects’ main messages and how they motivate this paper research.
- Page 5, the optimization procedure in Figure 2 is not so clear to me. The flowchart shows two configurations are generated at the beginning, i.e., baseline and morphing airfoil. The two then merge into one stream for optimization, which means that two designs are optimized. However, if the solution is not converged, the modification only applies to the baseline. What about the morphing concepts? Do you also monitor the convergence of the morphing concept optimization?
- Page 6, I’m not sure if it is reasonable to change Eqn. 2 to Eqn. 4 as I think at least the wing area would somehow play a role here. Or perhaps in this analysis, the wing area is fixed? Then I would suggest to include a table with all the essential design variables and the design space in this analysis.
- Line 339, “A hybrid optimizer is used in this study. as shown in Figure 5.” à “A hybrid ….., as shown in Figure 5”. The period changed to a comma.
- Page 12, Line 363, the authors mentioned the flight envelope of UAS-S45. It would be helpful to provide a figure on the flight envelope of the UAS-S45 and highlight the boundary conditions.
- Page 13, in figure 7, the two Re of 1.2e6 and 2.4e6 are used; however, in table 5, the flight conditions don’t cover 1.2e6. Please revise this for consistency.
- Page 13, in figure 7, the angle of attack (α) should be explained in the figure or the caption.
- Line 376, the authors state, “These differences in the drag coefficients variations are found at angles of attack higher than 8° …. ”; however, in figure 7, the differences of drag coefficients appear along with the full range of the α in considerations. Can you please comment on this?
- Line 385, typo of (). Please revise.
- Line 389, “….. (0.43c) at an angle of attack of 0° ”, should be 2°, as observed in figure 8(a).
- Page 14, the resolution of figure 9 is very poor, and figure 9 (b) is cut off. The same for figure 15 and figure 21.
- Page 18, to which case of DNLE is figure 14 referring?
- Page 21, to which case of MTE is figure 19 referring?
Overall, the optimization shows that the morphing concept can improve the airfoil’s aerodynamic performance. I think it would also be of high interest to see how these improvements are reflected at the performance level, such as the typical fuel savings with the optimized morphing concepts for a given flight mission. Do you see any chance to add the analysis in this perspective?
Author Response
Dear Reviewer 1, please find here the file containing our answers to all your comments. Many thanks for your useful corrections that helped us very much in improving our paper quality, the authors
Author Response File: 
Author Response.pdf
Reviewer 2 Report
General Comments:
The topic of the paper is therefore pertinent to Applied Sciences and well reflected in the title. Both abstract and manuscript state (claimed) novelty and findings of the work quite clearly; however, the latter needs restructuring with more subsections in order to favor a logical flow that drives the readers from motivations to conclusions while giving all relevant details in between. In particular, the literature survey in the Introduction shall position the work within the state-of-the-art research and practice in the field and aerospace community while stressing its key novelty accordingly: it is not clear what the latter really is, since rather extensive studies have indeed been performed on droop-nose leading edge and morphing trailing edge already. Multi-point and multi-objective optimization with hybrid techniques and surrogate models has also been investigated already, recently including transonic flow as well as aeroelastic effects of a flexible camber too (e.g., see the review of Li and the works of Haftka, Lee, Kumar, Butler, Fusi, Sha, Secanell, Kuder, Berci, Saeed, Wu, Thangeswaran, Geva… just to name a few that should somehow be considered in a more comprehensive literature review discussing and assessing all potentially relevant effects). Moreover, the optimization problems formulation is not entirely clear, as the definition of the flight points appear incomplete; the settings of the aerodynamic solvers are not specified and the convergence of the optimization outcome is not shown. Please, note that all data shall be declared and results shall always be reproducible by the reader (of course, assuming the latter has appropriate resources and skills) at any time.
In the light of the above, the manuscript shall undergo a significant revision in its logical sub-structures, novelty emphasis and results validation, while giving all relevant information to understand the problem and reproduce the outcome.
Specific Comments:
Line 2: are particle swarm and pattern search algorithms actually coupled or rather combined? Even after reading Section 2.4, this is not clear to me: do both algorithms exchange data while running (in parallel: coupled approach) or does the pattern search algorithm just refine (in series: combined approach) the optimum already obtained by the particle swarm algorithm?
Line 17: from a practical perspective, how do you use the single-objective optimal results then? (in fact, a multi-objective optimization seems more suitable and was typically performed)
Line 19: after reading Section 3, the optimization results based on Xfoil do not seem “validated” but rather “supported” with CFD, since no direct comparison and/or optimization results with the latter are shown; the only proper “validation” seems partially given in Figure 7 only. In particular, from a methodological perspective: what happens if CFD does not confirm the results form Xfoil eventually? (in order to avoid this, tuning strategies have been formulated)
Line 26: in fact, these considerations are not part of the presented work and shall be moved to the Introduction then (as per the previous request of a more comprehensive literature review).
Lines 46-47: note that this is not true anymore, since multi-point optimization is now standard.
Line 62: however, real aircraft are not yet designed to achieve an optimal aerodynamic performance “for all conditions of a flight envelope” but rather for a few points in its flight mission (in fact, note the latter shall not be confused/interchanged with the “flight envelope”).
Line 79: did you mean discrete control surfaces? They are not discussed in the paper, in fact...
Line 144: please remove “used is shown in Figure 2 that”.
Line 148: please remove “in the use of”.
Line 151: are the optimization algorithms really “coupled” or rather “combined”? (see before)
Line 167: this paper does not consider a “broad range of flight conditions for the UAV operation within its flight envelope”, which would include aircraft maneuvers and gust responses.
Page 6: please remove Equation (3), since unused later on; then, only indent new paragraphs. Note that the “new” quantity defined in Equation (4) is the endurance aerodynamic efficiency and shall not be confused with the endurance itself (i.e., please chose another suitable symbol).
Line 178-179: I do not agree, since aircraft weight also influences airfoil design through the required steady lift; in fact, this is a conceptual issue that I find in both optimization problem formulations: the aircraft level-flight equilibrium shall also be imposed as physical constraint (see https://www.icas.org/ICAS_ARCHIVE/ICAS2012/PAPERS/154.PDF as direct example, or https://link.springer.com/article/10.1007/s00158-014-1066-2); otherwise, it looks like flow conditions are not completely specified and it is not clear to me how the objective functions can be evaluated (e.g., what is the angle of attack?). N.B.: in this particular case of an existing UAV, the full UAS-S45 shall be trimmed considering all its components... although I understand your current focus is on the wing’s root section only (for demonstration) and I leave this 3D extension to the future works already mentioned at the very end of the paper Conclusions.
Line 193-194: note that some referenced works disagree, from a “flow-driven” perspective.
Line 225: please identify and explain both Bezier’s and PARSEC’s parameters in Figure 3.
Table 4: the caption is incorrect (identical to that of Table 2).
Line 251: please add a proper Xfoil subsection and explain the code settings used in the simulations (e.g., panels distribution, assumed turbulence level, free or enforced flow transition); also, all symbols in all equations shall be then defined either in the text or (preferably) in a comprehensive Nomenclature to be added in the most appropriate part of the manuscript.
Line 263: so, are non-converged solutions lost entirely without bringing any information in?
Line 269: please add a proper CFD subsection and explain the code settings used in the simulations (e.g., mesh size and elements, boundary conditions, numerical scheme); all data shall be declared in full and results shall always be reproducible by the journal readers at any time.
Line 308: please add a proper PSO subsection and make sure all settings are given in full.
Line 339: please add a proper subsection for the hybrid optimization scheme and fully clarify whether it is actually “coupled” or rather “combined” (see before); yet, where is the loop exit?
Table 5: in the absence of an equilibrium equation, these conditions are actually incomplete…
Figure 6: are MTE Case I and II shapes swapped, in fact? (at least according to Section 3.2)
Line 372: please add a proper subsection for Xfoil’s “validation” with CFD results, where all three flight conditions in Table 5 shall be considered as viscous flows are inherently nonlinear; for the very same reason, note that this partial a-priori validation does not necessarily hold for the optimized airfoil shape anyway. Figures 8 shall coherently include Xfoil’s results too and an additional figure shall then compare the pressure distributions relative to data in Figure 9 (which is not a “validation” per se, but rather a CFD-based support to interpret Xfoil’s results).
Line 427: please add a proper subsection for the “minimum drag” optimization problem, that also increases the airfoil’s (smoother) stall angle without modifying its linear region; to this respect, initial and final (optimal) airfoil shapes shall be shown in a dedicated figure. Another figure shall then be added to show the convergence trend of the hybrid optimization results.
Line 461: please add a proper subsection for the “maximum endurance aerodynamic efficiency” optimization problem; initial and optimal airfoil shapes shall be given in a dedicated figure and another one shall be added to show the convergence trend of the optimization process.
Figure 15: not a “validation” of Xfoil’s results, rather a CFD-based support to interpret them.
Line 501: please add a proper subsection for the “minimum drag” optimization problem, that increases the airfoil’s zero-lift with minor changes in its stall angle; to this respect, initial and final (optimal) airfoil shapes shall be shown in a dedicated figure while another one shall give the convergence trend of the hybrid optimization results.
Line 522: please add a proper subsection for the “maximum endurance aerodynamic efficiency” optimization problem; initial and optimal airfoil shapes shall be given in a dedicated figure and another one shall be added to show the convergence trend of the optimization process.
Figure 21: not a “validation” of Xfoil’s results, rather a CFD-based support to interpret them.
Author Response
Dear Reviewer, thank you very much for the revisions of our paper, that helped us in improving the paper quality. Many thanks once again and wish you a ''Happy New Year 2021'' with the realization of your wishes and excellent health of you and your family, the authors
Author Response File: Author Response.pdf
Round 2
Reviewer 1 Report
Thank you for addressing my comments and your detailed responses.
With the current revision, I would like to recommend this paper to be published.
Author Response
Thank you very much for your revisions as well as for the approval for publication of our paper, the authors
Reviewer 2 Report
Dear Authors, thanks for the editorial changes and improved paper structure (as kindly suggested); however, many answers to major key points read "Since only 10 days have been given to submit the revised paper and our access to laboratory is also limited due to the covid-19 pandemic, any addition of data and results will take more time than 10 days" and those items (also including my text you highlighted in red) are hence still open: of course, we all do understand the current unfortunate situation... but the latter cannot overrule the technical and scientific requirements of journal/MDPI publications.
If you are not granted enough time and your access to laboratory is also limited, I suggest taking the time necessary to complete the work and resubmit it later.
N.B.: it is OK to neglect effects, but you still need to mention them in the literature survey and justify why/how irrelevant/inapplicable for your work.
Author Response
Please see the attachment
Author Response File: Author Response.pdf
Round 3
Reviewer 2 Report
(see the attachment)
Comments for author File: Comments.pdf
Author Response
Please see the attachment.
Author Response File: Author Response.pdf
