The Impact of Distributed Propulsion on the Aerodynamic Characteristics of a Blended-Wing-Body Aircraft

Round 1

Reviewer 1 Report

1.       Title of the manuscript can be considered for shortening. A short and crisp title will be really great.

2.       Use of recent literature is advisable. Kindly please take that into consideration and revise if possible.

3.       Use of words like “I” and “We” can be avoided. I suggest the authors to rewrite such words. For instance, in Line No: 89- “We based on . . .”

4.       Two typical status – featuring climbing and gliding state has been analyzed. It should, however, be noted that most of the time, the flight will be on a cruise and hence the reviewer suggests you to perform the studies for cruise conditions as well in the future. Even though it is represented for the later cases why figure 9 comparison does not include that – What is the variation in that value?

5.       In figure 9, why the test case results were limited to 9 degrees? Is there any specific reason behind that?

6.       Line No: 330-331 – test has been performed at full turbulence condition. Kindly mention the turbulence values for better understanding. It will be better if the value of turbulence under which both the CFD has been performed and the test data has been observed is presented in the manuscript.

7.       Line No: 317 – I think it refers to Table 2 rather than Table 3. Kindly clarify.

8.       Based on table.2. the values of C_PK obtained from the Flight test and the CFD data show the variation of more than 12%. Hence Line No: 321-322 needs to be rewritten including the change in variation observed in the gliding values.

9.       Line No:327-328, “… which is about 13% of the C_L value in cruise (0.38)”. Kindly add citations for better understanding.

10.   In section 3.3, the use of words “low-throttle and high-throttle” is very generic and some more information regarding the same will be appreciated as it will enhance better understanding for the readers.

11.   At low throttle condition, is there any physical reason behind the dip in the CL between 10-to-20-degree angle of attack.

12.   Line No: 434-435 – The CL Max of the clean airframe seems to be around 1+ as per Fig.13. Even though after 10 degrees the slope of the lift coefficient decreases gradually the maximum lift coefficient seems to be happening at 20 degrees. Kindly clarify.

13.   Based on figure. 13 – explain the short plateau period happening for modified cases with low throttle and high throttle and why it is absent in the clean airframe configuration.

14.   Line no:464 – Usually the scaled models are expressed in terms of ratio and not in %. Kindly please modify the statement “…8.8% scaled-down . . .”

15.   Line No: 477 – Not only the maximum lift coefficient has improved overall coefficient of lift for the high throttle model is high throughout the test range.

 

 

Author Response

Respond to Referee #1

1. Title of the manuscript can be considered for shortening. A short and crisp title will be really great.

Reply:

Thanks for your suggestion, the title is changed to “The impact of distributed propulsion on the aerodynamic performance of a blended wing body aircraft”.

 

2. Use of recent literature is advisable. Kindly please take that into consideration and revise if possible.

Reply:

We have added recent pieces of literature, such as ref. 3~5.

 

3. Use of words like “I” and “We” can be avoided. I suggest the authors to rewrite such words. For instance, in Line No: 89- “We based on . . .”

Reply:

Thank you for pointing this out. We have checked the article and corrected it

 

4. Two typical status – featuring climbing and gliding state has been analyzed. It should, however, be noted that most of the time, the flight will be on a cruise and hence the reviewer suggests you to perform the studies for cruise conditions as well in the future. Even though it is represented for the later cases why figure 9 comparison does not include that – What is the variation in that value?

Reply:

Due to the limitation of battery capacity, the DPD cannot fly for a long time, so there is no result of the cruise in the flight test. Moreover, this part mainly focuses on the influence of API on the aerodynamic forces of DPD and verifies the accuracy of numerical simulation methods. The climbing and gliding correspond to the high-throttle and low-throttle states of the propulsion system respectively, and the cruising throttle is between them. The comparison of the climbing and gliding states can illustrate the influence of API on aerodynamic forces. After verifying that the CFD calculation results were consistent with the test flight results, subsequent studies were carried out by CFD, focusing on the aerodynamic power consumption at the cruise state. We explain this in the first paragraph of Section 3.1

 

5. In figure 9, why the test case results were limited to 9 degrees? Is there any specific reason behind that?

Reply:

The data points in figure 9 are from the results of the flight test. Due to safety concerns, the angle of attack has been limited to less than 10 degrees in flight tests.

 

6. Line No: 330-331 – test has been performed at full turbulence condition. Kindly mention the turbulence values for better understanding. It will be better if the value of turbulence under which both the CFD has been performed and the test data has been observed is presented in the manuscript.

Reply:

The flight height was less than 100 meters, so the DPD was under the boundary layer of the atmosphere, which was supposed to all turbulent flows. The Reynolds number of the flight based on the mean chord length was about 10^6, therefore, a turbulence model of SA was used in the numerical simulations. We added the Reynolds numbers based on the chord length of the wing root and wing tip, as well as the turbulence intensity in Sec. 3.1, to give an intuition of the turbulence of the incoming flows.

 

7. Line No: 317 – I think it refers to Table 2 rather than Table 3. Kindly clarify.

Reply:

Thank you for pointing out the error and we have corrected it.

 

8. Based on table.2. the values of CPK obtained from the Flight test and the CFD data show a variation of more than 12%. Hence Line No: 321-322 needs to be rewritten including the change in variation observed in the gliding values.

Reply:

Thank you for pointing out the problem. Although there is a difference in C_PK between CFD and flight test (less than 0.003), this difference has little effect on the calculation of C_L. Taking the 4° result as an example, for every 0.001 change in C_PK, C_L changes by 0.0005, which is negligible with respect to the C_L at 4° (0.38). We checked the results in both the glide phase and climb phase, and the errors of C_L in the CFD calculation are both within 5% compared to the flight test data. Therefore, it is appropriate to say that “the error of in the CFD calculation is within 5%”.

 

9. Line No:327-328, “… which is about 13% of the CL value in cruise (0.38)”. Kindly add citations for better understanding

Reply:

The cruise lift coefficient is given according to the balance between gravity and lifts when the aircraft is cruising. We have added explanations in the article.

 

10. In section 3.3, the use of words “low-throttle and high-throttle” is very generic and some more information regarding the same will be appreciated as it will enhance better understanding for the readers.

Reply:

Thank you for your suggestion. We have clarified the definition of “low throttle” and “high throttle” in the first paragraph of Section 3.3. We define the state “low throttle” in which the NPR is 1.008. and we also define the state” high throttle” in which the NPR is 1.029.

 

11. At low throttle condition, is there any physical reason behind the dip in the CL between 10-to-20-degree angle of attack.

Reply:

It is a good question. The dip in the CL is probably caused by the flow separation. But there is no more detailed flow visualization experiment. We can not explain it in detail. It is a great challenge to conduct in the future if possible.

12. Line No: 434-435 – The CL Max of the clean airframe seems to be around 1+ as per Fig.13. Even though after 10 degrees the slope of the lift coefficient decreases gradually the maximum lift coefficient seems to be happening at 20 degrees. Kindly clarify.

Reply:

Thank you for pointing out the error and we have corrected this part.

 

13. Based on figure. 13 – explain the short plateau period happening for modified cases with low throttle and high throttle and why it is absent in the clean airframe configuration.

Reply:

We can qualitatively explain this phenomenon from the fluorescence filament display experiment. It can be seen that this plateau occurs at a larger angle of attack, about 10-15 degrees, when the flow has separated in some areas of the wing surface, as can be seen in Fig.14. When part of the area is separated, the lift in this part decreases, but the unseparated area increases the lift due to the increase in the angle of attack. The combined effect of the two will cause the lift coefficient to deviate from the previous linear segment, the exact value of which depends on the magnitude of the contribution of the separated and unseparated regions to the lift, respectively.

 

14. Line no:464 – Usually the scaled models are expressed in terms of ratio and not in %. Kindly please modify the statement “…8.8% scaled-down . . .”

Reply:

Thank you for pointing out this problem and we have corrected it.

 

15. Line No: 477 – Not only the maximum lift coefficient has improved overall coefficient of lift for the high throttle model is high throughout the test range

Reply:

Thank you for pointing out that. we have added this to the conclusion

Author Response File: Author Response.doc

Reviewer 2 Report

The authors have written a well-documented journal paper. However, they could make some improvements like:

1. May add error bars in the experimental results in the graph

2.  Analysis methods may be organized differently. Sometimes it is very hard to follow.

3. Authors should explicitly mention why a specific test procedure (ground, flight, cfd or wind tunnel) is chosen for a specific operating condition.  

Author Response

Respond to Referee #2

1. May add error bars in the experimental results in the graph

Reply:

Thank you for your advice. Due to financial reasons, the flight test was conducted only once. Therefore, there was no error bar in the figures.

 

2. Analysis methods may be organized differently. Sometimes it is very hard to follow

Reply:

We have added a paragraph at the beginning of section 2 to summarize the choice of specific method. And we also revised some unclear descriptions to make the method more organized.

 

3. Authors should explicitly mention why a specific test procedure (ground, flight, cfd or wind tunnel) is chosen for a specific operating condition.

Reply:

Thank you for pointing that out. The flight test is mainly used to study the influence of distributed propulsion on the aerodynamic forces of the aircraft and verify the accuracy of CFD calculation. The flight time of the aircraft is limited by the battery capacity, so the test results do not include cruising status results. After the accuracy of CFD calculation is verified by flight test, CFD is used for more detailed aerodynamic power consumption analysis in cruise state. Wind tunnel tests are used to study the stall characteristics of the aircraft at high angles of attack because they are safer than flight tests and are closer to real flight conditions than CFD. We have explained this in detail in Section 2.

Author Response File: Author Response.doc