Technological Aspects of a Reparation of the Leading Edge of Helicopter Main Rotor Blades in Field Conditions

Round 1

Reviewer 1 Report

The authors investigate the application of repair of local disbonding of fragments of the anti-erosion layer of helicoper rotors in the field. Overall the study is complete and scientificly sound. The language of some passages could be improved.

The resolution of the figures could be improved, e.g. figure 16.

There are some minor formating errors, e.g. line 369.

Author Response

Dear Madame/Sir

Thank you for very helpful and valuable comments. We have revised our paper accordingly to your suggestions and feel that your comments helped to improve our manuscript.

 

The authors investigate the application of repair of local disbanding of fragments of the anti-erosion layer of helicopter rotors in the field. Overall the study is complete and scientifically sound. The language of some passages could be improved.

 

Thank you for your comment. The manuscript has been revised according to the suggestions of all reviewers.

1. The resolution of the figures could be improved, e.g. figure 16.

The resolution of figures with graphs has been improved.

2. There are some minor formatting errors, e.g. line 369.

Thank you for this remark. The error has been corrected.

 

In addition, the article has been corrected in accordance with the comments of the other two reviewers. We have added the missing descriptions of technological processes along with the justification of the processes and research used. Sections 4 and 7 have been significantly rebuilt.

Reviewer 2 Report

In this manuscript, the concept of repair technology feasible in field conditions was proposed for repairing the leading edge of helicopter main rotor blades. Tests were conduct to identify the hardware material and the polymer materials used in the leading edge. Technological process of repairing the leading edge of rotor blade and results of post-repair edge tests were presented in the manuscript. However, several questions should be revealed before publication.

Here are some specific comments/suggestions:

1. In section 5.1, the selected system “Bonding DP490” which was further tested did not meet the basic criterion - fracture surfaces of bonding layers. Furthermore, “Boning DP490” was the adhesive finally used in the proposed technology. This is a serious logical error.
2. In section 4.2, how to determine the cleaning is completed?
3. In section 5.2.1, the mechanical properties of the bonding 2216B/A + BR primer samples after aging is unstable in the strength tests. There is a lack of theoretical analysis on the occurrence of this phenomenon.
4. In Figure 20c, the stress of the bonding DP490 samples without aging rises slowly with the displacement, which is different from the results shown by the other three experiments. What is the reason for this phenomenon?
5. For Figure 3(b), Mg2 is not defined.
6. In Figure 5, (a) is the sample of bonding ply, and (b) is the sample of rubber. However, in the description on lines 177 to 179, (a) is the sample of rubber, and (b) is the other. Please check it.
7. In the analysis of Figure 6, it is mentioned that the DSC curves for the adhesive show an endothermic (melting) peak at 65°C with very low enthalpy. However, endothermic (melting) peak at 65°C appears in Figure 6(b), which represents the DSC curve of the rubber. Is there any inconsistency here?
8. According to the content of the manuscript, the 90 should be revised as -90 on line 183.
9. For Figure 7(c), there is no explanation about the purpose of using isopropyl alcohol. Please add more information.
10. For Figure 10(a), there is no description about how the assembly tool works and how to use it. It is an important part of the reparation. Please describe in detail.
11. “Figure 11” has two callouts, and there misses a full stop on line 473.

Author Response

Dear Madame/Sir

Thank you for very helpful and valuable comments. We have revised our paper accordingly to your suggestions and feel that your comments helped to improve our manuscript.

 

In this manuscript, the concept of repair technology feasible in field conditions was proposed for repairing the leading edge of helicopter main rotor blades. Tests were conduct to identify the hardware material and the polymer materials used in the leading edge. Technological process of repairing the leading edge of rotor blade and results of post-repair edge tests were presented in the manuscript. However, several questions should be revealed before publication.

 

Thank you for your comment. The manuscript has been revised according to the suggestions of all reviewers.

 

1. In section 5.1, the selected system “Bonding DP490” which was further tested did not meet the basic criterion - fracture surfaces of bonding layers. Furthermore, “Boning DP490” was the adhesive finally used in the proposed technology. This is a serious logical error.

After your remark, the paragraph 5.1 has been rebuilt for better understanding preliminary selection of the bonding:

“The first, and simplest selection of bonding system, consists of the verification on the nature of the damage (which components of the bonding layer was damaged) during the static tearing off of the shield (Figure 10). It was assumed that further, more advanced testing are feasible only in those bonding systems in which the decohesive fracture type in bonding layer or rubber. Because it would indicate that, the surface preparation and primer were correctly selected and that the maximum transferring load is via bonding layer or rubber.

The test results:

1. Bonding 2216B/A - deadhesive fracture surfaces between metal and the bonding layer.
2. Bonding 2216B/A + Rafil primer – decohesive fracture surfaces of the primer.
3. Bonding 2216B/A + BR-127 primer - decohesive fracture surfaces of the bonding layer.
4. Bonding DP490 – no damage of the bonding layer, decohesive fracture surface of the rubber.
5. Bonding DP490 + Rafil primer - decohesive fracture surfaces of the primer.
6. Bonding DP490 + BR-127 primer - deadhesive fracture surfaces between metal and the bonding layer.

According to above criterion, only the following systems were further tested:

• Bonding 2216B/A + BR-127 primer;
• Bonding DP490”

 

 

 

 

2. In section 4.2, how to determine the cleaning is completed?

 

The develop of method surface preparation has been laboratory tested using the water-break test method.  After your remark, the paragraph has been expanded:

“The development of surface preparation method has been laboratory tested using the water-break test method, using distilled water with accordance to [29,30,31]. During the repair of actual main rotor blade, this test will not be possible because it would significantly complicate the bonding process - it is difficult to dry the water in the slots as well as check the drying was done correct. It was assumed that surface preparation and cleaning must be performed according to developed method. This guarantee a properly prepared surface for bonding.”

 

Additionally paragraph has been expanded to include the description of figures 7d and 7e:

 

“The surfaces have to be dried by heat gun (Figure 7d) after chemical cleaning (shield with acetone and rubber with isopropyl alcohol). Although the fact that the solvents used evaporate easily, drying requires quite a long time to get rid of them from porous structures - sandblasted metal and rubber. Based on experience, 10 min at 35^oC was assumed to be sufficient. After the cooling down of the surfaces the primer is applied by brush (7e). The primer is cured in ambient condition during 8 hours [32, 33].”

 

In general, the entire section 4 has been improved and rebuilt.

 

3. In section 5.2.1, the mechanical properties of the bonding 2216B/A + BR primer samples after aging is unstable in the strength tests. There is a lack of theoretical analysis on the occurrence of this phenomenon.

Thank you for your comment. After your remark, the paragraph 5.2.1 has been expanded:

“The reason of the large discrepancy in the strength test results of the joint (Figure 15 b) may be the degradation of the polymeric materials in the joint. Degradation is referred to as the partial disintegration of a polymer - not into low molecular weight products, but into fragments of high but lower molecular weight than the initial polymer. Factors initiating degradation may be physical or chemical interactions such as radiation, temperature, or exposure to chemicals including water [52-55].”

 

4. In Figure 20c, the stress of the bonding DP490 samples without aging rises slowly with the displacement, which is different from the results shown by the other three experiments. What is the reason for this phenomenon?

 

Thank you for your comment. After your remark, the paragraph in which reference to Figure 20 (now  Fig. 19) is made has been expanded:

 

“The test samples were taken from a main rotor blade operated on a helicopter. There-fore, the factory bonding was exposed to actual atmospheric conditions. Similarly, aged DP490 bonded samples were exposed to variable climatic chamber conditions. In these cases the maximum force occurred at a displacement of about 2.5 mm during tearing off of the shield from the leading edge.   

            The maximum force occurred at a displacement of approximately 5 mm during test of bonding DP490 samples without aging.

            It was assumed that the faster force increase is due to internal stresses in the bonding ply and at the rubber-bonding ply border what is the result of aging processes under environmental conditions.”

 

5. For Figure 3(b), Mg2 is not defined.

Thank you for your remark. Mg2 is bending moment related to the induced drag. The information, after your remark, has been supplemented in the manuscript.

 

6. In Figure 5, (a) is the sample of bonding ply, and (b) is the sample of rubber. However, in the description on lines 177 to 179, (a) is the sample of rubber, and (b) is the other. Please check it.

 

Thank you for this remark. The error has been corrected:

“The bonding ply sample was taken out of the surface of the rubber after the shield was detached from the rubber, as per Figure 5a. The rubber sample was cut out from the top of the leading edge, as shown in Figure 5b.”

 

7. In the analysis of Figure 6, it is mentioned that the DSC curves for the adhesive show an endothermic (melting) peak at 65°C with very low enthalpy. However, endothermic (melting) peak at 65°C appears in Figure 6(b), which represents the DSC curve of the rubber. Is there any inconsistency here?

After your remark, the paragraph in which reference to Figure 6 is made has been rebuilt:

“Glass transition temperatures were determined for both rubber and bonding ply. It is -26°C for rubber and -11°C for bonding ply. Moreover, the DSC curves for the rubber show an endothermic peak at 65°C with low enthalpy.  It can supposed that this peak is related to the moisture content of the rubber. For bonding ply in the temperature range -20°C to 110°C, no permanent transformations were observed due to the action of both the simulated temperature changes alone during service and those resulting from the thermal cycle during curing. This indicates that both materials are thermally stable over the studied temperature range “.

 

8. According to the content of the manuscript, the 90 should be revised as -90 on line 183.

Thank you for this remark. The error has been corrected.

9. For Figure 7(c), there is no explanation about the purpose of using isopropyl alcohol. Please add more information.

After your remark, the paragraph 4 has been rebuilt. This will allow a better understanding of the repair technology.

 “The pieces of old bonding ply remain on the rubber after separation of metal shield from rubber. There are two ways to remove them: mechanically or chemically. The mechanical attempts by sandpaper were causing damage to the rubber. So the chemical method was applied. According to the manual [12] for the cleaning of the rubber on the leading edge gasoline or isopropyl alcohol can be used. The tests have shown that the gasoline is not effective for removal of the old bonding ply residue, so the isopropyl alcohol was chosen. Cleaning was done by wiping of the rubber surface by cotton cloth slightly dampened in isopropyl alcohol. Isopropyl alcohol evaporates quickly after cleaning so it does not pose further threat to the rubber and to the new joint.”

In general, the entire section 4 has been improved and rebuilt.

 

10. For Figure 10(a), there is no description about how the assembly tool works and how to use it. It is an important part of the reparation. Please describe in detail.

 

Thank you for this remark. The paragraph has been expanded to the description of Fig. 10a (now Fig. 9a):

“The composite back parts of assembly tool (Figure 9a) are installed on the trailing edge of blade as first. Next, the bonded shield is compressed to the leading edge using the composite front part of assembly tool by hand after applying adhesive paste on the bond-ed surfaces of leading edge under repair. Then, the front part of assembly tool is joined to the back part of assembly tool, using fastener (M6). Therefore shield is pressing to leading edge. In order to additionally pressing the shield to top and bottom of the leading edge bar clamps are installed as a last step. The excessed adhesive paste squeezed out is removed by wooden spatula.”

 

In general, the entire section 4 has been improved and rebuilt.

 

11. “Figure 11” has two callouts, and there misses a full stop on line 473.

 

Thank you for this remark. The error has been corrected.

Reviewer 3 Report

Refer to the comments

Comments for author File: Comments.pdf

Author Response

Manuscript considers the technological aspects of a reparation of the leading edge of main rotor blades for helicopter. As the title of this work, contents include detailed discussions just for a technical report rather than a paper. And, the work organization are not systematic in length and without even brief comments for each items in Figure et al. Reviewer recommends authors of this article to reduce as compact and precise as possible to be paper with the help of native English speaker.

 

Dear Madame/Sir

Thank you for very valuable comments. Referring to your comments, the article has been partially redrafted in terms of the arrangement of sub-chapters. We have added the missing descriptions of technological processes along with the justification of the processes and research used. In addition, the article has been corrected in accordance with the comments of the other two reviewers. Sections 4 (Repair technology) and 7 (Conclusions) have been significantly rebuilt. The revised manuscript has been sent to the editor.

Round 2

Reviewer 2 Report

  In this manuscript, the concept of repair technology feasible in field conditions was proposed for repairing the leading edge of helicopter main rotor blades. It has certain practical significance for the rapid maintenance of the leading edge of the main rotor of the helicopter. I would recommend acceptance of this manuscript.