Converting a Fixed-Wing Internal Combustion Engine RPAS into an Electric Lithium-Ion Battery-Driven RPAS

Round 1

Reviewer 1 Report

This study presents the development and ground tests of an electric propulsion system for a fixed wing UAV and the target application of this UAV is the precision agricultural. However, the contribution and novelty of this study is not enough. The commercial electric UAVs have been widely used in lots of applications including the precision agricultural more than one decade.

 

Authors have used already existing components and assembled a system. Authors should clearly explain what the new contributions are in a point-wise manner. There should be research level scientific contributions. Not merely assembling a system.

 

The literature survey is not comprehensive. This article contains a lot of fundamental descriptions and discussions, but there is no state-of-the-art regarding the electric UAVs or new propulsion systems.

 

The developed UAS is quite complicate and expensive. It is not suitable for civil applications and the precision agricultural.

 

The calculation of the required power or thrust for cruising should be based on the aircraft performance of the developed UAV. The conducted tests are not rigorous and contain many unreasonable assumptions. For example, the thrust test on the ground could not obtain the dynamic thrust of the UAV in cruising phase. The weight of the batteries will decrease the payload weight.

 

6. This study does not complete the real flight test which is a very important process to proof that the developed UAV could demonstrate the capability to achieve the desired purposes.  

Author Response

Reviewer(s)' Comments to Author:

Reviewer: 1

Comments R1.1:

This study presents the development and ground tests of an electric propulsion system for a fixed wing UAV and the target application of this UAV is the precision agricultural. However, the contribution and novelty of this study is not enough. The commercial electric UAVs have been widely used in lots of applications including the precision agricultural more than one decade.

Authors Reply: Authors thank the reviewer’s comments. In order to improve the understanding of the manuscript and in order to better communicate the objectives pursued by the authors, the sections Featured Application, Abstract and Introduction have been modified and extended. Novelty and contribution of this study is better explained. It is not a simple application of an electric RPA to precision agriculture which, as the reviewer says, is widely used. It is a deep modification of an RPA system that was obsolete and unused for almost twenty years, designed for a totally different application (aerial target for military use), propelled with gasoline (pollutant) and which has been given a second life with improved performance.

Please, see Sections Featured Application, Abstract and Introduction.

 

Comments R1.2:

Authors have used already existing components and assembled a system. Authors should clearly explain what the new contributions are in a point-wise manner. There should be research level scientific contributions. Not merely assembling a system.

Authors Reply: Continuing with the previous replay, Authors would like to resume some of the new contributions that have resulted after this work:

The life time of a series of aircrafts, that were obsolete and declassified, has been extended to be used in other applications. The new developments applied in this RPAS have been carried out by the authors, meaning a significant improvement in the know-how of both research centers (INTA and UHU) Among these new developments should be noted: Change to a more efficient engine, less heavy and bulky, with a greater ratio of torque vs size. A dedicated software has been developed in order to monitor and control the parameters related to the new power system. A new and less consuming telemetry module has been designed for this RPAS. This telemetry module is able to stablish communication between the GCS and the RPAS BMS and motor controller. 3D printed parts such as battery case, telemetry module chassis, propeller cone and motor controller housing has been designed and printed. Propeller cone and motor controller housing has been specially designed for a good aerodynamic response and for improving the cooling of the electric motor and the motor controller. Modernization of the fly control system and geolocation system have been also incorporated in this RPAS. The modification of the type and material of the propeller, reaching a higher performance have to be noted. With the change of engine, it has been possible to ensure the possibility to reach a higher altitude because electric motors do not require oxygen as an oxidizer to operate. Another novelty to highlight has been obtaining an RPAS with two important characteristics: its low noise level and its lower infrared signature. Important features in certain applications.

 

Comments R1.3:

The literature survey is not comprehensive. This article contains a lot of fundamental descriptions and discussions, but there is no state-of-the-art regarding the electric UAVs or new propulsion systems.

Authors Reply: Authors thank the reviewer’s comment. A new bibliographic search on the state of the art regarding UAV has been carried out.

Please, see Section Introduction.

 

Comments R1.4:

The developed UAS is quite complicate and expensive. It is not suitable for civil applications and the precision agricultural.

Authors Reply: Authors do not intend to build a less expensive RPAS than those that can be found today in the market. It has been tried to transform an obsolete RPAS model and try to modify its characteristics to make it more versatile, both for use in precision agriculture applications as in many others where the low noise level, the height reached, the non-contamination, the low infrared signature and other characteristics are decisive when deciding whether to use one type of aircraft or another. Moreover, the result of having changed the propulsion system from internal combustion engine to electric engine, does not make the system more complicated. On the other hand, it should be remembered that both INTA and UHU are two public research organizations whose ultimate goals are not competition with the industry but the training of future researchers.

 

Comments R1.5:

The calculation of the required power or thrust for cruising should be based on the aircraft performance of the developed UAV. The conducted tests are not rigorous and contain many unreasonable assumptions. For example, the thrust test on the ground could not obtain the dynamic thrust of the UAV in cruising phase. The weight of the batteries will decrease the payload weight.

Authors Reply: The transformations made in the RPAS maintain the main airworthiness characteristics of the aircraft. So, a thrust test with the original ICE configuration and with the final configuration of the electric ALO has been driven in order to ensure the global behavior of the complete system and to be able to compare the on ground thrust for both the original and the final electric configurations. The 3-blade propeller of carbon fiber has same thrust than the 2-blade original one. The used thrust test bench is the same we use for other similar aircrafts. Experimental results show a thrust comparison between the Electric ALO and ICE configuration. Thrust results confirm that both configurations reach around 12kg thrust at 60% Motor Input which will be enough for the launching needs. Since the modifications performed in original ALO do not affect the final weights nor the surfaces, the flight envelope for both configurations are the same. With this static thrust test, it can be assumed that the performance of the modified ALO will be the same when in flight.

Please, see Sections 2.3.2. Thrust Test Bench, 3.2. Thrust tests and Discussion.

 

Comments R1.6:

This study does not complete the real flight test which is a very important process to proof that the developed UAV could demonstrate the capability to achieve the desired purposes.

Authors Reply: Authors agree with the reviewer's comment. It wasn’t possible to perform real flight tests due to the lack of a launch window at the El Arenosillo Experimentation Center (https://en.wikipedia.org/wiki/El_Arenosillo) due to the current use of airspace in military exercises. Flight tests will be conducted as soon as possible. However, the results obtained with static tests make us expect that the results will be satisfactory.

 

Reviewer 2 Report

The paper deals with the conversion of a fixed wing ice drone into an electric one.

The topic is of interest but the paper needs major revisions before being considered for publication. The main criticalities (minors and majors) encountered in the paper are:

There are only 2 affiliations, not 5. lines 14-18: RPA and INTA have not been defined. Keywords include several repetitions. line 65: references should start with [1], not [9]. Introduction: the section is very poor. Literature review about drones / precision agriculture has to be enlarged (there are hundreds of papers published on MDPI, etc. to date). Which was the application of the designed electric drone? line 115: ‘has’ instead of ‘have’. Please, check the English grammar. line 380: please avoid the use of ‘we’ lines 146-212: the format of the bulleted list is confusing. There are too many figures, e.g. are Figs. 4,12 useful? Several figures can be joined (es. Figures 5 and 6). Please, improve the resolution of the figures. The main problem is that Authors claimed that the drone was designed for precision agriculture purpose but no in-field tests for precision agriculture were described (e.g. which data have been acquired?, etc.). This omission will affect the acceptance of the paper.

Author Response

Reviewer(s)' Comments to Author:

Reviewer: 2

Comments R2.1:

The paper deals with the conversion of a fixed wing ICE drone into an electric one. The topic is of interest but the paper needs major revisions before being considered for publication.

Authors Reply: Authors really thank this Reviewer’s comments and valorization of the paper. All Reviewer’s recommendations have been addressed and authors have resubmitted the manuscript with the proposed changes.

 

Comments R2.2:

There are only 2 affiliations, not 5. lines 14-18

Authors Reply: Authors apologize for this mistake and in the manuscript’s revised version, only 2 affiliations appear.

Please, see new affiliations section.

 

Comments R2.3:

RPA and INTA have not been defined.

Authors Reply:  In spite of the fact that INTA definition appears previously in the affiliation, INTA has been defined again. RPA and RPAS have been defined as well. 

See Section Featured Application.

 

Comments R2.4:

Keywords include several repetitions.

Authors Reply: Although some of the keywords may seem the same, the fact is that there are substantial differences between them. The authors wanted to emphasize the differences among them: A few years ago, in military areas, this type of aircraft without a pilot was known as unmanned aerial vehicles (UAV). It is a name that is currently in disuse. In unmanned aircraft, the complete system is almost more important than only the aircraft itself. The system includes the aircraft, the communications link and the ground station. For that reason, the term UAS (Unmanned Aerial System) was invented and UAV was still used when only wanted to talk about the aircraft. But to try to organize this, a few years ago (2011), ICAO (International Civil Aviation Organization) created the UAS Special Group and the first thing they did was to think of a nomenclature that made sense. Generically, they are called unmanned aircraft (UA) without specifying whether they are piloted or not. Within this nomenclature, depending on whether or not there is a remote pilot, they would be autonomous aircraft or manned aircraft by remote control (RPA, Remotely Piloted Aircraft). To refer to the entire system, the UA renamed UAS (Unmanned Aerial System) and RPA becomes RPAS (Remotely Piloted Aircraft System).

 

Comments R2.5:

line 65: references should start with [1], not [9].

Authors Reply: Authors apologize for this mistake and in the manuscript’s revised version, references start with [1].

 

Comments R2.6:

Introduction: the section is very poor.

Authors Reply: Authors thank this reviewer’s suggestion. The Introduction section has been rewritten and extended  

See Section Introduction.

 

Comments R2.7:

Literature review about drones / precision agriculture has to be enlarged (there are hundreds of papers published on MDPI, etc. to date).

Authors Reply:  According to reviewer’s suggestions, authors have included and contextualized new references.

Please, see Section Introduction and References [4][5][6][7][8][9][10][11][12][13][14][15] [16][17][18].

 

Comments R2.8:

Which was the application of the designed electric drone?.

Authors Reply: The original ALO (Light Observation Plane), developed by INTA in the 1990s was designed as an aerial target and in reconnaissance and surveillance missions at low cost. This new electric RPAS will have several applications, main one is to be used in precision agriculture for the automated detection, geolocation and counting of olive trees from high resolution multispectral images in large sized crop applications. But also, it is designed to be used as a wildlife surveillance and monitoring system in the surroundings of the Doñana National Park. And finally, another important application is to use it as a mobile platform for testing different energy systems such as advanced batteries, supercapacitors and fuel cells.

Thanks to the comments of the Reviewer, the Introduction section has been modified better highlighting the applications of the electric RPA.

 

Comments R2.9:

line 115: ‘has’ instead of ‘have’. Please, check the English grammar.

Authors Reply: Authors apologize for this mistake and in the manuscript’s revised version, English grammar has been checked.

 

Comments R2.9:

line 380: please avoid the use of ‘we’

Authors Reply:

Authors apologize for this mistake. It has been corrected in the manuscript’s revised version.

 

Comments R2.10:

lines 146-212: the format of the bulleted list is confusing

Authors Reply: Authors apologize for this. The format of the bullet has been modified in order to a better understanding.

Please, see Section 2.1.2 ALO RPAS Description.

 

Comments R2.11:

There are too many figures, e.g. are Figs. 4,12 useful?.

Authors Reply:  Authors thank this reviewer’s suggestion. Figure 12, together with Figures 10 and 11, have been moved to Annex 1, following this option also suggested in "Instructions for Authors" in Applied Science. Authors would like to leave Figure 4 to show how the monitoring stations were almost 30 years ago, in a nostalgic way compared to the modern GCS that can now be replaced by a smart phone.

 

Comments R2.12:

Several figures can be joined (es. Figures 5 and 6).

Authors Reply: Authors thank reviewer’s suggestion and Figures 5 and 6 have been joined, as well as Figures 8 and 9 and 15 and 16.    

 

Comments R2.13:

Please, improve the resolution of the figures.

Authors Reply: The resolution of some of the figures has been improved, but others are figures of the 90s, difficult to improve. Some others, such as Figures 10, 11 and 12, as explained, have been moved to Annex 1

 

Comments R2.13:

The main problem is that Authors claimed that the drone was designed for precision agriculture purpose but no in-field tests for precision agriculture were described (e.g. which data have been acquired? etc.). This omission will affect the acceptance of the paper.

 

Authors Reply: Authors thank the exhaustive revision of the paper done by the Reviewer. The authors would like to say that the objective of this manuscript is to describe the process followed to carry out the deep transformation of the aerial platform, from gasoline to electricity, within the scope of the TECNOLIVO Project (this European project has not yet been completed. It will end on May 31, 2020). Of course, numerous in-field tests related to precision agriculture have been developed in the Project. These tests have been carried out with commercial RPAS of rotary wing (quadcopters) and the results of them are in the process of being published soon. Some of them have already been published. In any case, following the recommendations of this Reviewer, new bibliographic references to results already published of the Project have been included in the manuscript, in the same way, data of the equipment embarked on the aerial platforms have been incorporated as well as some of the tests performed.

 

Reviewer 3 Report

Manuscript "Converting a fixed wing ICE RPAS into an electric LiPo battery driven RPAS for Precision Agricultural Purposes" by F. Llerena et al., submitted to Applied Sciences.

 

This work describes the recent results in novel remotely piloted aircraft systems. This is extremely important for precision agricultural techniques and other applications required miniaturized aerial systems.

 

This is a good contribution in the field, and I support this work, and believe it’s worth of publishing after some revision.

 

English is good, but further workout with the help of native-speaking colleagues would be beneficial.

 

Title: remove all abbreviations. Spell them out or rephrase the title.

 

Featured Application: same, spell the abbreviations out

 

Abstract: Needs stressing the novelty.

 

Introduction: Clearly list the major results and stress the novelty. This is not an absolutely novel technique, so the reader should clearly and immediately see what is the novelty of your results.

 

Figures: Add a brief (several lines) comments directly in the figure caption. Stress the main features the reader see in the figures.

As a recommendation, consider shortening the paper by removing some excessive technical details (e.g. figs. 10, 11, 12 etc.). Since the journal's format permits Supplementary Material, consider moving excessive technical details to the Supplementary Files.

 

Consider reducing the number of images. E.g., figs. 5 and 6; 8 and 9; 15, 16 and 17 etc. may be combined.

 

Table 6. Total efficient emission for EALO is not zero, since generation of electric energy may involve fuel-consuming techniques. Please comment on this.

 

What is important, make a direct economical comparison of the two systems, taking into account the costs of electric and ICE systems, as well as the costs of fuel/electric energy.

References. References are very important section of the paper, and the importance of the results should be supported by timely references. I recommend adding several recent references:

 

About novel materials for batteries and aerospace systems:

"In-Situ Synthesized Si@C Materials for the Lithium Ion Battery: A Mini Review". Nanomaterials 2019, 9(3), 432; https://doi.org/10.3390/nano9030432

"Advanced Materials for Next Generation Spacecraft", Adv. Mater. 30 (2018) 1802201. https://doi.org/10.1002/adma.201802201; "Explore space using swarms of tiny satellites", Nature 562 (2018) 185-187. https://doi.org/10.1038/d41586-018-06957-2

"Nanoscience Supporting the Research on the Negative Electrodes of Li-Ion Batteries". Nanomaterials 2015, 5(4), 2279-2301; https://doi.org/10.3390/nano5042279

"Recent progress and perspectives of space electric propulsion systems based on smart nanomaterials". Nature Commun. 9, 879, 2018. https://www.nature.com/articles/s41467-017-02269-7

 

About importance of miniaturization for advanced aerospace systems:

"Explore space using swarms of tiny satellites". Nature 562, 185, 2018. https://doi.org/10.1038/d41586-018-06957-2

* * *

Overall, I support this paper and recommend it for acceptance in Applied Sciences after revision.

Author Response

Reviewer(s)' Comments to Author:

Reviewer: 3

Comments R3.1:

This work describes the recent results in novel remotely piloted aircraft systems. This is extremely important for precision agricultural techniques and other applications required miniaturized aerial systems.

This is a good contribution in the field, and I support this work, and believe it’s worth of publishing after some revision.

Authors Reply: Authors really thank Reviewer comments and valorization of the paper. All Reviewer’s recommendations have been addressed and authors have resubmitted the manuscript with the proposed changes.

 

 

 

Comments R3.2:

English is good, but further workout with the help of native-speaking colleagues would be beneficial.

Authors Reply: Authors thank Reviewer recommendation. In the revised version of the manuscript, English grammar has been checked.

 

Comments R3.3:

Title: remove all abbreviations. Spell them out or rephrase the title.

Authors Reply: Authors have removed abbreviations (ICE and LiPo) spelling them out but Authors would like to leave “RPAS” because the use of writing "RPAS" in titles of scientific articles in this field is widespread. As examples, some MDPI publications:

Cuxart, B. Wrenger, B. Matjacic, and L. Mahrt, “Spatial Variability of the Lower Atmospheric Boundary Layer over Hilly Terrain as Observed with an RPAS,” Atmosphere (Basel)., vol. 10, no. 11, p. 715, Nov. 2019, doi: 10.3390/atmos10110715. Rey-Caramés, M. Diago, M. Martín, A. Lobo, and J. Tardaguila, “Using RPAS Multi-Spectral Imagery to Characterise Vigour, Leaf Development, Yield Components and Berry Composition Variability within a Vineyard,” Remote Sens., vol. 7, no. 11, pp. 14458–14481, Oct. 2015, doi: 10.3390/rs71114458. Booysen et al., “Towards Multiscale and Multisource Remote Sensing Mineral Exploration Using RPAS: A Case study in the Lofdal Carbonatite-Hosted REE Deposit, Namibia,” Remote Sens., vol. 11, no. 21, p. 2500, Oct. 2019, doi: 10.3390/rs11212500.

Some ELSEVIER publications:

Corraro, F. Corraro, and E. Filippone, “Performance Verification of an Enhanced Traffic Alerting System for RPAS Integration in ATM,” IFAC-PapersOnLine, vol. 51, no. 9, pp. 168–173, Jan. 2018, doi: 10.1016/j.ifacol.2018.07.028. Usach and J. A. Vila, “Reconfigurable Mission Plans for RPAS,” Aerosp. Sci. Technol., vol. 96, p. 105528, Jan. 2020, doi: 10.1016/j.ast.2019.105528.

 

Comments R3.4:

Featured Application: same, spell the abbreviations out.

Authors Reply: Authors have removed abbreviations (RPA and INTA) spelling them out

 

Comments R3.5:

Abstract: Needs stressing the novelty.

Authors Reply: Thanks to the recommendations of Reviewer, the novelties of the manuscript have been stressed in the Abstract section.

 

Comments R3.6:

Introduction: Clearly list the major results and stress the novelty. This is not an absolutely novel technique, so the reader should clearly and immediately see what is the novelty of your results.

Authors Reply: Referring to the previous recommendation, the novelties and major results have been stressed in the Introduction section of the manuscript.

 

 

Comments R3.7:

Figures: Add a brief (several lines) comments directly in the figure caption. Stress the main features the reader see in the figures.

Authors Reply: Authors have added some more information in most of figure captions

 

Comments R3.8:

As a recommendation, consider shortening the paper by removing some excessive technical details (e.g. figs. 10, 11, 12 etc.). Since the journal's format permits Supplementary Material, consider moving excessive technical details to the Supplementary Files.

Authors Reply: Authors thank this reviewer’s suggestion. Figures 10, 11 and 12 have been moved to Annex 1, following this option also suggested in "Instructions for Authors" in Applied Science.

 

Comments R3.9:

Consider reducing the number of images. E.g., figs. 5 and 6; 8 and 9; 15, 16 and 17 etc. may be combined.

Authors Reply: Authors thank this reviewer’s suggestion and Figures 5 and 6 have been combined, as well as Figures 8 and 9 and 15 and 16.

 

Comments R3.10:

Table 6. Total efficient emission for EALO is not zero, since generation of electric energy may involve fuel-consuming techniques. Please comment on this.

Authors Reply: Authors thank the reviewer’s comment. The authors would like to explain that since the 1990s, INTA produces electricity in a renewable way. This comes from several photovoltaic fields of different technologies as well as from a wind turbine. All these renewable energy sources are integrated in a microgrid that can also store excess of energy in different types of batteries and in the form of hydrogen. The reviewer might like more information about this microgrid in the following publication: https://ieeexplore.ieee.org/document/8849016. This is the reason why we consider the electric RPAS as a zero-emission device, in this case, generation of electric energy doesn’t involve fuel-consuming techniques.

 

Comments R3.11:

What is important, make a direct economical comparison of the two systems, taking into account the costs of electric and ICE systems, as well as the costs of fuel/electric energy.

Authors Reply: Authors do not intend to build a less expensive RPAS than those that can be found today in the market. It has been tried to transform an obsolete RPAS model and try to modify its characteristics to make it more versatile, both for use in precision agriculture applications as in many others where the low noise level, the height reached, the non-contamination, the low infrared signature and other characteristics are decisive when deciding whether to use one type of aircraft or another. Moreover, the result of having changed the propulsion system from internal combustion engine to electric engine, does not make the system more complicated. On the other hand, it should be remembered that both INTA and UHU are two public research organizations whose ultimate goals are not competition with the industry but the training of future researchers.

 

Comments R3.12:

References. References are very important section of the paper, and the importance of the results should be supported by timely references. I recommend adding several recent references:

About novel materials for batteries and aerospace systems:

"In-Situ Synthesized Si@C Materials for the Lithium Ion Battery: A Mini Review". Nanomaterials 2019, 9(3), 432; https://doi.org/10.3390/nano9030432;

"Advanced Materials for Next Generation Spacecraft", Adv. Mater. 30 (2018) 1802201. https://doi.org/10.1002/adma.201802201;

"Nanoscience Supporting the Research on the Negative Electrodes of Li-Ion Batteries". Nanomaterials 2015, 5(4), 2279-2301; https://doi.org/10.3390/nano5042279;

"Recent progress and perspectives of space electric propulsion systems based on smart nanomaterials". Nature Commun. 9, 879, 2018. https://www.nature.com/articles/s41467-017-02269-7

About importance of miniaturization for advanced aerospace systems:

"Explore space using swarms of tiny satellites". Nature 562, 185, 2018. https://doi.org/10.1038/d41586-018-06957-2.

Authors Reply: The authors agree with this Reviewer about the importance of References, in particular, about battery references. Although the references suggested by the Reviewer are very specific in their use in space applications and satellites, they have been duly referenced in the text of the manuscript due to its importance in matters of component miniaturization.

 

Comments R3.13:

Overall, I support this paper and recommend it for acceptance in Applied Sciences after revision.

Authors Reply: The authors would like to thank Reviewer again for his constructive comments as well as his recommendation for this manuscript to be accepted in Applied Sciences after review.

 

Round 2

Reviewer 1 Report

It is suggested to define the RPAS (Remoted Piloted Aircraft System) and to compare the differences between RPAS and UAV, especially for drones. Actually, the RPAS is a transition product and it is almost out of date since the autopilot system is available and affordable for most users. If the authors want to promote the RPAS and give it the second life, it is required to show the pros and cons of the RPAS and the drones. The reasons why the RPAS was obsolete and unused for almost twenty years need to be discussed in the article. Could the challenges and problems of the RPAS be solved with the modification of the engine system developed in this study? There are many successful long endurance fixed wing UAVs and some of them has adopted the electric propulsion system. Therefore, the comparison of fixed wing UAVs and rotorcrafts is not adequate. It is suggested to compare with the existing long endurance fixed wing UAVs. The research level scientific findings and improvements are still not enough although the authors have make a lot of effort to update the new avionics and flight control unit. This study shows the rough ground test results, but the results do not support by the theory of aircraft design and aircraft performance. For a journal paper, the scientific finding and novelty of this study must be improved. The real flight test does not carry out to proof that the developed UAV could demonstrate the capability to achieve the desired purposes. Without the real flight test, the result of this study is incomplete.  

Author Response

It is suggested to define the RPAS (Remoted Piloted Aircraft System) and to compare the differences between RPAS and UAV, especially for drones. Actually, the RPAS is a transition product and it is almost out of date since the autopilot system is available and affordable for most users. If the authors want to promote the RPAS and give it the second life, it is required to show the pros and cons of the RPAS and the drones. The reasons why the RPAS was obsolete and unused for almost twenty years need to be discussed in the article. Could the challenges and problems of the RPAS be solved with the modification of the engine system developed in this study?

Authors would like to say that in the Introduction section differences among UAS, UAV, RPA, RPAS and drones have been defined. Anyway and, in order to avoid any misunderstanding, authors wanted to emphasize the differences among them: A few years ago, in military areas, this type of aircraft without a pilot was known as unmanned aerial vehicles (UAV). It is a name that is currently in disuse. In unmanned aircraft, the complete system is almost more important than only the aircraft itself. The system includes the aircraft, the communications link and the ground station. For that reason, the term UAS (Unmanned Aerial System) was invented and UAV was still used when only wanted to talk about the aircraft. But to try to organize this, a few years ago (2011), ICAO (International Civil Aviation Organization) created the UAS Special Group and the first thing they did was to think of a nomenclature that made sense. Generically, they are called unmanned aircraft (UA) without specifying whether they are piloted or not. Within this nomenclature, depending on whether or not there is a remote pilot, they would be autonomous aircraft or manned aircraft by remote control (RPA, Remotely Piloted Aircraft). To refer to the entire system, the UA renamed UAS (Unmanned Aerial System) and RPA becomes RPAS (Remotely Piloted Aircraft System). This is very well explained in some of the references included in the manuscript, nonetheless, authors have included a new reference where these difficult definitions are really well explained (S. I. Granshaw, “RPV, UAV, UAS, RPAS … or just drone?,” Photogramm. Rec., vol. 33, no. 162, pp. 160–170, 2018, doi: 10.1111/phor.12244.), we recommend it.

When we refer to the RPAS called ALO being a system that was obsolete, we were referring to the fact that, in particular, that aircraft was out of use in INTA for the applications for which, in its day, it was designed. These were target aircraft and reconnaissance aircraft. The ALO was soon replaced by other more modern models and was removed from service (https://www.inta.es/INTA/en/quienes-somos/historia/los-rpa/). 20 years later, the authors wanted to demonstrate the possibility of changing the propulsion system based on fossil fuels and internal combustion engine, for another based on electric propulsion and batteries. Instead of buy a new RPA or design and construct one, it was decided to take advantage of the existence of several units of the ALO that was out of use in order to make experiments at a lower cost. If this were possible, the next step would be to replace part of the batteries and hybridize the system with hydrogen and fuel cells. In this field, both INTA and UHU have a vast experience. The main advantages of using an electric RPAS over an internal combustion engine, as described in the manuscript (see Introduction section), are reliability, efficiency, lower noise, reduction of heat losses, as well as getting an energy supply free of polluting emissions when this electricity is produced from renewable energies (in Discussion Section it was referenced this point citing: https://ieeexplore.ieee.org/document/8849016)

There are many successful long endurance fixed wing UAVs and some of them has adopted the electric propulsion system. Therefore, the comparison of fixed wing UAVs and rotorcrafts is not adequate. It is suggested to compare with the existing long endurance fixed wing UAVs.

Authors thank the reviewer’s comments. For a better understanding, and after a new bibliographic search, two tables have been included in the manuscript. One of them shows a comparison between electric and ICE engines (both fixed wings and rotary wings UAVs). The other is a comparison between advantages and disadvantages of fixed wings and rotary wings UAVs.

Moreover, a comparison between the ALO RPAS and several commercial fixed wing RPAS has been also included (see Conclusions section).

The research level scientific findings and improvements are still not enough although the authors have made a lot of effort to update the new avionics and flight control unit.

Authors thank the reviewer’s comments. With the aim to meet with reviewer’s suggestions, regarding the aerial platform, authors have completed the state of art including a comparison of the main techniques for acquiring image measurable data and the different models of ICE and electric RPAS used in last two decades are analysed.

Focusing on electric platforms, the paper included a comparative between battery (differentiating into different technologies), supercapacitor and hydrogen-based energy system.

In relation to the novelties of the developed work regarding commercial solutions, this second revision demonstrate that the developed electric ALO overpasses in terms of flight autonomy and surface to be treated is the commercial UAVS (multicopters) models currently used. Additionally, this transformation has allowed additional innovations like: 1) more efficient engine, less heavy and bulky, 2) a greater ratio of torque vs size; 3) a dedicated software to monitor and control the parameters related to the new power system; and 4) a new and less consuming telemetry module.

This study shows the rough ground test results, but the results do not support by the theory of aircraft design and aircraft performance. For a journal paper, the scientific finding and novelty of this study must be improved. The real flight test does not carry out to proof that the developed UAV could demonstrate the capability to achieve the desired purposes. Without the real flight test, the result of this study is incomplete.

Authors thank the reviewer’s comments, and we would like to explain that the flight test are expected for the second semester of 2020. Due to the high rate of occupation of INTA aerial space, the authorization for flight tests takes around 6-9 moths. On the other hand, the aim of the paper was to propose a methodology to transform a fixed-wing unmanned aerial system, used as an aerial target and powered by an internal combustion engine, into an electric RPA for use in the cultivation of olive trees and other species that occupy large areas of land. The main reason for carrying out this transformation is to increase the flight autonomy of the RPA in order to be able to take measurements of the different parameters that affect this large-scale crop. Based on this, authors have considered that it would be interesting to share with the scientific community the first experimental results that justify the decision to transform an ICE to an electric RPA in the context of the project’s requirements. Of course, and following with the reviewer’s suggestions, as soon as authors have permission to develop fly tests, they will prepare a second contribution where they will include a comparison of how different electric power sources like battery, supercapacitor and fuel cell have effect on the RPA behaviour.

 

Reviewer 2 Report

There are still Authors' comments in the submitted file!!!

The Introduction is confusing as there is a mixing between the project and the manuscript. In my opinion, the project description has to be included in subsection 1.1.

Authors solved the literature review of precision agriculture in 3 lines! There are several topics related to UAV in precision agriculture that need to be addressed (e.g. 2D, 3D point cloud, multispectral camera, thermal, etc.)

..for Precision Agricultural Purposes.. has to be removed from the title as the manuscript only deals with the conversion of the vehicle. Moreover, Authors claimed that the new vehicle will be used for several purposes.

Please, avoid the use of ‘we’.

Figures 5-7 can be removed

The caption of the figure on page 12 is missing. However, the figure can be removed.

Are figures 17 and 27 both useful? I do not think so. The same for figures 11, 13, 29 and 30.

Author Response

There are still Authors' comments in the submitted file!!!

Authors have been asked to submit the new manuscript with all the corrections made using the "change control"

The Introduction is confusing as there is a mixing between the project and the manuscript. In my opinion, the project description has to be included in subsection 1.1.

Authors thank the reviewer’s comments. In order to improve the understanding of the manuscript, the Introduction section has been rewritten. Focusing on the description of the study of the transformation of the primitive aircraft into an electric one. With this purpose, and following the reviewer’s advice, the Project TecnOlivo description has been included in a new subsection 1.1

Authors solved the literature review of precision agriculture in 3 lines! There are several topics related to UAV in precision agriculture that need to be addressed (e.g. 2D, 3D point cloud, multispectral camera, thermal, etc.)

In spite of the sentence “..for Precision Agricultural Purposes..” has been removed from title and even though the main objective of this study refers to the transformation of the UAV’s energy system, authors have included new references on precision agriculture as well as a review of the state of the art of sensors used in this subject. A table with differences between hyperspectral and multispectral imaging, spectroscopy and RGB imagery has also been included.

“..for Precision Agricultural Purposes..” has to be removed from the title as the manuscript only deals with the conversion of the vehicle. Moreover, Authors claimed that the new vehicle will be used for several purposes.

According to reviewer’s comment, title has been changed as explained in previous paragraph.

Please, avoid the use of ‘we’.

Authors apologize for this mistake. Use of “we” has been removed from the manuscript.

Figures 5-7 can be removed

According to reviewer’s suggestion, both figures have been removed.

The caption of the figure on page 12 is missing. However, the figure can be removed.

It has been checked again that the caption of the figure is already present. However, authors would like to leave this figure because it shows one of the novelties and contributions of this study: A new and less consuming telemetry module which has been designed for this RPAS. This telemetry module is able to stablish communication between the GCS and the RPAS BMS and motor controller.

Are figures 17 and 27 both useful? I do not think so. The same for figures 11, 13, 29 and 30.

According to reviewer’s suggestion, Figure 27 has been deleted. Authors have considered to leave Figure 17 because it shows how is the launcher of the new RPAS, in order to show those readers who are not experts in this topic, what a launching ramp is like.

Figure 11 has been deleted. Authors would suggest to leave Figure 13 because is also a novel contribution and can help to a better understanding.

Figures 29 and 30 have been grouped in one. Authors would like to show how is the order of sizes of the devices that have been substituted.

Authors really would like to thank this Reviewer’s comments which have improved significantly this paper. We really appreciate your effort and dedication of time.

 

Reviewer 3 Report

Accept in present form

Author Response

Authors really thank this Reviewer for accepting the manuscript in its present status.

Round 3

Reviewer 1 Report

Sections 3 and 4 are not well organized. The objectives and requirements of battery tests, thrust tests, and launching dummy tests are not shown at the beginning of Section 3. More details about the connections between these tests need to be provided. There are lots of paragraphs with only one or two sentences.

 

For the thrust tests, the flight conditions of cruise phase are required to determine the cruise speed, endurance, and required thrust according to total drag. For launching dummy tests, the stall speed and required takeoff speed also are required to verify that the developed system can reach the desired parameters and specifications of the aircraft. The test results need to be supported by the theory of aircraft design and aircraft performance.

 

Table 6 should be verified by the comment in Point 2.

 

The quality and resolution of most figures are not enough.

Author Response

Comments R1.1:

• Sections 3 and 4 are not well organized. The objectives and requirements of battery tests, thrust tests, and launching dummy tests are not shown at the beginning of Section 3. More details about the connections between these tests need to be provided. There are lots of paragraphs with only one or two sentences.

Authors Reply: Authors thank the reviewer’s comments. For a better understanding, sections 3 and 4 have been reorganized including objectives of performed tests based in previous works on the same UAV carried out by the authors. In order to a better clarification, a definition of the different phases of the main flight plan has been included in order to determine the necessary energy consumption. These have been summarized in the new table 10. The process by which these values are reached is described profusely in Annex 2 (sorry, in Spanish but easily understandable). Authors do not consider necessary to include these developments in the manuscript as it leaves the initial scope of this work.

 

Comments R1.2:

• For the thrust tests, the flight conditions of cruise phase are required to determine the cruise speed, endurance, and required thrust according to total drag. For launching dummy tests, the stall speed and required takeoff speed also are required to verify that the developed system can reach the desired parameters and specifications of the aircraft. The test results need to be supported by the theory of aircraft design and aircraft performance.

Authors Reply: Authors thank the reviewer’s comments. Again, we refer the reviewer to the work included in Annexes 2, 3 and 4. Before the transformation of the internal combustion engine UAV into an electric one, a thorough study of the characteristics of the system (geometric, mass, aerodynamics, power plant, system feeding, ...), has been carried out.

Then, it has been proceeded to analyze the most common maneuvers that are performed in the operation of the platform: cruise flight, ascending flight, symmetrical horizontal turn and gliding flight. To do this, a study of both the available and required power in each of the maneuvers is first carried out. We can learn from it the physical limitations of the platform, such as the loss speed, the flight envelope, the ascending speed and angle of rise, the different flight ceilings or the gliding flight under different conditions.

In order to attend this reviewer’s comment, the new figure 22 showing the available power and required power for each flight altitude has been included in the manuscript. In it we can see the flight envelope of the UAV (efficiency, cruising speed, autonomy, loss speed, maximum aerodynamic speed, etc.), for each altitude.

 

Comments R1.3:

• Table 6 should be verified by the comment in Point 2.

Authors Reply: This comment has been also included in the previous answers.

 

Comments R1.4:

• The quality and resolution of most figures are not enough.

Authors Reply: Authors would like to confirm that the resolution of all the figures that are possible has been improved, but unfortunately others figures date from last century (in the 90s), so it is difficult to improve them. Authors would thank the reviewer to number the figures he/she refers and we will do all our best to improve them.

Reviewer 2 Report

I thank the Authors for the extra work they did.

In my opinion, the manuscript is now suitable for publication.

Author Response

Authors really thank this Reviewer for accepting the manuscript in its present status.