drones

Article Development of a Solar-Powered Unmanned Aerial Vehicle for Extended Flight Endurance

Yauhei Chu †, Chunleung Ho †, Yoonjo Lee † and Boyang Li *

Department of Aeronautical and Aviation Engineering, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong, China; [email protected] (Y.C.); [email protected] (C.H.); [email protected] (Y.L.) * Correspondence: [email protected]; Tel.: +852-340-082-31 † Authors have contributed equally.

Abstract: Having an exciting array of applications, the scope of unmanned aerial vehicle (UAV) application could be far wider one if its flight endurance can be prolonged. Solar-powered UAV, promising notable prolongation in flight endurance, is drawing increasing attention in the industries’ recent research and development. This work arose from a Bachelor’s degree capstone project at Hong Kong Polytechnic University. The project aims to modify a 2-metre wingspan remote-controlled (RC) UAV available in the consumer market to be powered by a combination of solar and battery-stored power. The major objective is to greatly increase the flight endurance of the UAV by the power generated from the solar panels. The power system is first designed by selecting the suitable system architecture and then by selecting suitable components related to solar power. The flight control system is configured to conduct flight tests and validate the power system performance. Under fair



experimental conditions with desirable weather conditions, the solar power system on the aircraft
 results in 22.5% savings in the use of battery-stored capacity. The decrease rate of battery voltage

Citation: Chu, Y.; Ho, C.; Lee, Y.; Li, during the stable level flight of the solar-powered UAV built is also much slower than the same B. Development of a Solar-Powered configuration without a solar-power system. Unmanned Aerial Vehicle for Extended Flight Endurance. Drones Keywords: solar-powered; UAV; endurance extension; flight experiments 2021, 5, 44. https://doi.org/ 10.3390/drones5020044

Academic Editor: 1. Introduction Abdessattar Abdelkefi With widening the application scope of unmanned aerial vehicle (UAV) as the driving force, the development of solar-powered UAV recently has attracted more attention in Received: 30 April 2021 academia and commercial industries. A critical factor limiting the scope of application Accepted: 21 May 2021 Published: 24 May 2021 of conventional battery-powered electric UAVs is their energy storage capability. The conventional UAV is powered by the energy stored in batteries on board to maintain the

Publisher’s Note: MDPI stays neutral propulsion and functioning of flight control electronics. The amount of carried electric with regard to jurisdictional claims in power limits its flight range before takeoff. Although increasing the size of the battery published maps and institutional affil- or installing more batteries can increase the energy storage capacity, the weight of the iations. aircraft also increases. In turn, more power is consumed to carry the extra weight, resulting in the flight range being not necessarily prolonged. A possible approach to overcome this physical limit is the use of solar power as an energy source on UAV. Solar-powered UAV, using solar cells installed onboard, captures solar energy reaching the aircraft surface during daylight. Such generated power is supplied to the motor to propel the aircraft and Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. other electronics or to recharge the battery on board. The battery supplies power when in This article is an open access article darkness or under clouds. distributed under the terms and This project is aimed at the development of a small fixed-wing hand-launched solar- conditions of the Creative Commons powered UAV. A remote-controlled (RC) model glider for leisure purpose available on Attribution (CC BY) license (https:// the consumer market, a 759-2 Phoenix 2000 RC plane, is modified to be powered by a creativecommons.org/licenses/by/ hybrid of solar power and battery-stored power. This project suggested key points for 4.0/). the modification of an existing conventional RC plane to be solar-powered. The most

Drones 2021, 5, 44. https://doi.org/10.3390/drones5020044 https://www.mdpi.com/journal/drones Drones 2021, 5, 44 2 of 19

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modification of an existing conventional RC plane to be solar-powered. The most signifi- significantcant advantage advantage of modifying of modifying an existing an existing plan planee over over building building a new a new one one is the is thesaving saving on ondevelopment development time time and and cost cost on the on airframe, the airframe, making making solar-powered solar-powered UAV UAV available available to more to moreusers userswith witha more a more straightforward straightforward design design and andbuilding building process. process. Numerous Numerous adaptive adaptive de- designssigns are are required required for forsuch such modification, modification, for instance, for instance, modifying modifying the power the power system system archi- architecturetecture to include to include solar solar cells. cells. TheThe primaryprimary objectiveobjective ofof thisthis workwork isis to increase the flightflight duration of the UAV. FlightFlight teststests areare conductedconducted toto comparecompare thethe powerpower systemsystem performanceperformance ofof thethe RCRC planeplane betweenbetween thethe manufacturer’smanufacturer’s defaultdefault versionversion andand thethe solar-powersolar-power modifiedmodified version.version. ObjectivesObjectives areare alsoalso setset forfor thethe flightflight controlcontrol system.system. FlightFlight teststests forfor determiningdetermining thethe flightflight rangerange wouldwould lastlast forfor an extended period. period. To To ensure ensure a a fair fair comparison, comparison, the the flight flight control control system system shall shall be becapable capable of flying of flying the the aircraft aircraft autonomously autonomously without without human human intervention. intervention. The The finished finished so- solar-poweredlar-powered UAV UAV is is shown shown in in Figure Figure 1.1 .

FigureFigure 1.1. TheThe solar-poweredsolar-powered UAVUAV builtbuilt inin this this project project named named ‘Sun’. ‘Sun’.

2.2. LiteratureLiterature ReviewReview 2.1.2.1. ExamplesExamples ofof Solar-PoweredSolar-Powered UAVUAV ProjectProject TheThe firstfirst reviewed reviewed solar-powered solar-powered UAV UAV project project is AtlantikSolar, is AtlantikSolar, which which was conducted was con- byducted Oettershagen by Oettershagen et al. [ 1et] ofal. the[1] of Autonomous the Autonomous Systems Systems Lab, Lab, Swiss Swiss Federal Federal Institute Institute of Technologyof Technology Zurich. Zurich. Having Having completed completed an 81-hour an 81-hour continuous continuous flight flight that coveredthat covered a distance a dis- oftance 2338 of km, 2,338 the km, UAV the established UAV established a new world a new record. world Itrecord. has a conventionalIt has a conventional glider configu- glider ration,configuration, having ahaving wingspan a wingspan of 5.69 m of and 5.69 a m total and mass a total of mass 6.93 kg. of 6.93 A new kg. methodology A new methodol- was developedogy was developed as a result as of a the result project of forthe theprojec designt for of the solar-powered design of solar-powered UAVs for energetically UAVs for robustenergetically perpetual robust flight perpetual in sub-optimal flight in meteorological sub-optimal meteorological conditions. The conditions. project started The project with energeticstarted with system energetic modelling system to modelling consider theto consider variations the invariations operating in conditionsoperating conditions and local meteorologicaland local meteorological conditions. conditions. The airframe The and airframe energy and generation energy generation and storage and system storage was sys- de- signedtem was according designed to according the parameters to the obtainedparameters from obtained the energetic from the system energetic modelling. system Power mod- systemelling. Power components, system suchcomponents, as the Maximum such as the Power Maximum Point Trackers Power Point (MPPT) Trackers and a (MPPT) battery managementand a battery system, management were custom-made system, were and custom-made are capable and of regulating are capable the of energy regulating flow and the providing detailed energy flow information. The flight control system was designed with energy flow and providing detailed energy flow information. The flight control system state-space models. With the design of aircraft systems completed, the design of the whole was designed with state-space models. With the design of aircraft systems completed, the UAV was preliminarily verified with lab tests and short flight tests. design of the whole UAV was preliminarily verified with lab tests and short flight tests. The second solar-powered UAV project reviewed is that conducted by Morton et al. [2] The second solar-powered UAV project reviewed is that conducted by Morton et al. at The Centre for Distributed Robotics, University of Minnesota. A 4-metre wingspan [2] at The Centre for Distributed Robotics, University of Minnesota. A 4-metre wingspan solar UAV for the objective of low altitude aerial sensing applications was developed. solar UAV for the objective of low altitude aerial sensing applications was developed. The The power required for level flight of that UAV was estimated to be below 46 W. It was power required for level flight of that UAV was estimated to be below 46 W. It was capable capable of a maximum of 180 W solar power generation. The captured solar power is over of a maximum of 180 W solar power generation. The captured solar power is over 300% 300% of the power required for level flight. The airframe design aimed at satisfying the of the power required for level flight. The airframe design aimed at satisfying the appli- application of low altitude aerial sensing. Then, power required for level flight was derived cation of low altitude aerial sensing. Then, power required for level flight was derived from the aerodynamic characteristics of the airframe. After that, the power system was designed.from the aerodynamic Some power systemcharacteristics electronics, of the such airframe. as the MPPT, After werethat, the custom-built power system to satisfy was project-specific requirements. The solar power system consists of 64 SunPower C60 solar Drones 2021, 5, 44 3 of 19

designed. Some power system electronics, such as the MPPT, were custom-built to satisfy project-specific requirements. The solar power system consists of 64 SunPower C60 solar cells and a custom-built MPPT with an average efficiency of 91.59%. For the flight control system, it was designed according to the mission requirements. After the design of aircraft Drones 2021, 5, 44 systems, the weight distribution of the plane was determined. 3 of 19

2.2. Power System Components cells andThe aprimary custom-built objective MPPT with of integrating an average efficiency a solar ofpower 91.59%. system For the into flight a control UAV is to increase system, it was designed according to the mission requirements. After the design of aircraft thesystems, range the by weight providing distribution an extra of the power plane was source determined. during flight. In addition to the power sys- tem components in conventional UAV, extra components are required. The review article about2.2. Power power System devices Components for solar-powered aircraft applications by Safyanu et al. [3] is mainly referredThe primaryto in understanding objective of integrating and determining a solar power the system components into a UAV needed is to increase for the solar power the range by providing an extra power source during flight. In addition to the power system. It is also shown in reputable solar-powered UAV projects [1,2,4] that photovoltaic system components in conventional UAV, extra components are required. The review (PV)article cells about and power Maximum devices for Power solar-powered Point Tracker aircraft applications(MPPT) are by required Safyanu et for al. [the3] solar power system.is mainly The referred PV tocells in understanding collect solar and energy determining and convert the components it into neededelectric for energy; the the MPPT trackssolar power the maximum system. It is power also shown point in reputable of the PV solar-powered cells and extracts UAV projects the [maximum1,2,4] that power from thephotovoltaic PV cells. (PV) cells and Maximum Power Point Tracker (MPPT) are required for the solar power system. The PV cells collect solar energy and convert it into electric energy; the MPPT tracks the maximum power point of the PV cells and extracts the maximum power 2.3.from Photovoltaic the PV cells. (PV) Cells Voltage and current are produced by the PV cell when the light hits its surface. The 2.3. Photovoltaic (PV) Cells relationship between insolation—the amount of energy from the sun that reaches the Voltage and current are produced by the PV cell when the light hits its surface. The Earth—andrelationship betweenthe amount insolation—the of produced amount voltage of energy and fromcurrent the of sun a thatPV cell reaches is represented the by an I-VEarth—and curve (Current–Voltage the amount of produced Curve). voltage I-V and curves current vary of a PVamong cell is different represented kinds by an and models of PVI-V cells curve but (Current–Voltage are mainly Curve).similar I-V [5]. curves An I-V vary curve among represents different kinds the and voltage–current models of relation- shipPV cells at butparticular are mainly insolation, similar [5]. Anand I-V multiple curve represents I-V curves the voltage–current are included relationship in a diagram to illus- at particular insolation, and multiple I-V curves are included in a diagram to illustrate the trateperformance the performance of a PV cellat of different a PV cell insolation, at different as shown insolation, in Figure2 as. shown in Figure 2.

Figure 2. I-V curve of a typical PV cell. Figure 2. I-V curve of a typical PV cell. Solar Cell Efficiency Tables (Version 54) created by Green et al. [6] are referenced duringSolar this project’sCell Efficiency solar cell model Tables selection (Version process. 54) The created Efficiency by TablesGreen list et the al. current [6] are referenced PV cell models of the highest confirmed energy conversion efficiency in their respective during this project’s solar cell model selection process. The Efficiency Tables list the cur- classifications. Common types of PV cells, such as silicon crystalline cells, silicon thin-film rentcells, PV gallium cell models arsenide (GaAs)of the highest cells, and confirmed copper indium energy gallium conversion selenide (CIGS) efficiency cells [7 ],in their respec- tiveare includedclassifications. in the Efficiency Common Tables. types The of highest PV cells, achievable such efficiencyas silicon of crystalline each category cells, of silicon thin- filmPV cell cells, can gallium be compared. arsenide The selection (GaAs) of cells, PV cell and model copper for this indium project gallium will be discussed selenide (CIGS) cells [7],in detail are included in the following in the section. Efficie Whenncy Tables. multiple The PV cellshighest are connected achievable in series,efficiency they of each cate- form a PV array (or solar array). The nominal total voltage of the PV array is defined by gorythe nominal of PV voltagecell can per be cell compared. and the number The ofselection cells. Usually, of PV the cell same model model for of PV this cell project will be discussedis installed inin andetail array, in so the that: following PV array nominal section. total When voltage multiple = the nominal PV cells voltage are perconnected in se- ries,cell × theynumber form of cellsa PV [2 ].array (or solar array). The nominal total voltage of the PV array is

defined2.4. Maximum by the Power nominal Point Tracker voltage (MPPT) per cell and the number of cells. Usually, the same model of PV cell is installed in an array, so that: PV array nominal total voltage = the nominal An MPPT is to extract maximum power from the PV cells. The maximum power voltagepoints P MPPper(P cellMPP ×= number IMPP × V ofMPP cells) on I-V[2]. curves are where I = IMPP and V = VMPP. The

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output power of a PV array varies with insolation. As insolation changes from time to time, the MPPT is needed to continuously track the fluctuating PMP on the I-V curves of the respective insolation [3]. For UAV applications in which PV arrays are installed on the aircraft at different angles, each solar array should be connected to its dedicated MPPT. PV arrays at different angles capture different cross-sections of solar radiation and experience different levels of insolation, such that the PV arrays can operate on the optimal point on their respective I-V curve [2]. The primary performance indicator of MPPT is power conversion efficiency, which reflects the ratio between the input power from PV cells and the output power of the MPPT. There exist many algorithms for MPPT of which the operation principles and performance vary. MPPT methods can be classified into indirect and direct methods or intelligent and conventional methods. Indirect methods obtain PV characteristics based on mathematical relationships, and no meteorological conditions are required; direct methods take meteo- rological conditions into account. Intelligent methods are more complex, expensive, and efficient than conventional methods [8]. Solar-powered UAV projects that are centred in such areas and have sufficient resources and expertise in electronic engineering involve custom-made MPPTs for optimised performance [1,2].

2.5. Power Management for Solar-Powered UAV Research is conducted for the design of the power system of the solar-powered plane of this project. A battery, PV cells, and an MPPT, as the foregoing describes, constitute the major components of the power system. The motor is the main consumer of power inflight, while the battery and PV cells are the power sources. It is investigated how components in the power system should be arranged and connected to supply a combination of electric power from that stored in the battery and that generated by the PV cells in-flight. The research on power management for hybrid power UAV by Strele [9] is reviewed. The two categories of power management for hybrid power UAV are Passive Power Man- agement (PPM) and Active Power Management (APM). In PPM systems, the alternative power source and the battery are directly connected in parallel. The voltage of both power sources should be roughly the same. The combined power is transmitted to the ESC, thus enabling the motor to propel the aircraft. The advantage of the PPM system is that it is weight saving, as no extra electronic components are needed to control the power flow. The demerits include compromising power system safety and efficiency, since the power flow in the system is not controlled. For APM systems, supervisory electronic components are employed to control power flow. The complexity of APM systems greatly varies. In an example of a simple APM system design, a relay is used to control which source (fuel cell or battery) delivers power to the motor. The switching between power sources is controlled manually by the remote operator engaging the battery and disengaging the fuel cell during high power consumption flight phases (takeoff and initial climb), disengaging the battery and engaging the fuel cell during low power consumption flight phases (cruising). However, it is doubted that if this power system architecture can be applied to a solar-powered UAV. It requires further testing on whether the MPPT can directly supply power to the ESC without a battery connected in the circuit [9]. In more complex APM system designs, a microprocessor controls multiple DC-to-DC converters that are connected to the power sources. The more electronics employed in an APM system, the more precise the power flow can be controlled, meanwhile and the more weight, cost, and power consumption are induced to the aircraft [9].

2.6. Unmanned Aerial System A UAV consists of the airframe, sensors, payload, ground control station, and wire- less communication system [10]. The implementation of autopilot with support from the ground control station allows the aircraft to be flown autonomously or be controlled Drones 2021, 5, 44 5 of 19

Drones 2021, 5, 44 5 of 19 2.6. Unmanned Aerial System A UAV consists of the airframe, sensors, payload, ground control station, and wireless communication system [10]. The implementation of autopilot with support from the ground controlremotely. station A UAV allows system the aircraft is composed to be flown of several autonomously sub-systems or be in cont a singlerolled aircraft. remotely. Experimen- A UAV systemtal results is composed show correlation of several betweensub-systems charging in a single voltage aircraft. and Experimental capacity, yet someresults sub-systems show cor- relationare optional between depending charging voltage on the typeand capacity, of the UAV yet some [11]. sub-systems However, these are optional sub-systems depending can be oncategorised the type of intothe UAV three [11]. main However, features: these unmanned sub-systems aircraft, can be communication, categorised into and three Ground main features:Control unmanned Station [12 aircraft,]. communication, and Ground Control Station [12]. ToTo receive receive the the signal signal and and to toapply apply the the autono autonomousmous features features for forthe the flight, flight, an autopilot an autopilot is necessaryis necessary to implement to implement on a UAV. on a UAV. During During the flight the flightcomputer computer selection, selection, a UAV a must UAV fulfil must fulfil the following criteria in relation to the targeted mission: small dimensions and the following criteria in relation to the targeted mission: small dimensions and weight, low weight, low price, waypoint following capabilities, auto takeoff and landing capabilities, price, waypoint following capabilities, auto takeoff and landing capabilities, and configurable. and configurable. Open-source autopilots are preferred over sophisticated commercial Open-source autopilots are preferred over sophisticated commercial autopilots, as the differ- autopilots, as the difference in price is enormous. Figueredo [11] has shown the work of ence in price is enormous. Figueredo [11] has shown the work of comparing the autopilot so- comparing the autopilot solutions in the market by categorising the evaluation points of lutions in the market by categorising the evaluation points of physical specifications, sensors, physical specifications, sensors, and autopilot function. and autopilot function. Sensors provide the functionality to maintain the flight without human input. The Sensors provide the functionality to maintain the flight without human input. The selec- selection of sensors may depend on the mission required by the UAV. The greater the tion of sensors may depend on the mission required by the UAV. The greater the number of number of sensors, the greater the possibility that the UAV could gain stable control. Yet, sensors, the greater the possibility that the UAV could gain stable control. Yet, it could increase it could increase the burden of setting up the components on the airframe and covering the burden of setting up the components on the airframe and covering the excessive loads. the excessive loads. The suggested sensors from André et al. [13] are a GPS receiver, Thegyro, suggested acceleration, sensors magnetic, from André pressure, et al. [13] ultrasonic are a GPS sensor receiver, or sonar, gyro, infrared acceleration, sensor magnetic, and RGB pressure,camera ultrasonic or another sensor image or sensor. sonar, infrared sensor and RGB camera or another image sensor. BasedBased on on the the level level of of autonomy autonomy for for flight flight co control,ntrol, the the forms forms can can be be divided divided into into three three levels.levels. The The level level of autonomy of autonomy may mayapply apply in rela intion relation to the toUAV the mission; UAV mission; however, however, in recent in days,recent the days, UAS thesystem UAS generally system generallyutilises part utilises or whole part autonomous or whole autonomous to gain control to gain for steady control flight.for steady Semi-autonomous flight. Semi-autonomous is a guidance system is a guidance having both system ground having control both and ground autonomous control featureand autonomous for the flight featurecontrol system for the. flightDuring control the critical system. portions During of the the pre-flight critical portionsflight, takeoff, of the andpre-flight landing, flight, the control takeoff, input and from landing, the ground the control are required. input from the ground are required.

3.3. Power Power System System Design Design 3.1.3.1. Selection Selection of ofPV PV Cell Cell TheThe category category of of monocrystalline monocrystalline silicon silicon cell cell isis deemed deemed suitable suitable for for this this project. project. The The PVPV cell cell model model SunPower SunPower C60 C60 (Figure (Figure 3),3), which which is is a a monocrystalline monocrystalline silicon silicon cell, cell, is is chosen chosen forfor this this project project considering considering cost, cost, accessibility, accessibility, energy energy efficiency, efficiency, weight, size,size, andand flexibility.flexibil- ity.The The following following sections sections will will explain explain the the reason reason for suchfor such selection selection in detail. in detail. The specificationsThe specifi- cationsof the of SunPower the SunPower C60 solar C60 cellsola isr showncell is shown in Table in1 Table. 1.

(a) (b)

FigureFigure 3. 3.PVPV cell cell model model used used in in this this work work and and its its installation. installation. (a ()a )SunPower SunPower C60 C60 PV PV cell; cell; (b (b) )Instal- Installa- lationtion of PV cell withwith hothot shrinkshrink plasticplastic wrap.wrap. Drones 2021, 5, 44 6 of 19

Table 1. Specifications of SunPower C60 solar cell (extracted from manufacturer datasheet).

Parameter Value Mass 0.008 kg Size 0.125 × 0.125 m Thickness 1.65 × 10−4 m

From the Solar Cell Efficiency Tables (Version 54) created by Green et al. [6], the sili- Drones 2021, 5con, 44 crystalline cell manufactured by Kaneka has an efficiency of 26.7 ± 0.5%, which is the 6 of 19 second highest. The monocrystalline silicon cell is a mature technology that has a theoretical efficiency of 29% [14]. It is also easily accessible from the consumer market at an affordable cost to this project.Table Silicon 1. Specifications cells are selected of SunPower widely C60 as solar the cell PV (extracted cells for fromsolar-powered manufacturer aircraft datasheet). [15], such that the monocrystalline silicon PV cell is chosen for this project. The SunPower C60 solar cell has an efficiency betweenParameter 21.8% and 22.5%, which is higher than Value the typical efficiency of the existing monocrystaMasslline silicon cell of 15% to 20% [3]. 0.008 kg × Considering the shape and areaSize of the wings, a total of 22 PV cells 0.125were installed0.125 m on Thickness 1.65 × 10−4 m the aircraft, with 11 on each wing (six on the upper side and five on the lower side), as shown in Figure 4. TheFrom total the weight Solar the Cell PV Efficiency cells is 0.176 Tables kg. (Version All 22 PV 54) cells created were by connected Green et al. [6], the in series and formsilicon a single crystalline PV array. cell Acco manufacturedrding to the by Kanekamanufacturer has an efficiency datasheet, of one 26.7 Sun-± 0.5%, which Power C60 solar cellis the outputs second around highest. 0.5 The V monocrystalline under the solar silicon radiation cell is of a 300 mature to 1000 technology W/m2. that has a Considering that theoreticalsome PV cells efficiency were ofinstalled 29% [14 ].at Itthe is alsobackside easily of accessible the wings, from it was the consumer antici- market pated that the PVat array an affordable would have cost a to total this output project. of Silicon less than cells 11 are V. selected widely as the PV cells for A single PV arraysolar-powered is used on aircraft the plane. [15], such Cons thatidering the monocrystalline that there is very silicon little PV difference cell is chosen for this project. The SunPower C60 solar cell has an efficiency between 21.8% and 22.5%, which in angle of the installation position of the PV cells on the wings, the effect of optimising is higher than the typical efficiency of the existing monocrystalline silicon cell of 15% to the amount of sunlight20% [3 ].captured by separating into several PV arrays is very small. Indi- vidual MPPTs are alsoConsidering needed for the each shape PV and array. area of theHaving wings, a asingle total of PV 22 PV array cells saves were installed the on the weight of the MPPT.aircraft, The with PV 11 cells on each are wingstuck (six onto on thethe upper wings side with and double-sided five on the lower tape. side), To as shown in protect the cells andFigure to 4ensure. The total more weight secure the PVinst cellsallation, is 0.176 a protective kg. All 22 PV film cells is wereadded. connected It is a in series transparent plasticand book form wrap a single that PV shrinks array. According when heat to theis applied. manufacturer The plastic datasheet, film one is SunPowerfirst C60 2 fixed on the wingssolar with cell super outputs glue around covering 0.5 V all under PV thecells; solar then, radiation it is blown of 300 towith1000 a W/mhot wind. Considering gun such that it shrinksthat some to follow PV cells the were curvature installed of at the backsidewings. of the wings, it was anticipated that the PV array would have a total output of less than 11 V.

Figure 4. PV array connection in power system. Figure 4. PV array connection in power system. A single PV array is used on the plane. Considering that there is very little difference 3.2. Selection of Maximumin angle ofPower the installation Point Tracker position (MPPT) of the PV cells on the wings, the effect of optimising the Most importantly,amount the of sunlightMPPT model captured that by suit separatings the output into several voltage PV of arrays the PV is very array small. and Individual MPPTs are also needed for each PV array. Having a single PV array saves the weight of the charging voltagethe MPPT.of the Thebattery PV cellsshould are be stuck selected. onto the The wings total with output double-sided voltage of tape. the ToPV protect the array was anticipatedcells and to be to ensurebelow more11 V, secure and installation,the charging a protectivevoltage of film 3S isLiPo added. battery It is a is transparent between 12 and 12.6plastic V. Since book wrapthe total that voltage shrinks whenof the heat PV isarray applied. is not The sufficient plastic film to meet is first the fixed on the minimum chargingwings voltage with superof the gluebattery, covering the MPPT all PV cells; should then, include it is blown the withfunction a hot of wind boost- gun such that ing the voltage. Anotherit shrinks criterion to follow for the MPPT curvature selection of the wings.is power conversion efficiency. The higher the MPPT efficiency, the more power captured by the PV array can be transferred 3.2. Selection of Maximum Power Point Tracker (MPPT) to the power system. Most importantly, the MPPT model that suits the output voltage of the PV array and the charging voltage of the battery should be selected. The total output voltage of the PV array was anticipated to be below 11 V, and the charging voltage of 3S LiPo battery is between 12 and 12.6 V. Since the total voltage of the PV array is not sufficient to meet the minimum charging voltage of the battery, the MPPT should include the function of boosting the voltage. Another criterion for MPPT selection is power conversion efficiency. The Drones 2021, 5, 44 7 of 19 Drones 2021, 5, 44 7 of 19

higherConsidering the MPPT efficiency, the above the more criteria, power capturedthe MPPT by themodel PV array Genasun can be transferred GVB-8-Li-14.2V to Boost theMPPT power was system. selected for this project, as shown in Figure 5. With a ‘Minimum Panel Voltage for Charging’Considering of the 5 V above and a criteria, ‘Maximum the MPPT Recomme modelnded Genasun Panel GVB-8-Li-14.2V Vmp’ of 13 V, Boost this MPPT can MPPTaccept was a supply selected of for 5–13 this V project, from asthe shown PV array. in Figure The5. Withvoltage a ‘Minimum input from Panel the Voltage PV array into the forMPPT Charging’ was estimated of 5 V and ato ‘Maximum be < 11 V, Recommended such that the Panel MPPT Vmp’ was of 13 considered V, this MPPT suitable can for the accept a supply of 5–13 V from the PV array. The voltage input from the PV array into designed PV array of this project. The ‘Minimum Panel Voltage for Charging’ has to be the MPPT was estimated to be <11 V, such that the MPPT was considered suitable for themet designed such that PV the array MPPT of this starts project. to Thecharge ‘Minimum the battery. Panel VoltageFor the for battery Charging’ side, has the to MPPT gives bea constant met such 14.2 that theV to MPPT charge starts the to battery, charge theas long battery. as the For minimum the battery side,charging the MPPT voltage of 5 V is givessupplied a constant at the 14.2 panel V to terminal. charge the The battery, MPPT as longstep ass up the the minimum voltage charging supplied voltage by the PV array ofto 5give V is suppliedan output at theof panel14.2 V, terminal. which Theslightly MPPT exceeds steps up the the voltagecharging supplied voltage by for the the 3S LiPo PVbattery array of to 12–12.6 give an output V. Though of 14.2 that V, which being slightly given, exceeds Genasun the chargingGVB-8-Li-14.2V voltage for Boost the MPPT has 3S LiPo battery of 12–12.6 V. Though that being given, Genasun GVB-8-Li-14.2V Boost MPPTthe closest has the specifications closest specifications with the with requiremen the requirementsts of ofthis this project project among among the the MPPTs MPPTs available availableon the consumer on the consumer market. market. This This MPPT MPPT also also has has a highhigh electrical electrical efficiency efficiency of 95–97% of 95–97% com- comparedpared to toother other MPPTs MPPTs available available on on the the market. market.

FigureFigure 5. 5.Genasun Genasun GVB-8-Li-14.2V GVB-8-Li-14.2V Boost Boost MPPT. MPPT. 3.3. Design of Power System Architecture 3.3. Design of Power System Architecture A PPM system with one PV array, one MPPT, and one battery was considered suitable for theA power PPM system ofwith the UAV.one PV The array, major one advantage MPPT, of and the one PPM battery system was is that considered it is suita- weightble for saving, the power as it requiressystem noof extrathe UAV. electrical The component major advantage to control theof the power PPM flow. system The is that it is effectweight of weightsaving, saving as it requires is prominent no extra on the electrical plane of thiscomponent project, the to weightcontrol of the which power is flow. The relativelyeffect of small.weight The saving additional is prominent electrical component on the plane would of add this significant project, weightthe weight to the of which is plane. Moreover, the PPM system has lower complexity. It does not involve the customisa- tionrelatively of electronic small. components The additional for the electrical purpose of component power distribution, would inadd which significant advanced weight to the knowledgeplane. Moreover, in electrical the andPPM electronic system engineering has lower andcomplexity. more research It does and not development involve the customi- timesation would of electronic be required. components for the purpose of power distribution, in which advanced knowledgeThe disadvantages in electrical of a PPMand systemelectronic included engi theneering compromise and more on power research system and safety development sincetime thewould power be flow required. in the system is not controlled. However, as the solar-powered plane in thisThe project disadvantages is only for research of a purposesPPM system but not included designed forthe regular compromise and frequent on use,power system power system safety was not considered a prioritised concern. safety since the power flow in the system is not controlled. However, as the solar-powered 3.4.plane Power in Systemthis project Design is Verification only for research purposes but not designed for regular and fre- quentPreliminary use, power tests system were conducted safety was to verifynot considered the viability a ofprioritised the whole powerconcern. system design. Firstly, it was verified that the power system functioned. The power system is set up3.4. according Power System to the Design connection Verification shown in Figure6, and the whole setup was brought to open space. The LED light on the MPPT indicated that it was in ‘Low Current Charging’ mode.Preliminary Secondly, it wastests verified were conducted that the solar to power verify system the viability could recharge of the thewhole battery. power system Thedesign. power Firstly, system it setup was wasverified left in that an open the spacepower with system the throttle functioned. of the motor The switchedpower system is set off.up Theaccording battery voltageto the connection increased after shown 40 minutes. in Figu Thisre shows6, and that the the whole energy setup stored was in brought to theopen battery space. increased The LED after light being on charged the MPPT by the indica MPPT,ted which that meansit was the in power‘Low Current system Charging’ workedmode. Secondly, properly. Thirdly, it was itverified was tested that that the the so powerlar power flow system in the parallel could connectionrecharge the battery. The power system setup was left in an open space with the throttle of the motor switched off. The battery voltage increased after 40 minutes. This shows that the energy stored in the battery increased after being charged by the MPPT, which means the power system worked properly. Thirdly, it was tested that the power flow in the parallel connection between the MPPT, the battery, and the ESC was as expected. The battery charging current is 0 with the motor switched on and is greater than 0 with the motor switched off. When Drones 2021, 5, 44 8 of 19

Drones 2021, 5, 44 8 of 19

between the MPPT, the battery, and the ESC was as expected. The battery charging current is 0 with the motor switched on and is greater than 0 with the motor switched off. When the motor is on, all current from the MPPT is drawn by the ESC, and there is no current the motor is on, all current from the MPPT is drawn by the ESC, and there is no current for chargingfor charging the battery. the battery. Therefore, Therefore, the power the system power design system was design concluded was toconcluded be viable, andto be viable, noand significant no significant modification modification was needed. was Theneeded. building The process building of the process solar-powered of the solar-powered UAV couldUAV becould commenced. be commenced.

FigureFigure 6. 6.Power Power system system topological topological overview. overview.

ToTo calculate calculate the the power power contribution contribution distribution distributi fromon the from battery the or battery PV cells, or a voltage–PV cells, a volt- currentage–current sensor sensor FrSky FAS40SFrSky FAS40S was connected was connected to the main to the lead main of the lead battery of the to measurebattery to meas- the output from the onboard battery; and a power module was connected to the ESC to ure the output from the onboard battery; and a power module was connected to the ESC measure the power consumption of the whole aircraft. The data were delivered directly by theto measure RC data link the andpower could consumption be saved as log of the data whole by the aircraft. transmitter. The data were delivered directly by the RC data link and could be saved as log data by the transmitter. 4. Flight Control System Configuration 4. FlightAs the Control project System is set for Configuration solar-powered UAV, the flight performance is aimed for low-altitudeAs the long-enduranceproject is set for (LALE) solar-powered flight, which UAV, is not the limited flight performance by takeoff and is landing aimed for low- conditions.altitude long-endurance The objective for (LALE) auto flight flight, is to which provide is a not fixed limited circular by route takeoff so that and the landing flight condi- performance of the solar power system can be analysed under fair comparison. The flight tions. The objective for auto flight is to provide a fixed circular route so that the flight control objective is to perform as much auto flight as possible during the flight test. Taranis X9Dperformance was the RC of modelthe sola selectedr power for system this project, can be as analysed it overrides un theder minimumfair comparison. requirements. The flight con- trol objective is to perform as much auto flight as possible during the flight test. Taranis X9D 4.1.was Autopilot the RC model System selected for this project, as it overrides the minimum requirements. Based on the autopilot requirements, several autopilot models from the market were compared.4.1. Autopilot The System open-source autopilot models were selected as it has a comparatively cheap price and accessibility to modify the feature for designing the UAV flight control Based on the autopilot requirements, several autopilot models from the market were system. The specifications were compared with the method set by Figuerido [11]. As acompared. result, Pixhawk The open-source legacy was selected, autopilot as itmodels has features were suitableselected foras theit has project a comparatively with reasonablecheap price price. and accessibility to modify the feature for designing the UAV flight control system.The RCThe plane specifications bought from were the market compared comes wi withth fourthe servosmethod and set a motorby Figuerido with DF13 [11]. As a cablesresult, to Pixhawk directly connect legacy towas the selected, RC receiver. as it Several has features components suitable were for added the project to the flight with reason- controlable price. system to enable the autopilot function. To ensure the safety of the plane during flight,The more RC sensors plane werebought added from for the data market monitoring. comes Forwith the four operation, servos and each a telemetry motor with DF13 wascables connected to directly to a 6-pinconnect DF13 to connectorthe RC receiver. for autopilot Several within components the UAV andwere a micro-USBadded to the flight cablecontrol to connect system with to enable the PC the from autopilot GCS. function. To ensure the safety of the plane during Pixhawk has sensors and an interface that supports the global navigation satellite flight, more sensors were added for data monitoring. For the operation, each telemetry system (GNSS) and receive those signals to communicate via the u-box. The real-time kinematicwas connected (RTK) GPS to a receiver 6-pin DF13 is available. connector However, for autopilot it still requires within an the external UAV GPS and sensor a micro-USB tocable collect to andconnect transmit with the the GPS PC data from to GCS. the flight computer. Therefore, M8N GPS (Galileo- readyPixhawk E1B/C (NEO–M8N)) has sensors module and an was interface selected. that To tunesupports the flight the controlglobal surfacenavigation and satellite system (GNSS) and receive those signals to communicate via the u-box. The real-time kin- ematic (RTK) GPS receiver is available. However, it still requires an external GPS sensor to collect and transmit the GPS data to the flight computer. Therefore, M8N GPS (Galileo- ready E1B/C (NEO–M8N)) module was selected. To tune the flight control surface and to ensure the safety of the actuators, a switch was added as a safety element. Whenever the UAV is powered on, the switch must be pressed to move the servos and motor. The Drones 2021, 5, 44 9 of 19 Drones 2021, 5, 44 9 of 19

to ensure the safety of the actuators, a switch was added as a safety element. Whenever finished Autopilotthe UAVSystem is powered Diagram on, is theshown switch in Figure must be 7. pressed All the tocomponents move the servos are powered and motor. The through the batteryfinished elimination Autopilot Systemcircuit Diagramof the power is shown module. in Figure This7. Allsetup the allows components the plane are powered to be fully controlledthrough by the the battery pilot eliminationas well as circuitthe GCS. of the power module. This setup allows the plane to be fully controlled by the pilot as well as the GCS.

Figure 7. Diagram of theFigure autopilot 7. Diagram system of the connections. autopilot system connections.

The power moduleThe power from modulethe autopilot from the system autopilot Pixhawk system Pixhawk is the primary is the primary sensor sensor to ex- to extract the voltage and current data; however, there could be few methods to add the sensor that tract the voltage and current data; however, there could be few methods to add the sensor will generate the same function. The power module transmits the data to the GCS by that will generatetelemetry. the same As function. there are two The different power module sources (RCtransmits and GCS) the to data extract to the the GCS data, by it requires telemetry. As thereextra are effort two to different synchronise sources the data (R inC orderand GCS) of time to intoextract one the single data, datasheet. it requires extra effort to synchronise the data in order of time into one single datasheet. 4.2. Flight Modes 4.2. Flight Modes Using semi-autonomous control for this project, pilot input from the ground would Using semi-autonomousbe given for takeoff control and landing,for this whileproject, auto-pilot pilot input would from take the control ground for undergoingwould the mission. Setting the mission to fly in a fixed route and altitude would help maintain a be given for takeoff and landing, while auto-pilot would take control for undergoing the controlled environment to compare the power consumption of the two UAVs. A total of five mission. Settingmodes the mission were selected to fly to in be a appliedfixed route for this and project. altitude Manual would mode, help FBWA maintain mode, a stabilise controlled environmentmode, auto to mode,compare and the RTL power mode co arensumption settled on both of the the two RC controllerUAVs. A andtotal Mission of five planner. modes were selectedThe to be setting applied was for set this by project. initialising Manual the logical mode, switchesFBWA mode, and mixer, stabilise especially mode, for the auto mode, and flightRTL mode modes. are The settled frequency on both boundary the RC controller for channel and 5, whichMission was planner. assigned for the flight The settingmode, was wasset by carefully initialising distributed the logical for five switches different modesand mixer, to make especially sure that for the the signals are flight modes. Thenot overlappingfrequency boundary each other. for channel 5, which was assigned for the flight mode, was carefullyThe distributed display screen for five of differen the RC transmittert modes towas make set sure up to that read the the signals data more are easily during the flight test. Data of battery voltage, current, timer, and another timer for counting not overlapping each other. the duration of the flight was selected. Nonetheless, ‘special functions’ were set up for The displaythe screen voice indicatorof the RC as transmitter well as the timerwas set and up log. to Althoughread the data not necessary, more easily a voice dur- indicator ing the flight test.would Data help of thebattery pilot tovoltage, identify current, the selection timer, of and flight another mode without timer for checking counting through the the duration of displaythe flight screen. was selected. Nonetheless, ‘special functions’ were set up for the voice indicator as well as the timer and log. Although not necessary, a voice indicator would help the4.3. pilot Ground to identify Control the Station selection Setup of flight mode without checking through the display screen. The radio connection between the “Mission Planner” and UAV was simply done by connecting the micro-USB cable to the computer. All the necessary drivers needed are 4.3. Ground Controlautomatically Station Setup installed during the process when carefully following the steps recommended The radio connection between the “Mission Planner” and UAV was simply done by connecting the micro-USB cable to the computer. All the necessary drivers needed are automatically installed during the process when carefully following the steps recom- mended from the Ardupilot website. With appropriate telemetry hardware, monitoring the data of vehicle status during the operation, recording, and analysing of telemetry logs and operating the vehicle in first-person view is available. Drones 2021, 5, 44 10 of 19 Drones 2021, 5, 44 10 of 19

Miscellaneous settings shall be taken care to support the flight and avoid any mis- from the Ardupilot website. With appropriate telemetry hardware, monitoring the data ofleading vehicle errors status that during could the operation,result in a recording, failsafe mode. and analysing The flight oftelemetry mode could logs be and calibrated by operatingsetting the the mode vehicle allocation in first-person the viewsame is as available. the RC in Mission Planner. On the other hand, the calibrationMiscellaneous shall be settings checked shall under be taken the care “Mandatory to support theHardware” flight and for avoid acceleration, any mis- compass, leadingradio frequency, errors that could and resultservo in output. a failsafe Moreover, mode. The checking flight mode the could data be flow calibrated from bythe sensors of settingtelemetry, the mode GPS, allocation and flight the samedata asin thethe RC heads-up in Mission area Planner. from On the the Mission other hand, Planner the main page calibration shall be checked under the “Mandatory Hardware” for acceleration, compass, were also important. These calibrations shall be checked every time before any flight test radio frequency, and servo output. Moreover, checking the data flow from the sensors of telemetry,as the signal GPS, efficiency and flight data vary in by the weather, heads-up location, area from theetc. Mission Planner main page were alsoLastly, important. collecting These the calibrations most accurate shall be data checked of power every timeconsumption, before any flightvoltage, test and current asmeasured the signal by efficiency power vary module by weather, requires location, manual etc. calibration to minimise the error. Compar- ing Lastly,data from collecting different the most sensors accurate (one data from of power the power consumption, module voltage, and andone currentfrom the voltage– measuredcurrent sensor) by power and module analysing requires the manual difference calibration in data to minimise measurement the error. could Comparing be utilised to set data from different sensors (one from the power module and one from the voltage–current sensor)up a manual and analysing calibration the difference unit. in data measurement could be utilised to set up a manualIt calibrationwas determined unit. that the RC plane has optimal flight performance at a ground speedIt was of 15 determined m/s, which that is the measured RC plane by has the optimal onboard flight GPS. performance It was also at a recommended ground that speedsubsequent of 15 m/s, flight which tests is measuredshould be by conduc the onboardted at GPS. an altitude It was also of recommended around 100 m. that subsequentFigure flight 8 shows tests should the maximum be conducted boundary at an altitude of flight of around marked 100 m. by overlapping the path recordedFigure 8in shows Mission the maximumPlanner when boundary the of‘Sun’ flight UAV marked was by flown. overlapping The boundary the path marked in recorded in Mission Planner when the ‘Sun’ UAV was flown. The boundary marked in the yellowthe yellow line shows line toshows be the to safety be boundarythe safety where boundary the RC signalswhere andthe GCS RC connectionssignals and are GCS connec- steadytions are and steady strong. and In addition, strong. the In minimumaddition, altitudesthe minimum for avoiding altitudes any obstacles for avoiding during any obstacles theduring flight the for UAVflight to for be flownUAV wereto be marked flown aswere well. marked as well.

FigureFigure 8. 8.Available Available airspace airspace marked marked in the map.in the map.

The primary role of autopilot is to make the UAV loiter over a certain time or have a The primary role of autopilot is to make the UAV loiter over a certain time or have a certain number of turns in the same circular route. Based on the marking of airspace in the firstcertain two flightnumber tests, of the turns fundamental in the routesame was circular set up toroute. have controlBased onceon the the UAVmarking reaches of airspace in athe certain first altitude. two flight The tests, waypoints the fundamental were marked on ro theute map was with set up desired to have altitudes. control Based once the UAV onreaches this factor, a certain the distance altitude. and The altitude waypoints were set were according marked to the on calculated the map ascending with desired or altitudes. descendingBased on this angle factor, between the the distance waypoints. and Thealtitu waypointde were radius set according was set to to be the 30 m.calculated The ascend- UAVing or starts descending to make a turnangle once between it reaches the the waypoints. waypoint boundary. The waypoint The loiter radius radius was setset to be 30 m. as 30 m on waypoint 4. The UAV starts to make a turn once it reaches the waypoint boundary. The loiter radius The first waypoint was set on the right top, allowing the pilot to have 325.5 m to reachwas set the as altitude 30 m ofon 100waypoint m after 4. takeoff, as shown in Figure9. After a big turn from waypointThe (Wp)first waypoint 2 to Wp 3, thewas UAV set on was the assigned right top, to have allowing 10 loiter the turns pilot on to Wp4. have Wp4 325.5 m to reach the altitude of 100 m after takeoff, as shown in Figure 9. After a big turn from waypoint (Wp) 2 to Wp 3, the UAV was assigned to have 10 loiter turns on Wp4. Wp4 was inten- tionally set away from the runway to avoid any crash with other UAV flights. Once the loitering is finished, the flight would perform a round turn over the runway and some forest area to gradually descend and to be ready for landing. When the UAV is flying parallel with the runway ay a low altitude of around 10 m, the pilot would take back control from auto mode to either FBWA, stabilise, or manual mode. The flight data related Drones 2021, 5, 44 11 of 19

was intentionally set away from the runway to avoid any crash with other UAV flights. Drones 2021, 5, 44 Once the loitering is finished, the flight would perform a round turn over the runway 11 of 19 and some forest area to gradually descend and to be ready for landing. When the UAV is flying parallel with the runway ay a low altitude of around 10 m, the pilot would take back control from auto mode to either FBWA, stabilise, or manual mode. The flight data relatedto power to power consumption consumption data data will will be be logged logged only duringduring loitering loitering to ensureto ensure that thethat the exper- experimentalimental environment environment was was as as similar similar asas possible. possible.

FigureFigure 9. 9.Waypoints Waypoints marked marked on map on formap auto-mode. for auto-mode.

5. Experiment Results and Discussion 5. Experiment Results and Discussion 5.1. Power System Performance Parameters 5.1. Power System Performance Parameters The MPPT, battery, and ESC are connected in parallel. The symbols used to represent the powerThe systemMPPT, performance battery, and are ESC listed are in Tableconnected2. The ESCin parallel. is powered The by symbols both the MPPT used to represent andthe battery.power VsystemOC and performanceISC are PV array are performance listed in Table indicators 2. The and ESC are is measured powered when by theboth the MPPT PV array is not connected to the rest of the power system. Vs, Is, and Ps are the PV array and battery. and are PV array performance indicators and are measured when outputsthe PV when array the is PV not array connected is connected to the to the rest MPPT. of the All aircraftpower systems system. draw , power , and from are the PV the ESC: the propulsion system (i.e., motor) is connected to the ESC, and the flight control systemarray isoutputs connected when to the the battery PV array elimination is connected circuit of the to ESC.the SuchMPPT. that, All the aircraft ESC inputs systems draw indicatepower thefrom power the consumptionESC: the propulsion of the whole system UAV. The(i.e., MPPT motor) outputs is connected indicate the to power the ESC, and the generationflight control of the system solar power is connected system (i.e., to PV the array batte andry MPPT) elimination as it is the circuit connection of the point ESC. Such that, betweenthe ESC the inputs solar powerindicate system the andpower the UAV’sconsum powerption system. of the whole UAV. The MPPT outputs indicateTo determine the power the extentgeneration to which of solarthe solar power power assists system in the power (i.e.,supply PV array of the and UAV, MPPT) as it is the output of the MPPT should be examined. If the solar power system works and does the connection point between the solar power system and the UAV’s power system. supply power to the UAV, the MPPT outputs should be greater than 0. Alternatively, the effectiveness of the solar power system can also be indicated by the state of the battery. Table 2. The symbol representation used in power system analysis. Vb corresponds to a percentage of battery stored capacity. Under the same test conditions, the decrease in Vb when the solar power system is present (on ‘Sun’) should be smaller than that whenSymbol it is absent (on ‘Eclipse’). Less power shouldRepresentation be drawn from the battery when the MPPT is connected, as the MPPTVoltage supplies partof PV of thearray power (or requiredvoltage by input the ESC. of MPPT) The MPPT shall not be connected directlyCurrent to output any kind of ofPV sensors, array (or as theyvoltage would of MPPT) interfere, and the MPPT would stop working. Thus, Vs, Is, Ip, and Vp cannot be measured Power output of PV array (or power input of MPPT) directly with sensors. Alternatively, Ip can be calculated when Ib and Im are known: Voltage output of MPPT
Ip = Im − Ib, when Im > CurrentIp . output of MPPT (1) Power output of MPPT

Voltage of battery Current output of battery Voltage of ESC Current input of ESC Power input of ESC PV array open-circuit voltage PV array short-circuit current

To determine the extent to which solar power assists in the power supply of the UAV, the output of the MPPT should be examined. If the solar power system works and does supply power to the UAV, the MPPT outputs should be greater than 0. Alternatively, the Drones 2021, 5, 44 12 of 19 Drones 2021, 5, 44 12 of 19

Table 2. The symbol representation used in power system analysis. effectiveness of the solar power system can also be indicated by the state of the battery. Symbol Representation corresponds to a percentage of battery stored capacity. Under the same test conditions, Vs Voltage of PV array (or voltage input of MPPT) the decrease in when the solar power system is present (on ‘Sun’) should be smaller Is Current output of PV array (or voltage of MPPT) than that when itP sis absent (on ‘Eclipse’).Power Less output power of PV arrayshould (or be power drawn input from of MPPT) the battery when the MPPT is connected,Vp as the MPPT supplies partVoltage of the output power of MPPT required by the ESC. I Current output of MPPT The MPPT shallp not be connected directly to any kind of sensors, as they would in- Pp Power output of MPPT terfere, and the VMPPTb would stop working. Thus,Voltage , of, battery , and cannot be measured I Current output of battery directly with sensors.b Alternatively, can be calculated when and are known: Vm Voltage of ESC Im Current input of ESC = −, (when ). (1) Pm Power input of ESC VOC PV array open-circuit voltage I PV array short-circuit current 5.2. Power SystemSC Tests

5.2.Tests Power on System the Testspower system were conducted to investigate the performance of the solar powerTests system on the (i.e., power PV array system and were MPPT) conducted under to investigatedifferent solar the performance radiation levels of the and to in- vestigatesolar power the system relationship (i.e., PV arraybetween and MPPT)the input under and different output solar power radiation of the levels MPPT. and to investigateThe performance the relationship of the between PV array the input that and consists output of power 22 PV of thecells MPPT. installed on both wings The performance of the PV array that consists of 22 PV cells installed on both wings was tested. and of the PV array were measured under different solar radiation atwas different tested. VtimesOC and ofI day,SC of thewith PV no array load were (i.e., measured the MPPT) under connected. different solar To determine radiation at the MPPT different times of day, with no load (i.e., the MPPT) connected. To determine the MPPT performance, it was connected to the whole power system, and its electrical outputs ( performance, it was connected to the whole power system, and its electrical outputs (Vp andand Ip)) were measured measured with with results results shown shown in Figure in Figure 10. 10.

FigureFigure 10. 10. Results of of power power system system tests, tests, showing showing the MPPT the outputMPPT power output and power the product and the of short- product of short-circuitcircuit current current and open-circuit and open-circuit voltage voltage of the PV of array the PV under array different under solar different radiation solar intensities. radiation intensities. Under the solar radiation level of 500 to 900 W/m2, the MPPT has satisfactory perfor- 2 mance,Under having the ansolar output radiation power betweenlevel of 87.2%500 to and 900 92.8% W/m of, the VOC MPPT× ISC of has the satisfactory PV array. perfor- mance,The PV having array and an the output MPPT power could supply between 3.74 87.2% to 4.83 and W under 92.8% the of stationary VOC × ISC condition of the PV of array. The PVthe array UAV. However,and the MPPT the MPPT could performance supply 3.74 is unsatisfactory to 4.83 W under the the solar stationary radiation condition inten- of the 2 UAV.sity of However, 100 to 300 W/mthe MPPT, having performance output power is betweenunsatisfactory 0% and 26.5%under of theVOC solar× ISC radiation. The inten- MPPT is not activated (at ‘standby’ mode) when the solar radiation level is at 100 W/m2 sity of 100 to 300 W/m2, having output power between 0% and 26.5% of VOC × ISC. The and started charging the battery (at ‘low current charging’ mode) at 300 W/m2 of solar 2 MPPTradiation is not intensity. activated (at ‘standby’ mode) when the solar radiation level is at 100 W/m and started charging the battery (at ‘low current charging’ mode) at 300 W/m2 of solar radiation intensity.

5.3. Flight Experiments 5.3.1. Flight Test Overview The primary objective of this project is to increase the flight endurance of the RC plane by modifying it with a solar power system. To reduce drag and save weight, we fixed all the avionics system and sensors into the fuselage of the 759-2 Phoenix 2000 RC plane. The finished interior diagram is shown in Figure 11. To validate if the modified Drones 2021, 5, 44 13 of 19

5.3. Flight Experiments 5.3.1. Flight Test Overview The primary objective of this project is to increase the flight endurance of the RC plane Drones 2021, 5, 44 13 of 19 by modifying it with a solar power system. To reduce drag and save weight, we fixed all the avionics system and sensors into the fuselage of the 759-2 Phoenix 2000 RC plane. The finished interior diagram is shown in Figure 11. To validate if the modified solar-powered solar-poweredUAV ‘Sun’ meets UAV this ‘Sun’ primary meets objective, this primary a fair objective, experiment a fair should experiment be done should to compare be done its performanceto compare its with performance an unmodified with an same unmodifi type RCed planesame namedtype RC ‘Eclipse’. plane named ‘Eclipse’.

Figure 11. Diagram of fuselage interior for solar-powered UAV ‘Sun’.‘Sun’.

Flight experiments were conducted to comparecompare the power system performance of the two aircrafts. Two approaches werewere used to validate if the modifiedmodified aircraft cancan meetmeet thethe project objective: • Comparing the change in the battery stored capacity before and after a periodperiod inin which ‘Sun’ and ‘Eclipse’ areare underunder thethe samesame flightflight path;path; • Observing the MPPT output power during the flight of ‘Sun’. • Observing the MPPT output power during the flight of ‘Sun’. Both ‘Sun’ and ‘Eclipse’ followed the same flightflight plan inin thethe flightflight test.test. DuringDuring thethe period when data are logged, ‘Sun’ and ‘Eclipse’ are completelycompletely controlled by the flightflight computer under loiter mode, which controls the plane to perform circuits around a point. computer under loiter mode, which controls the plane to perform circuits around a point. Both ‘Sun’ and ‘Eclipse’ were flown on the same day immediately after the other one. This Both ‘Sun’ and ‘Eclipse’ were flown on the same day immediately after the other one. This is to minimise the difference in weather condition, especially solar radiation. is to minimise the difference in weather condition, especially solar radiation. Two flight experiments were conducted, as shown in Table3. In each experiment, Two flight experiments were conducted, as shown in Table 3. In each experiment, ‘Sun’ and ‘Eclipse’ were flown to compare the change in battery stored capacity (flight #1.1 ‘Sun’ and ‘Eclipse’ were flown to compare the change in battery stored capacity (flight vs. #2.1 and flight #1.2 vs. #2.2). In each flight, the initial battery voltage V was taken #1.1 vs. #2.1 and flight #1.2 vs. #2.2). In each flight, the initial battery voltage b,i was taken at an instant t , and the final battery voltage V was taken at t , as shown, in Table4 . at an instant 1 , and the final battery voltage b, f was taken at 2, as shown in Table 4. Both instants were within the period when the, aircraft was performing circuiting in a Both instants were within the period when the aircraft was performing circuiting in a reg- regular and stable manner. This ensures that the ‘Sun’ and ‘Eclipse’ were performing ular and stable manner. This ensures that the ‘Sun’ and ‘Eclipse’ were performing the the same manoeuvre when voltage data are logged. The duration (∆t = t2 − t1) is the same manoeuvre when voltage data are logged. The duration (∆ = −) is the same for same for logging voltage data from each flight. The change in Vb in each flight can be loggingfairly compared. voltage data from each flight. The change in in each flight can be fairly com- pared. Table 3. Overview of flights conducted and experimental comparisons. Table 3. Overview of flights conducted and experimental comparisons. Flight Experiment Flight Experiment UAVUAV Model Mission Obtained Data Number Number Mission Obtained Data Number Number Model #1.1 #1 Change of stored battery #1.1 #1 Changecapacity of and stored output bat- ‘Sun’ CircleCircle flight flight for for10 #1.2 #2 ‘Sun’ 10 turns with tery capacityfrom MPPT and out- #1.2 #2 turns with 80 m #2.1 #1 80 m height and Changeput from of MPPT stored ‘Eclipse’height 80and m 80 radius m ra- #2.1#2.2 #1 #2 Changebattery of capacitystored bat- ‘Eclipse’ dius #2.2 #2 tery capacity

Drones 2021, 5, 44 14 of 19

Drones 2021, 5, 44 14 of 19 Table 4. Flight plan of the UAVs for flight test.

TableFlight 4. Flight Stage plan of the Takeoff UAVs for flight Climbtest. Circling Descend Landing Flight mode Stabilise Auto Loiter Auto Stabilise Flight Stage Takeoff Climb Circling Descend Landing Time t1 t2 Flight mode Stabilise Auto Loiter Auto Stabilise Logged Data Vb,i Vb,f Time t1 t2 Logged Data V V The independent variable of this experimentb,i is b,fthe solar power system and other fac- torsThe remained independent unchanged variable or of maintained this experiment as similar is the solar as possible. power system ‘Eclipse’ and otheris built by unin- factorsstalling remained all the solar unchanged power or system maintained components as similar asfrom possible. ‘Sun’. ‘Eclipse’ The wings is built with by PV cells in- uninstallingstalled are allreplaced the solar by power unmodified system components wings of from the ‘Sun’.same Themodel wings of with the PVRC cells plane, and the installedMPPT is are removed. replaced byThis unmodified ensures that wings the of thetwo same UAVs model used of thein this RC plane,experiment and the are identical MPPTexcept is for removed. the presence This ensures of the that solar the twopower UAVs system. used in this experiment are identical except for the presence of the solar power system. The flight test was conducted on 12 April 2021 in Hong Kong Model Engineering The flight test was conducted on 12 April 2021 in Hong Kong Model Engineering Club,Club, New New Territories, Territories, Hong Hong Kong, Kong, between between 2 p.m. and 2 PM 3:30 and p.m. 3:30 The solarPM. radiationThe solar was radiation was betweenbetween 750 750 and and 900 900 W/m W/m2, and2, and it was it mostlywas mostly sunny. sunny. One of the One finished of the flight finished paths flight is paths is shownshown in in Figure Figure 12. 12.

FigureFigure 12. 12.Flight Flight path path record record of flight of flight #2.2 of #2.2 ‘Eclipse’. of ‘Eclipse’.

5.3.2. Measurement of Power System Performance in the Flight Test 5.3.2. Measurement of Power System Performance in the Flight Test MPPT output current Ip was measured in real time during flight with onboard sensors. Ip is indirectlyMPPT output obtained current from Ib and wasIm measured(Equation in (1)). real A time voltage–current during flight sensor with was onboard sen- connectedsors. is to indirectly the main lead obtained of the battery, from which and measures (EquationVb and I b(1)).. A power A voltage–current module is sensor V I connectedwas connected to the ESC, to the which main measures lead of them and battery,m. which measures and . A power mod- When the UAVs were performing circuiting, the battery voltage Vb was measured ule is connected to the ESC, which measures and . (initial battery voltage V and final battery voltage V ). At each battery voltage, the When the UAVsb ,werei performing circuiting,b, f the battery voltage V was measured respective battery stored capacity can be determined. The changes in battery voltage (initial battery voltage V and final battery voltage V ). At each battery voltage, the re- (∆Vb = Vb, f − Vb,i) were compared, between the flights of ‘Sun’, and ‘Eclipse’. Therefore, thespective performance battery of stored the solar capacity power system can be could determined. be determined. The changes in battery voltage (∆V = V, −V,) were compared between the flights of ‘Sun’ and ‘Eclipse’. Therefore, the per- 5.3.3.formance Flight of Test the Results solar power system could be determined. In all of the four test flights, the initial (t1) and final (t2) time points when the battery voltages were taken were of the same duration: 4 min 45 s apart. In these periods, the UAVs 5.3.3. Flight Test Results were under stable circuiting condition. The actual duration in which the UAVs entered ‘loiter’In mode all of thethe flight four computer test flights, and the circuited initial for (t 101) turnsand final is longer (t2) than time 4 minpoints 45 s, when such the battery thatvoltages the period were between taken were t1 and of t2 isthe completely same duration: within the 4 periodmin 45 when s apart. the UAVs In these were periods, the circuiting.UAVs were The under circuiting stable durations circuiting vary condition. in each flight The (the actual shortest duration being 6 in min which 5 s and the UAVs en- thetered longest ‘loiter’ being mode 7 min of 27 the s), whichflight was computer probably and caused circuited by difference for 10 in turns wind is direction longer than 4 min and speed. Shown in Table5, it was concluded that less battery capacity was consumed 45 s, such that the period between t1 and t2 is completely within the period when the UAVs were circuiting. The circuiting durations vary in each flight (the shortest being 6 min 5 s and the longest being 7 min 27 s), which was probably caused by difference in wind di- rection and speed. Shown in Table 5, it was concluded that less battery capacity was con- sumed by ‘Sun’ with solar power system. Flight #2.1 and #2.2 of ‘Eclipse’ demonstrated that during stable circuiting without a solar power system, an average of 38% of battery- Drones 2021, 5, 44 15 of 19

Drones 2021, 5, 44 15 of 19 by ‘Sun’ with solar power system. Flight #2.1 and #2.2 of ‘Eclipse’ demonstrated that during stable circuiting without a solar power system, an average of 38% of battery-stored capacity was required. Under largely similar experimental conditions, flight #1.1 and #1.2 storedof ‘Sun’ capacity performed was therequired. same stable Under circuiting, largely but similar part of experimental the power required cond wasitions, supplied flight #1.1 andby #1.2 the of solar ‘Sun’ power performed system. Anthe averagesame stable of 15.5% circuiting, of battery but stored part of capacity the power was required.required was suppliedTherefore, by the solarsolar power power system system. is capable An averag of savinge of 15.5% an average of battery of 22.5% stored of the capacity battery was required.stored capacity Therefore, in 4 minthe solar 45 s of power flight. system is capable of saving an average of 22.5% of the battery stored capacity in 4 min 45 s of flight. Table 5. Results of the comparison of battery stored capacity change in flights.

ParametersTable 5. Results #1.1 ‘Sun’ of the comparison #1.2 of ‘Sun’ battery stored capacity #2.1 ‘Eclipse’ change in flights. #2.2 ‘Eclipse’ V Initial Battery Voltage bParameters,i at t1 11.81 V 11.56 V #1.1 ‘Sun’ #1.2 12.02 ‘Sun’ V #2.1 ‘Eclipse’ 11.50 V #2.2 ‘Eclipse’ Final Battery Voltage Vb, f at t2 11.50 V 11.42 V 11.40 V 11.04 V VoltageInitial Drop ∆BatteryVb Voltage 0.31, at V t1 0.14 V 11.81 V 0.62 11.56 V V 12.02 V 0.46 V 11.50 V Initial BatteryFinal Percentage Battery Voltage , at t2 11.50 V 11.42 V 11.40 V 11.04 V 68% 55% 78% 49% Capacity atVoltage t1 Drop ∆ 0.31 V 0.14 V 0.62 V 0.46 V Final Battery Percentage Initial Battery Percentage Capacity49% at t1 43%68% 41%55% 78% 10% 49% Capacity at t2 Drop inFinal Battery Battery Percentage Percentage Capacity at t2 49% 43% 41% 10% 19% 12% 37% 39% DropCapacity in Battery Percentage Capacity 19% 12% 37% 39% Average Drop in Battery Average Drop in Battery Percentage Capacity15.5% 15.5% 38% 38% Percentage Capacity Battery Percentage Capacity Saved by Solar Power System 22.5% Battery Percentage Capacity 22.5% Saved by Solar Power System Apart from quantitatively determining the solar power system performance, graphical comparisonsApart fromwere quantitatively also done. Figure determining 13 shows the solar the power change system in battery performance, voltages graphical during the flights.comparisons The battery were alsovoltages done. should Figure 13show shows a constant the change decreasing in battery voltagestrend during during flights the of ‘Eclipse’flights. and The during battery flights voltages of should ‘Sun’ show(if the a power constant supplied decreasing by trendthe solar during power flights system of is less‘Eclipse’ than the and power during flightsrequired of ‘Sun’ by the (if the ESC). power By supplied observing by the the solar rate power of decrease system isin less battery voltagethan theduring power the required flights by of the ‘Sun’ ESC). and By observing‘Eclipse’, theit can rate be of decreasecompared in batteryif the voltagesolar power during the flights of ‘Sun’ and ‘Eclipse’, it can be compared if the solar power system on system on ‘Sun’ could help decrease the battery stored capacity use. ‘Sun’ could help decrease the battery stored capacity use.

FigureFigure 13. Moving 13. Moving average average of change of change in inbattery battery voltages voltages recorded recordedin in the the all all of of the the four four test test flights. flights. The The moving moving average average of of the voltage’sthe voltage’s raw raw data data is istaken taken with with a windowwindow of of 20 20 s; hence,s; hence, the movingthe moving average average curves startcurves at thestart 20th at second. the 20th The second. initial The and final time points taken for the calculation of battery capacity change are indicated as t1 and t2, respectively. initial and final time points taken for the calculation of battery capacity change are indicated as t1 and t2, respectively. The current curves shown in Figure 14 illustrate that the ESC input current was higher thanThe the current battery curves output currentshown forin mostFigure of the14 il time.lustrate Part that of the the ESC ESC input input current current was was higher than the battery output current for most of the time. Part of the ESC input current was supplied by the MPPT. Given that the MPPT output current equals subtracting bat- tery output current from ESC input current (Equation (1)), the MPPT output current was greater than 0 most of the time in the period between t1 and t2. Drones 2021, 5, 44 16 of 19

Drones 2021, 5, 44 16 of 19

Drones 2021, 5, 44 supplied by the MPPT. Given that the MPPT output current equals subtracting16 of 19 battery

output current from ESC input current (Equation (1)), the MPPT output current was greater than 0 most of the time in the period between t1 and t2.

Figure 14. ESC input current and battery output current comparison of flight #1.2 ‘Sun’ by moving FigureFigure 14. ESC 14. input ESC currentaverage input andcurrent with battery a and window output battery current of output10 comparisons. current of comparison flight #1.2 ‘Sun’ of by flight moving #1.2 average ‘Sun’ by with moving a window of 10average s. with a window of 10 s. Figure 15 shows the difference between the current consumed by the ESC and output Figure 15 shows the difference between the current consumed by the ESC and output Figure 15from showsfrom the the battery. the difference battery. The The MPPT between MPPT output outputthe current current current consumed is is greater greater thanbythan the 00 most ESCmost and of of the theoutput time. time. It It was was con- from the battery.cluded Theconcluded that MPPT the that outputsolar the power solar current power system is greater system supplied suppliedthan 0electrical most electrical of thepower power time. to to Itthe thewas UAV UAV con- inin mostmost flight cluded that theperiods. solarflight power periods. system supplied electrical power to the UAV in most flight periods.

Figure 15.Figure MPPT 15. outputMPPT output current current for flight for flight test test ‘Sun ‘Sun’’ #1.2 #1.2 obtained obtained by subtractingsubtracting the the battery battery output output current current from the from ESC the ESC input current (a small part of the calculated MPPT output current is negative and converted into 0). Figure 15. MPPTinput output current current (a small for flightpart of test the ‘Sun calculated’ #1.2 obtained MPPT outputby subtracting current isthe negative battery andoutput converted current into from 0). the ESC input current (a small part of the calculated MPPT5.4. output Results Discussioncurrent is negative and converted into 0). 5.4. Results Discussion From the power system tests results, the solar power system works satisfactorily under 5.4. Results DiscussionhighFrom solar the radiation. power system This means tests the results, solar power the so systemlar power can only system extend works battery satisfactorily life on the un- 2 From theder power highUAV system solar flight performanceradiation. tests results, This at solarthe means so radiationlar thepower solar intensities system power above works system 300 satisfactorily W/mcan onlyand extend works un- more battery life efficiently at solar radiation intensities above 500 W/m2. The poor MPPT performance at der high solaron radiation. the UAV This flight means performance the solar at power solar radiationsystem can intensities only extend above battery 300 W/m life 2 and works low solar radiation is likely to be caused by an incompatible MPPT model with the battery. more efficiently at solar radiation intensities above 500 W/m2. 2 The poor MPPT perfor- on the UAV flightDue performance to budget restriction, at solar radiation the MPPT intensities selected in thisabove project—Genasun 300 W/m and GVB-8-Li-14.2V works more efficientlymance atBoost solarat low MPPT—satisfies radiation solar radiation intensities the requirements is likely above to of be500 PV caused W/m array2 input . byThe an voltage, poor incompatible MPPT PV array perfor- inputMPPT current, model with mance at lowthe solar battery.and radiation battery Due charging is to likely budget voltage, to berestriction, caused but it does by the not an MPPT satisfy incompatible theselected battery MPPTin type. this It modelproject—Genasun is recommended with to GVB- the battery. Due8-Li-14.2V to budget Boost restriction, MPPT—satisfies the MPPT the selected requirements in this project—Genasun of PV array input GVB- voltage, PV array 8-Li-14.2V Boostinput MPPT—satisfies current, and battery the requirements charging voltage, of PV buarrayt it doesinput not voltage, satisfy PV the array battery type. It is input current,recommended and battery charging to be used voltage, with abu 12Vt it 4Sdoes lithi notum–iron–phosphate satisfy the battery battery,type. It butis the battery recommendedused to be in usedthe powerwith a system12V 4S islithi anum–iron–phosphate 11.1V 3S lithium–polymer battery, battery.but the batteryThe MPPT internal used in the powersensing system algorithm is an may 11.1V not 3S work lithium–polymer efficiently with battery. an incompatible The MPPT battery internal model. sensing algorithmAlthough may not work the comparison efficiently withof the an change incompatible in battery battery stored model. capacity shows that the so- Althoughlar the power comparison system isof capable the change of saving in battery battery stored stored capacity capacity, shows the that values the ofso- change in bat- lar power systemtery voltageis capable determined of saving batteryfrom the stored two flight capacity, experiments the values exhibit of change some in difference. bat- A pos- tery voltage determinedsible explanation from thewould two beflight the experimentsdifference in exhibit wind speedsome difference.in the two Aflight pos- experiments. sible explanation would be the difference in wind speed in the two flight experiments. Drones 2021, 5, 44 17 of 19

be used with a 12V 4S lithium–iron–phosphate battery, but the battery used in the power system is an 11.1V 3S lithium–polymer battery. The MPPT internal sensing algorithm may not work efficiently with an incompatible battery model. Although the comparison of the change in battery stored capacity shows that the solar power system is capable of saving battery stored capacity, the values of change in battery voltage determined from the two flight experiments exhibit some difference. A possible explanation would be the difference in wind speed in the two flight experiments. During the second flight experiment, the wind speed was higher at between 9 and 12 knots, compared to 3 to 6 knots during the first flight experiment. During circuiting in the second flight experiment, the aircraft required more power when it flew against the wind. As shown in Figure 13, the battery voltages fluctuated during circuiting. The periods when the voltages increase to reach small peaks are those when the aircraft was in the section of the circle that it faced head wind. As a result of higher windspeed, the fluctuation in battery voltage in the second flight experiment is greater than that in the first flight experiment. The battery voltage curves for the second flight experiment fluctuate more than those for the first flight experiment, so that the battery voltages taken during the data logging period could have greater error. In conclusion of the experimental data of changes in battery stored capacity, the use of solar power on this UAV does decrease the use of battery stored capacity during flight. From Figure 13, the battery voltage curves show that the rate of decrease in battery voltage in the test flights of ‘Sun’ is slower than that in the test flights of ‘Eclipse’. Therefore, from the graphical interpretation of battery voltage experiment data, it is concluded that the use of solar power system on the UAV leads to a slower rate of decrease in battery voltage during flights, hence extending the battery life during flights and extending the flight duration. The MPPT electrical output values determined from the flight test are slightly larger than those determined in the power system test. In the flight test, the MPPT output current can be as high as 1.2 to 1.4 A. A possible explanation would be the PV cells on the back side of the wings were exposed to more reflected sunlight when the aircraft was flying than they were tested on ground; hence, higher power was generated by the PV array. The relative angles of the PV cells attached to the wing change continuously during flight. This may result in fluctuating MPPT electrical output. In Table6, the average values of the experiment raw data of ESC input current Im, battery output current Ib, and battery voltage Vb between t1 and t2 of 4 min 45 s in three of the test flights are calculated. Data for flight #1.1 of ‘Sun’ are not included as they exhibit significant anomaly—the exact same current value repeating for abnormally long period. Therefore, the average MPPT output power and average electrical power required for circuiting during the period is obtained. The data obtained from flight #1.2 of ‘Sun’ show that having an average MPPT output power of 6.27 W, the solar power system can supply 15.4% of the electrical power required for circuiting.

Table 6. Average values of electrical parameters between t1 and t2.

Electrical Parameters #1.2 ‘Sun’ #2.2 ‘Eclipse’

Average ESC Input Current Im 3.51 A 4.34 A Average Battery Output Current Ib 2.97 A 4.34 A Average MPPT Voltage Vp (=V ) 11.59 V Nil b
Average ESC Voltage Vm = Vb = Vp 11.59 V 11.26 V Average MPPT Output Current 0.54 A Nil Ip (=Im − Ib) Average MPPT Output Power 6.27 W Nil Pp (= Vp × Ip) Average Power Required for Circuiting 40.68 W 48.87 W (= Vm × Im) Drones 2021, 5, 44 18 of 19

6. Conclusions The integration of solar power onto UAVs can extend the flight duration; hence, it has the potential of enabling a wider scope of application of UAVs. The main objective of this project is, by installing a solar power system on it, to extend the flight duration of 759-2 Phoenix 2000—a leisure purpose RC glider. The flight test ultimately verified that the project objective was met. Under fair experimental conditions with desirable weather conditions (solar radiation level over 700 W/m2), the installation of the solar power system on the aircraft results in a 22.5% of saving in the use of battery stored capacity. By comparisons of the battery voltage graphs during circuiting, the rate of decrease in battery voltage of the solar-powered UAV (‘Sun’) is much slower. When the solar power system is present, the possible flight duration supported by the same battery is increased. Therefore, the final flight test verified that the project objective is met. It is verified that the power system performance in an actual flight is largely consistent with the power system tests by comparing the flight test results and the power system tests. Discrepancies are in a reasonable range. In the final flight test, the output power of the solar power system (i.e., MPPT output power) has an average of 6.27 W under 800 W/m2 of solar radiation, which is higher than that measured in the power system tests but within a reasonable deviation. The following are suggested for future works on the development of low-cost solar- powered UAV. Firstly, an active power management technique can be tested with low-cost off-the-shelf components. Programmable microprocessors, such as Arduino, together with relays can be used in the connection between the solar power source and the battery power source. The power flow can be controlled, such as switching between which power source is used to power the UAV or cutting off all power sources from the load to let the solar power source recharge the battery. Furthermore, the overall efficiency of the systems with active power management and passive management can be compared. Secondly, more investigation on what autopilot settings (such as airspeed, altitude, turning radius) are optimal for extending the flight endurance can be done.

Author Contributions: Conceptualisation, Y.C., C.H. and Y.L.; methodology, Y.C., C.H. and Y.L.; validation, Y.C., C.H., Y.L. and B.L.; writing—original draft preparation, Y.C., C.H. and Y.L.; writing— review and editing, B.L.; visualisation, Y.C.; supervision and project administration, B.L. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the Undergraduate Capstone Project and Start-up Fund (P0034164) from the Department of Aeronautical and Aviation Engineering, The Hong Kong Poly- technic University. Conflicts of Interest: The authors declare no conflict of interest.

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