Article Experimental Heat Loads for Electrothermal Anti-Icing and De-Icing on UAVs

Richard Hann 1,2,* , Adriana Enache 3,4 , Mikkel Cornelius Nielsen 2, Bård Nagy Stovner 2, Jeroen van Beeck 4, Tor Arne Johansen 1 and Kasper Trolle Borup 2

1 Centre for Autonomous Marine Operations and Systems, Department of Engineering Cybernetics, Norwegian University of Science and Technology, 7491 Trondheim, Norway; [email protected] 2 UBIQ Aerospace, 7011 Trondheim, Norway; [email protected] (M.C.N.); [email protected] (B.N.S.); [email protected] (K.T.B.) 3 Aero-Thermo-Mechanics Department, Faculty of Applied Sciences, Universitè Libre de Bruxelles, 1050 Brussels, Belgium; [email protected] 4 Environmental and Applied Fluid Dynamics Department, Von Karman Institute for Fluid Dynamics, 1650 Sint-Genesius-Rode, Belgium; [email protected] * Correspondence: [email protected]

Abstract: Atmospheric in-flight icing on unmanned aerial vehicles (UAVs) is a significant hazard. UAVs that are not equipped with protection systems are usually limited to operations within visual line of sight or to weather conditions without icing risk. As many military and commercial UAV missions require flights beyond visual line of sight and into adverse weather conditions, energy- efficient ice protection systems are required. In this experimental study, two electro-thermal ice   protection systems for fixed-wing UAVs were tested. One system was operated in anti-icing and de- icing mode, and the other system was designed as a parting strip de-icing system. Experiments were Citation: Hann, R.; Enache, A.; Nielsen, M.C.; Stovner, B.N.; van conducted in an icing wind tunnel facility for varying at low Reynolds numbers. A Beeck, J.; Johansen, T.A.; Borup, K.T. parametric study over the ice shedding time was used to identify the most energy-efficient operation Experimental Heat Loads for mode. The results showed that longer intercycle durations led to higher efficiencies and that de-icing Electrothermal Anti-Icing and with a parting strip was superior compared to anti-icing and de-icing without a parting strip. These De-Icing on UAVs. Aerospace 2021, 8, findings are relevant for the development of energy-efficient systems in the future. 83. https://doi.org/10.3390/ aerospace8030083 Keywords: ; UAV; unmanned aerial vehicle; UAS; unmanned aerial system; atmospheric icing; in-flight icing; icing; anti-icing; de-icing; drone; RPAS; adverse weather Academic Editor: Hirotaka Sakaue

Received: 29 September 2020 Accepted: 12 March 2021 1. Introduction Published: 18 March 2021 In manned aviation, the history of in-flight icing research dates back to the 1940s [1].

Publisher’s Note: MDPI stays neutral For unmanned aerial vehicles (UAVs), icing research has a much shorter history and can with regard to jurisdictional claims in be considered an emerging research field. Although the first analysis and reports of at published maps and institutional affil- mospheric icing on UAVs date back to the 1990s [2], the research in this area has only iations. recently gained momentum [3]. The reason UAV icing is becoming a trending topic is partly linked to the rise of commercial applications of UAV technologies [4]. In particular, small and medium-sized fixed-wing UAVs with wingspans of a few meters are in the focus of new business opportunities. Examples for the application of such UAVs are package deliveries, search and rescue, environmental monitoring, and agriculture [4]. In addition, Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. UAV technology is adapted by many defense forces around the world [5]. Many UAV This article is an open access article missions require operations of unmanned in adverse weather conditions [6]. One distributed under the terms and of the main barriers to achieving an all-weather capability of UAVs is to mitigate the risk of conditions of the Creative Commons atmospheric in-flight icing [3]. Attribution (CC BY) license (https:// Atmospheric icing, or in-cloud icing, occurs when an aircraft travels through a cloud creativecommons.org/licenses/by/ containing supercooled liquid droplets that freeze upon impact with the [1]. 4.0/). The resulting ice accretions on the airframe have several hazardous effects: clogging

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resulting ice accretions on the airframe have several hazardous effects: clogging of pitot oftubes, pitot adding tubes, addingweight, weight,reducing reducing propeller thrust, thrust,and degrading and degrading aerodynamic aerodynamic perfor- performancemance [7,8]. [Ice7,8 ].that Ice forms that forms on the on leading-edge the leading-edge of lifting of lifting surfaces surfaces changes changes the theairfoil ge- geometryometry and and leads leads to to a adecrease decrease in in lift, lift, increase increase in in drag, drag, and and a ahigher higher stalling stalling risk risk [9]. [9]. ThereThere areare severalseveral keykey differencesdifferences betweenbetween icingicing onon mannedmanned andand unmannedunmanned aircraft.aircraft. AnAn overview overview of of the the special special technical technical and and operational operational challenges challenges of UAVs of UAVs in icing in icing conditions condi- istions given is ingiven [3]. in Differences [3]. Differences are for are example for example related related to airframe to airframe size, velocity, size, velocity, meteorological meteor- environments,ological environments, mission objectives,mission objectives, propulsion propulsion type, and type, more. and One more. important One important difference dif- betweenference between manned manned and unmanned and unmanned icing is theicing disparity is the disparity in the Reynolds in the Reynolds number regime.number Whileregime. manned While manned aircraft typically aircraft typicall operatey atoperate high Reynoldsat high Reynolds numbers numbers (Re = 10 (7Re–10 = 810),7 most–108), UAVsmost UAVs fly at fly significantly at significantly lower lower Reynolds Reynolds numbers numbers (Re =(Re 10 =5 –10105–106).6). This This difference difference in in thethe ReynoldsReynolds numbernumber regimeregime cancan havehave aa significant significant impact impact on on icing icing processes processes [ 10[10].]. ItIt is is thereforetherefore important important that that dedicated dedicated research research for for icing icing matter matter on on UAVs UAVs is is conducted conducted at at low low ReynoldsReynolds numbers. numbers. IceIce protectionprotection systemssystems (IPSs)(IPSs) mitigatemitigate the adverse effects effects of of ice ice on on aircraft aircraft and and are are a arelatively relatively new new technology technology on on UAVs UAVs [3]. [3 ].Electr Electrothermalothermal IPSs IPSs are areone one type type of system of system that thatcan canactively actively mitigate mitigate the hazards the hazards of icing of icing and andallow allow aircraft aircraft to operate to operate in all-weather in all-weather con- conditions.ditions. The Theadvantage advantage of such of suchsystems systems is that is they that are they lightweight are lightweight and a mature and a maturetechnol- technology.ogy. Other active Other activeIPS systems IPS systems that may that be may suitable be suitable for UAVs for UAVsrely on rely mechanical on mechanical or chem- or chemicalical ( (freezing point point depressants) depressants) principles principles for for ice ice protection. protection. Passive Passive systems areare anan emergingemerging technology technology that that aim aim to to use use special special surfaces surfaces to to reduce reduce or or prevent prevent ice ice accretions. accretions. AnAn IPS IPS cancan generallygenerally bebe operatedoperated inin twotwo differentdifferent modesmodes [[11]:11]: anti-icinganti-icing andand de-icing.de-icing. InIn anti-icing anti-icing mode, mode, the the surface surface of of the the airfoil airfoil is heatedis heated continuously continuously to avoid to avoid any any ice accretionice accre- attion any at time any time [12]. [12]. In de-icing In de-icing mode, mode, the surfacethe surface is heated is heated periodically, periodically, allowing allowing for icefor toice buildto build up inup between in between the the heating heating cycles. cycles. This This intercycle intercycle ice isice removed is removed from from the airframethe airframe by twoby two processes processes [13,14 [13,14].]. First, First, ice at ice the at interface the inte torface the to surface the surface is melted, is melted, resulting resulting in a liquid in a waterliquid layer. Second, layer. Second, ice is shed ice is from shed the from airframe the airframe with the with aid ofthe the aid aerodynamic of the aerodynamic forces. Theforces. ice sheddingThe ice shedding efficiency effici is mainlyency is depending mainly depending on the geometry on the geometry of the ice of shape the ice and shape the airspeedand the airspeed [15]. The [15]. energy The amount energy requiredamount requ for de-icingired for isde-icing typically is typically lower compared lower com- to anti-icing,pared to anti-icing, but the intercycle but the intercycle ice generates ice generates additional additional drag [16]. drag [16]. AA partingparting stripstrip (PS)(PS) isis a a special special heating heating zone zone that that is is continuously continuously heated heated and and can can be be usedused toto reducereduce thethe requiredrequired energy energy for for de-icing de-icing [ 11[11].]. WithoutWithout aa parting parting strip, strip, the the entire entire leading-edgeleading-edge will will be be covered covered by by ice ice (Figure (Figure1 ).1). A A parting parting strip strip located located near near the the stagnation stagnation pointpoint atat thethe leading-edgeleading-edge willwill separateseparate thethe iceice intointo anan upperupper andand aa lowerlower partpart (Figure(Figure2 ).2). ThisThis separationseparation increasesincreases thethe aerodynamicaerodynamic forcesforces onon thethe ice ice (Figure (Figure3 )3) and and consequently consequently increases ice shedding efficiency. The gap has a size of typically 2 to 3 mm. increases ice shedding efficiency. The gap has a size of typically 2 to 3 mm.

FigureFigure 1.1. ChordwiseChordwise cutcut into an ice shape that that formed formed after after 66 min min of of icing, icing, shows shows how how the the entire entire leading-edgeleading-edge is is covered covered by by ice. ice.

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Figure 2. Closeup of the leading-edge after 4 min of ice accretion with active parting strip heating. FigureFigure 2. 2.Closeup Closeup of of the the leading-edge leading-edge after after 4 4 min min of of ice ice accretion accretion with with active active parting parting strip strip heating. heating.

Figure 3. a b FigureFigure 3. Schematic3.Schematic Schematic layout layout layout of of ofthe the the heating-zones heating-zones heating-zones for for for the the the conventional conventional conventional de-icing de-icing de-icing (a ( )( )aand) and and de-icing de-icing de-icing with with with a aparting a parting parting strip strip strip (b ( ).().b ). There are several special design requirements for UAV IPSs [3]. Most importantly, ThereThere are are several several special special design design requirements requirements for for UAV UAV IPSs IPSs [3]. [3]. Most Most importantly, importantly, the systems need to be lightweight and energy-efficient. Electrothermal systems are well thethe systems systems need need to tobe be lightweight lightweight and and energy- energy-efficient.efficient. Electrothermal Electrothermal systems systems are are well well suited for UAVs since they are mature, lightweight, and can easily be retrofitted to existing suitedsuited for for UAVs UAVs since since they they are are mature, mature, lightweigh lightweight, andt, and can can easily easily be be retrofitted retrofitted to toexist- exist- [17–21]. Energy-efficiency is, however, a central challenge for electrothermal inging airframes airframes [17–21]. [17–21]. Energy-efficiency Energy-efficiency is, is,ho however,wever, a central a central challenge challenge for for electrothermal electrothermal systems,systems, as as they they require require relatively relatively high high am amountsounts of of energy, energy, compared compared to to other other IPSs IPSs like like chemicalsystems, oras mechanicalthey require systems relatively [13 ].high The am energyounts used of energy, for ice protectioncompared isto energyother IPSs lost forlike chemicalchemical or ormechanical mechanical systems systems [13]. [13]. The The ener energygy used used for for ice ice protection protection is energyis energy lost lost for for thethe propulsion propulsion system—using system—using an an electrothermal electrothermal IPS IPS consequently consequently reduces reduces the the range range and and endurancethe propulsion of the system—using UAV. Icephobic an coatingselectrothermal can reduce IPS consequently the ice adhesion reduces forces the andrange are and a enduranceendurance of ofthe the UAV. UAV. Icephobic Icephobic coatings coatings ca nca reducen reduce the the ice ice adhesion adhesion forces forces and and are are a a relativelyrelatively new new technology technology that is currentlycurrently underunder research research [22 [22].]. In In the the future, future, these these systems sys- mayrelatively be especially new technology energy-efficient that is incurrently combination under with research an active [22]. IPS In [the23]. future, these sys- temstems may may be be especially especially energy-efficient energy-efficient in incombination combination with with an an active active IPS IPS [23]. [23]. ElectrothermalElectrothermal systems systems for UAVsUAVs must must be be carefully carefully designed designed to minimizeto minimize the requiredthe re- heat toElectrothermal operate the system. systems The for goal UAVs must must be tobe run carefully an IPS designed with the minimumto minimize required the re- quired heat to operate the system. The goal must be to run an IPS with the minimum heatquired loads heat for to each operate specific the icing system. condition. The goal To must achieve be this,to run a good an IPS understanding with the minimum of the required heat loads for each specific icing condition. To achieve this, a good understand- underlyingrequired heat physical loads for processes each specific of anti-icing icing condition. and de-icing To achieve is required. this, a Ingood particular, understand- the ing of the underlying physical processes of anti-icing and de-icing is required. In particu- influenceing of the and underlying interlinkage physical of icing processes and IPS of parametersanti-icing and on de-icing the required is required. heat loads In particu- are of lar, the influence and interlinkage of icing and IPS parameters on the required heat loads significancelar, the influence for the and design interlinkage of energy-efficient of icing and systems. IPS parameters on the required heat loads are of significance for the design of energy-efficient systems. are ofD· significanceICE is an electrothermal for the design IPS of that energy-efficient has been developed systems. at the Centre for Autonomous D·ICE is an electrothermal IPS that has been developed at the Centre for Autonomous MarineD·ICE Operations is an electrothermal and Systems IPS at thethat Norwegian has been developed University at ofthe Science Centre andfor Autonomous Technology Marine Operations and Systems at the Norwegian University of Science and Technology (NTNUMarine AMOS)Operations and and commercialized Systems at the by Norweg UBIQ Aerospace.ian University The technologyof Science and is based Technology on an (NTNU AMOS) and commercialized by UBIQ Aerospace. The technology is based on an electrothermal(NTNU AMOS) heating and commercialized system and an by ice UBIQ detection Aerospace. algorithm The using technology thermal is signalsbased on [19 an]. electrothermal heating system and an ice detection algorithm using thermal signals [19]. Theelectrothermal systems use heating heating system zonesmade and an of ice carbon detection fibre fabrics.algorithm using thermal signals [19]. TheThe systems Insystems this study,use use heating heating two prototypes zones zones made made of theof ofcarbon D carbon·ICE fibre system fibre fabrics. fabrics. were tested in the icing wind tunnel facilityIn Inthis atthis study, the study, Technical two two prototypes prototypes Research of Centre ofthe the D·ICE D·ICE of Finlandsystem system were (VTT) were tested duringtested in inthe fall the icing 2019. icing wind wind This tunnel studytunnel facilityaimedfacility at to atthe determine the Technical Technical the Research required Research Centre heat Centre loads of ofFinland for Finland anti-icing (VTT) (VTT) andduring during de-icing fall fall 2019. for 2019. a This selection This study study of aimedmeteorologicalaimed to todetermine determine icing the conditions.the required required heat De-icingheat loads loads loadsfo rfo anti-icingr anti-icing for two and IPS and layoutsde-icing de-icing (conventionalfor for a selectiona selection andof of meteorologicalpartingmeteorological strip) wereicing icing compared.conditions. conditions. ADe-icing parametricDe-icing loads loads study for for two over two IPS the IPS layouts ice layouts shedding (conventional (conventional time was and con- and partingductedparting strip) for strip) both were were configurations compared. compared. A in Aparametric order parametric to better study study understand over over the the the ice ice effectshedding shedding of certain time time parameterswas was con- con-

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ducted for both configurations in order to better understand the effect of certain parame- tersducted on the de-icingfor both efficiency.configurations The experimein order tonts bett wereer understandconducted at the low effect Reynolds of certain numbers parame- (Re onters= 8–9 the on × de-icingthe105 )de-icing that efficiency.are efficiency. typical Thefor The small experiments experime to medium-sizednts were were conducted conducted fixed-wing at at low lowUAVs. Reynolds Reynolds numbersnumbers (Re = 8–9 × 10105)5 )that that are are typical typical for for small small to to medium-sized medium-sized fixed-wing fixed-wing UAVs. UAVs. 2. Methods 2. Methods 2.1. Test Setup 2.1. Test Setup The VTT icing wind tunnel [24] is an open-loop wind tunnel (Figure 4) situated inside The VTT icing wind tunnel [24] is an open-loop wind tunnel (Figure4) situated inside a large climateThe VTT test icing chamber wind [25].tunnel The [24] tunnel is an open-loopwas originally wind designed tunnel (Figure for investigations 4) situated inside of a large climate test chamber [25]. The tunnel was originally designed for investigations of winda large turbine climate icing test at low chamber velocities, [25]. upThe to tunnel a maximum was originally airspeed designed of v = 50 m/s.for investigations The internal of wind turbine icing at low velocities, up to a maximum airspeed of v = 50 m/s. The internal testwind section turbine has a icingsize ofat 0.65low velocities,× 0.65 × 1.0 up m. to The a maximum icing wind airspeed tunnel offacility v = 50 can m/s. operate The internal in test sectionsection hashas aa sizesize ofof 0.650.65× × 0.65 × 1.01.0 m. m. The The icing icing wind wind tunnel facility can operate inin the temperature range of T = −25 … +30 °C. A◦ 3 × 3 spray nozzle grid generates a droplet the temperaturetemperature rangerange of ofT T= =− −25 ...… +30+30 °C.C. A A 3 3 ×× 33 spray spray nozzle nozzle grid grid generates generates a droplet cloud with a liquid water content range of LWC = 0.1 … 1.0 g/m³ and3 a median (droplet) cloud with aa liquidliquid waterwater contentcontent rangerange ofofLWC LWC= = 0.1 0.1... … 1.01.0 g/mg/m³ andand a a median (droplet) volume diameter range of MVD = 12 … 30 μm. µ volume diameter range of MVD = 12 … . .. 30 30 μm.m.

FigureFigure 4. Schematics 4. Schematics of the of theicing icing wind wind tunnel tunnel facility facility with with the theice protection ice protection system. system. Figure 4. Schematics of the icing wind tunnel facility with the ice protection system. TheThe test testwing wing is a isrectangular a rectangular wing wing with with a chord a chord c = c0.45= 0.45 m and m and a span a span b = b0.65= 0.65 m. The m. The wingwing is basedThe is based test on wingthe on RG-15 the is a RG-15 rectangular airfoil, airfoil, which wing which is awith low-Reynolds is aa low-Reynoldschord c = airfoil 0.45 m airfoil specifically and a specifically span designed b = 0.65 designed m. for The UAVsforwing and UAVs is modelbased and on modelaircraft. the RG-15 aircraft. The wing airfoil, The was wingwhich manufact was is a manufactured low-Reynoldsured from fibre-reinfor fromairfoil fibre-reinforced specificallyced epoxy. designed In epoxy. this for In study,thisUAVs we study, andplaced model we the placed wingaircraft. the at theThe wing end wing at of the wasthe end tunnelmanufact of the for tunnelured better from foraccessibility betterfibre-reinfor accessibility to theced IPS epoxy. during to the In IPSthis testingduringstudy, (Figure we testing placed 4). (FigureThe the rectangular wing4). The at the rectangular wing end spansof the wing thtunnele spansentire for thetestbetter entiresection, accessibility test and section, therefore to the and IPS the therefore duringre- sultsthetesting are results representative (Figure are representative4). The of arectangular 2D IPS of model. a wing 2D IPS Th spanse model. electrothermal the Theentire electrothermal test heating section, for and the heating IPStherefore was for sup- the the IPS re- pliedwassults by suppliedarecarbon-fibre representative by carbon-fibreheating of zonesa 2D heating IPS that model. were zones integratedTh thate electrothermal were into integrated the wing heating intostructure for the the wing (Figure IPS structurewas 5). sup- Power(Figureplied was by5 supplied ).carbon-fibrePower and was heatingmonitored supplied zones and for thateach monitored were zone integrated individually. for each zoneinto Power the individually. wing delivery structure Power to the (Figure deliveryheat- 5). ing toPowerwas the regulated heating was supplied was via a regulated control and monitored board via a controlus foring each pulse-width board zone using individually. modulation pulse-width Power (PWM) modulation delivery [19]. (PWM) to the heat- [19]. ing was regulated via a control board using pulse-width modulation (PWM) [19].

FigureFigure 5. Schematic 5. Schematic layout layout of the of composite the composite wing wing structure. structure. Figure 5. Schematic layout of the composite wing structure. TwoTwo D·ICE D·ICE prototypes prototypes were were used used in this in this study: study: a conventional a conventional design design and and a parting a parting stripstrip design.Two design. TheD·ICE The conventional prototypes conventional designwere design used (Figur (Figurein ethis 3a) 3study: consisteda) consisted a conventional of one of one primary primary design heating-zone heating-zoneand a parting extendingextendingstrip design. over over theThe theleading conventional , edge, withdesign with a total(Figur a total widthe width3a) ofconsisted 5 of cm. 5 cm. A of secondary Aone secondary primary heating-zone heating-zoneheating-zone withwithextending a width a width ofover 5 of cm the 5 cmwas leading was located located edge, on onthewith theuppe a uppertotalr and width and lower lower of side 5 cm. side of Athe of secondarythe airfoil. airfoil. The heating-zone The parting parting stripwith (Figurea width3 b)of design5 cm was had located two primary on the zonesupper atand the lower leading side edge, of the each airfoil. with The a width parting of

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strip (Figure 3b) design had two primary zones at the leading edge, each with a width of 2.52.5 cm cm and and two two secondary secondary zones zones with with a a width width of of 5 5 cm cm each. each. A A thin thin heating heating element element acting acting asas a a parting parting strip strip was was located located between between the the two two primary primary zones. zones. IcingIcing conditions conditions werewere chosen chosen to to represent represent in-cloud in-cloud icing icing environments environments that that can can be be expectedexpected during during typical typical UAV UAV operations operations [3 ,[3,226].6]. These These conditions conditions can can be be considered considered moder- mod- atelyerately severe severe conditions. conditions.

2.2.2.2. Anti-Icing Anti-Icing ThisThis study study aims aims to to determine determine the the minimum minimum required required heat heat loads loads for for anti-icing anti-icing and and de- de- icing.icing. For For anti-icing, anti-icing, the the minimum minimum required required load load was was determined determined by by iteratively iteratively reducing reducing thethe powerpower toto the heating system system until until the the point point when when ice ice accretions accretions became became visually visually ob- observableservable on on the the heated heated surfaces. surfaces. The The tests tests we werere conducted on aa wingwing withwitha a conventional conventional IPSIPS design.design. A A flowchart flowchart of of the the iterative iterative procedur proceduree is shown is shown in Figure in Figure 6. The6. process The process started started with a heating load qstart that was high enough that in the first step no ice accretion with a heating load qstart that was high enough that in the first step no ice accretion oc- occurred. The value is then halved repeatedly until a heat flux was found where ice occurs. curred. The value is then halved repeatedly until a heat flux was found where ice occurs. The subsequent heat load was chosen as the mean between the load where ice occurs (qlow) The subsequent heat load was chosen as the mean between the load where ice occurs (qlow) and the load where no ice occurred (qhigh). The procedure continued until the difference and the load where no ice occurred (qhigh). The procedure continued until the difference between the two loads became smaller than a limit (q ≈ 0.3 kW/m2). In this study, the between the two loads became smaller than a limitlimit (qlimit ≈ 0.3 kW/m2). In this study, the limitlimit was was determined determined by by the the smallest smallest step step size size of of the the power power control control unit. unit. This This method method was was applied for the conditions specified in Table1. Each point was repeated at least two times applied for the conditions specified in Table 1. Each point was repeated at least two times to verify repeatability. to verify repeatability.

Figure 6. Flowchart of the minimum required anti-icing heat load procedure. Figure 6. Flowchart of the minimum required anti-icing heat load procedure.

Table 1. Anti-icing test matrix. Table 1. Anti-icing test matrix. Parameter Range Parameter Range Velocity v 25 m/s VelocityTemperaturev T 25 m/s−2, −5, −10 °C Temperature T −2, −5, −10 ◦C Reynolds number Re 9 × 105 Reynolds number Re 9 × 105 3 LiquidLiquid water contentwater contentLWC LWC 0.44 g/m0.443 g/m DropletDroplet diameter diameterMVD MVD 24 µm 24 μm AngleAngle of attack ofα attack α 0◦ 0°

2.3.2.3. De-Icing De-Icing ForFor de-icing, de-icing, thethe situation situation isis more more complex, complex, as as there there are are a a total total of of three three parameters parameters thatthat cancan bebe adjusted:adjusted: the heat load load during during the the heating heating cycle cycle qqde-icede-ice, the, the duration duration of ofthe the de- de-icingicing cycle cycle tde-icetde-ice, and, and the the intercycle intercycle time time between between heating heating cycles cycles tintercycletintercycle, time, time in inwhich which ice iceaccretes accretes on onthe the airfoil. airfoil. To determine To determine the optimal the optimal combination combination of these, of these, a parameter a parameter study studyhas been has beenperformed performed for the for conditions the conditions specified specified in Table in Table 2. In2 .these In these tests, tests, the thede-icing de-icing load loadqde-iceq de-iceand theand intercycle the intercycle time t timeintercycletintercycle were setwere andset the and resulting the resulting de-icing de-icing time tde-ice time wastde-ice then wasmeasured then measured manually. manually. The de-icing The time de-icing was defined time was as definedthe duration as the from duration the moment from the the momentheating thewas heating turned wason until turned the on moment until the when moment ice whenstarted ice visually started shedding visually shedding from the from the wing. Furthermore, the effect of the and airspeed on shedding times was investigated.

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Table 2. De-icing test matrix.

Parameter Range Velocity v 10, 18, 25 m/s Temperature T −2, −5, −10 ◦C Reynolds number Re 3 × 105, 6 × 105, 9 × 105 2 Heat load qde-ice (approx.) 9, 12, 18 kW/m Intercycle time tintercycle 120, 240, 360, 480 s Angle of attack α 0, 4, 8◦ Liquid water content LWC 0.44 g/m3 Droplet diameter MVD 24 µm

2.4. Measurement Errors An error propagation analysis was performed to estimate the measurement uncer- tainty for all the physical parameters presented in this study. The analysis accounts for random errors which are caused by unpredictable variations in the measurement sys- tem [27]. In these experiments, the considered random error sources were the human reading of instrumentation (the stopwatch reading for de-icing time measurements and caliper readings of ice thickness) and the precision of the measuring instrumentation. Systematic errors are considered negligible in this analysis, as several procedures were followed to reduce them (keeping the measuring instrumentation at testing conditions to avoid thermal influences or using two operators and stopwatches for shedding time measurements). To reduce random errors, several test repetitions in similar experimental conditions were conducted. The results presented in this study are the averaged values of generally three repetitions (in a few cases this number was reduced to two due to testing time constraints). Each repetition was an independent observation of the studied experimental condition. The estimation of random errors was reduced to a computation of uncertainty of the mean value, assuming a normal distribution of the errors (as a physical phenomenon is studied [28]), and statistical independence of the measurements. Given the small number of samples, the Student’s t-distribution was employed to obtain an estimation of the error in the mean. The calculation considered the number of samples and the standard deviation of the data points multiplied by Student’s t-factor [27], for a 90% confidence interval. The choice of 90% confidence interval was considered to give sufficiently accurate error estimation as the present study aims to compare the performance of several de-icing solutions and to show a proof of concept of the techniques. The described errors were added to form the measurement’s uncertainty, depicted in figures in absolute values by error bars, and reported in percentages of the measured values as following: the imposed temperature relative uncertainty ranged between ~2 and 10% for temperature values of −10 to −2 ◦C, respectively; the relative uncertainty of the airfoil’s angle of attack was estimated to ~1% for values of 0◦ to 8◦, and the wind speed uncertainty varied in the range of ~4 to 10% for values of 25 to 10 m/s. These uncertainties are based on the precision of the experimental set-up. For the anti-icing tests, the heat flux uncertainties extended between ~10 and 45% for measured values of 8 to 1.4 kW/m2, respectively, while for ice thickness the uncertainty ranged between ~5 and 25% for measured values of 13 to 1.4 mm, respectively. Regarding the de-icing experiments, the applied heat flux was imposed in a range of ~8 to 28 kW/m2 with a relative uncertainty of 3 to ~0.5. The measurement uncertainty of ice shedding time generally varied between ~4 and 42% as the measured values ranged between 80 and 5 s, respectively. One case was an exception, where the relative uncertainty of the shedding time was estimated at 125% (parting-strip de-icing at −2 ◦C ± 10%, 17.6 kW/m2 ± 3%). The considered contributing factors to this high uncertainty were a combination of low measured values (within 6 to 9 s) where factoring in for human error has a higher impact, high standard deviation within the test samples, and a limited number of samples. An increased number of test repetitions is required to decrease this measurement uncertainty; however, the measurement point is considered to present a general trend estimate. Aerospace 2021, 8, x FOR PEER REVIEW 7 of 16 Aerospace 2021, 8, x FOR PEER REVIEW 7 of 16

considered contributing factors to this high uncertainty were a combination of low meas- considered contributing factors to this high uncertainty were a combination of low meas- ured values (within 6 to 9 s) where factoring in for human error has a higher impact, high ured values (within 6 to 9 s) where factoring in for human error has a higher impact, high standard deviation within the test samples, and a limited number of samples. An in- Aerospace 2021, 8, 83 standard deviation within the test samples, and a limited number of samples. An7 of in- 15 creased number of test repetitions is required to decrease this measurement uncertainty; creased number of test repetitions is required to decrease this measurement uncertainty; however, the measurement point is considered to present a general trend estimate. however, the measurement point is considered to present a general trend estimate. 3.3. ResultsResults 3. Results ThisThis sectionsection presentspresents thethe resultsresults ofof thethe testteston onthe theIPS. IPS. Figure Figure7 7displays displays a a thermal thermal This section presents the results of the test on the IPS. Figure 7 displays a thermal imageimage ofof the the activated activated IPS IPS during during anti-icing anti-icing operation. operation. TheThe picturepicture showsshows thethe small small gap gap image of the activated IPS during anti-icing operation. The picture shows the small gap betweenbetween the the primary primary andand secondarysecondary heatingheating zones,zones, asas wellwell asas aa uniformuniform heatheat distribution.distribution. between the primary and secondary heating zones, as well as a uniform heat distribution. RunbackRunback ice ice has has formed formed downstream downstream of of the the heating heating zones. zones.Figure Figure8 8shows shows a a de-icing de-icing case, case, Runback ice has formed downstream of the heating zones. Figure 8 shows a de-icing case, momentsmoments beforebefore thethe leading-edgeleading-edge iceice shapeshape would would shed. shed. SeveralSeveral runbackrunback rivuletsrivulets withwith moments before the leading-edge ice shape would shed. Several runback rivulets with liquidliquid water water can can be be observed. observed. These These would would freeze freeze downstream downstream of of the the airfoil. airfoil. liquid water can be observed. These would freeze downstream of the airfoil.

Figure 7. Thermal image of an anti-icing test. The leading-edge is continuously heated, producing FigureFigure 7.7.Thermal Thermal imageimage ofof anan anti-icinganti-icing test.test. The leading-edge is continuously heated, producing runback water that refreezes downstream. runbackrunback waterwater that that refreezes refreezes downstream. downstream.

Figure 8. De-icing test, moments before the ice shedding. Liquid water is produced from the lead- Figure 8. De-icing test, moments before the ice shedding. Liquid water is produced from the lead- Figureing-edge 8. andDe-icing refreezes test, downstream. moments before the ice shedding. Liquid water is produced from the ing-edge and refreezes downstream. leading-edge and refreezes downstream. 3.1. Anti-Icing 3.1.3.1. Anti-IcingAnti-Icing The results for the anti-icing loads are shown in Figure 9. Two runs were conducted TheThe resultsresults forfor thethe anti-icinganti-icing loadsloads areare shownshown inin FigureFigure9 .9. Two Two runs runs were were conducted conducted each for T = −2 ◦°C and T = −5 °C.◦ Four runs were conducted at T = −10 ◦°C, since it showed eacheach for for T T = =− −2 °CC andand TT == −−55 °C.C. Four runs werewere conductedconducted at at T T = =− −10 °C,C, sincesince itit showedshowed a larger variation of the required heat fluxes. This large variation was likely related to the aa largerlarger variationvariation of the required required heat heat fluxes. fluxes. This This large large variation variation was was likely likely related related to the to difficulty to identify the exact heat flux when steady-state anti-icing is achieved. The re- thedifficulty difficulty to identify to identify the the exact exact heat heat flux flux when when steady-sta steady-statete anti-icing anti-icing is achieved. is achieved. The The re- sults show a strong linear correlation with the temperature. This behavior was expected resultssults show show a astrong strong linear linear correlation correlation with with the temperature. ThisThis behaviorbehavior waswas expectedexpected and can be explained with the larger temperature difference (between ambient and the andand cancan bebe explainedexplained withwith thethe largerlarger temperaturetemperature differencedifference (between(between ambientambient andand thethe freezing point) that needs to be overcome by the anti-icing system at lower temperatures. freezingfreezing point)point) thatthat needsneeds toto bebe overcomeovercome by by the the anti-icing anti-icing system system at at lower lower temperatures. temperatures. 3.2. Ice Thickness The ice thickness is a key parameter for de-icing and depends on ice accretion time and temperature for v = 25 m/s (Figure 10). The ice thickness showed a good linear fit with the ice accretion time for all temperatures. The highest ice accretion rate occurred at T = −10 ◦C, and was closely followed by T = −5 ◦C. At temperatures close to the freezing point (T = −2 ◦C), the ice accretion efficiency was lower most likely due to the formation of glaze ice. The resulting difference in ice thickness between the temperature was one

parameter that affected the shedding times for both IPSs. A remark is that together with thickness, other ice properties have varied (shape, density, and adhesion strength) for the tested temperature range. Aerospace 2021, 8, x FOR PEER REVIEW 8 of 16

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Figure 9. Minimum required heat fluxes for anti-icing at different temperatures.

3.2. Ice Thickness The ice thickness is a key parameter for de-icing and depends on ice accretion time and temperature for v = 25 m/s (Figure 10). The ice thickness showed a good linear fit with the ice accretion time for all temperatures. The highest ice accretion rate occurred at T = −10 °C, and was closely followed by T = −5 °C. At temperatures close to the freezing point (T = −2 °C), the ice accretion efficiency was lower most likely due to the formation of glaze ice. The resulting difference in ice thickness between the temperature was one parameter that affected the shedding times for both IPSs. A remark is that together with thickness, Figureother ice9. Minimum properties required have heatvaried fluxes (shape, for an densti-icingity, at anddifferent adhesion temperatures. strength) for the tested Figure 9. temperatureMinimum range. required heat fluxes for anti-icing at different temperatures. 3.2. Ice Thickness The ice thickness is a key parameter for de-icing and depends on ice accretion time and temperature for v = 25 m/s (Figure 10). The ice thickness showed a good linear fit with the ice accretion time for all temperatures. The highest ice accretion rate occurred at T = −10 °C, and was closely followed by T = −5 °C. At temperatures close to the freezing point (T = −2 °C), the ice accretion efficiency was lower most likely due to the formation of glaze ice. The resulting difference in ice thickness between the temperature was one parameter that affected the shedding times for both IPSs. A remark is that together with thickness, other ice properties have varied (shape, density, and adhesion strength) for the tested temperature range.

Figure 10. Ice thickness over ice accretion time at different temperatures. Figure 10. Ice thickness over ice accretion time at different temperatures.

3.3.3.3. Conventional De-Icing De-Icing Shedding times times for for the the conventional conventional de-icing de-icing system system were investigated were investigated for a range for of a range of ice accretion times, temperatures, and power settings (Figure 11). The results revealed ice accretion times, temperatures, and power settings (Figure 11). The results revealed three three mechanisms. First, an increase in de-icing heat flux led to a decrease in shedding mechanisms.times. This was First, most anlikely increase related in to de-icinga more rapid heat melting flux led process to a decrease at the ice/structure in shedding times. Thisinterface. was mostSecond, likely an increase related in to ice a accretion more rapid time melting led to an process increase at in the shedding ice/structure time. A interface. Second,2 min increase an increase in ice accretion in ice accretiontime led to timean approximate led to an 50% increase increase in in shedding shedding time.time. A 2 min increaseThis indicated in ice that accretion an increase time in led ice to thickn an approximateess, and likely 50% consequently increase higher in shedding aerody- time. This indicatedFigurenamic 10.shedding Ice that thickness anforces, increase over did ice not accretion in lead ice to thickness, time faster at differentshedding and temperatures. likelytimes. consequentlyInstead, a large ice higher thickness aerodynamic sheddingat the leading-edge forces, did might not be lead acting to fasteras an ice shedding bridge between times. Instead,the upper a and large lower ice thicknessice ac- at the leading-edge3.3.cretion Conventional regions. might Moreover, De-Icing be acting the ice aswas an likely ice bridge kept in betweenplace by the the stagnation upper and pressure, lower and ice accretion longer melting times were needed to achieve shedding. Third, the shedding times increase regions.Shedding Moreover, times for the the ice conventional was likely de-icing kept in system place by were the investigated stagnation for pressure, a range of and longer at lower temperatures. This was most likely related to the larger temperature difference meltingice accretion times times, were temperatures, needed to achieveand power shedding. settings (Figure Third, 11). the The shedding results revealed times increase at that had to be overcome by the IPS. lowerthree mechanisms. temperatures. First, This an wasincrease most in likelyde-icing related heat flux to the led largerto a decrease temperature in shedding difference that Aerospace 2021, 8, x FOR PEER REVIEW times. This was most likely related to a more rapid melting process at the9 ice/structureof 16 had to be overcome by the IPS. interface. Second, an increase in ice accretion time led to an increase in shedding time. A 2 min increase in ice accretion time led to an approximate 50% increase in shedding time. This indicated that an increase in ice thickness, and likely consequently higher aerody- namic shedding forces, did not lead to faster shedding times. Instead, a large ice thickness at the leading-edge might be acting as an ice bridge between the upper and lower ice ac- cretion regions. Moreover, the ice was likely kept in place by the stagnation pressure, and longer melting times were needed to achieve shedding. Third, the shedding times increase at lower temperatures. This was most likely related to the larger temperature difference that had to be overcome by the IPS.

Figure 11. SheddingFigure times11. Shedding of the conventional times of the conventional de-icing ice protectionde-icing icesystem protection for system three different for three temperatures: different T = −2◦ (a), temperatures: T = −2° (a), T = −5° (b), and T = −10° (c). T = −5◦ (b), and T = −10◦ (c). 3.4. Parting Strip De-Icing Further tests were conducted with variations of intercycle time, heat fluxes, and tem- peratures (Figure 12). Similar to the conventional IPS, decreasing heat fluxes increased shedding time. At T = −2 °C the shedding times between the lowest and the highest heat flux settings varied significantly less compared to lower temperatures. Furthermore, while an increase in intercycle time seemed to lead to increased ice shedding times for T = −5 °C (in accordance with the conventional IPS—although with a smaller impact), for T = −2 °C it led to decreased shedding times. We were not able to fully explain this behavior without further tests, but these effects may be related to measurement uncertainties (as the values vary in a range of ± 1 s), the difference in ice shapes, or the low adhesion forces of glaze ice cases. The continuous power requirement of the parting strip scaled linearly with ambient temperature as Pparting strip = [7.1, 20.2, 45.6 W] for temperatures T = [−2, −5, −10 °C], respectively. The values for the parting strip were obtained in a similar approach as the minimum heat fluxes for anti-icing of the IPS.

Figure 12. Shedding times of the parting strip de-icing ice protection system for three different temperatures: T = −2° (a), T = −5° (b), and T = −10° (c).

3.5. Parting Strip vs. Conventional De-Icing To showcase the effect of the parting strip on ice shedding, a comparison between the conventional system, the parting strip system with deactivated parting strip, and ac- tivated parting strip were conducted (Figure 13). The tests were performed at T = −2 °C and tintercycle = 4 min at relatively similar heat fluxes q = [10.5 … 12.5 kW/m2]. The results showed that there is little difference between the conventional system and the deactivated parting strip. The latter shed ice slightly later compared to the conventional system even though it used almost 20% higher heat loads. This was likely related to the unheated gap on the parting strip system (as the parting strip itself was deactivated) which delayed the

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Aerospace 2021, 8, 83Figure 11. Shedding times of the conventional de-icing ice protection system for three different 9 of 15 temperatures: T = −2° (a), T = −5° (b), and T = −10° (c).

3.4. Parting Strip De-Icing 3.4. Parting Strip De-Icing Further tests were conducted with variations of intercycle time, heat fluxes, and tem- peratures (Figure 12).Further Similar tests to werethe conventional conducted withIPS, variationsdecreasing ofheat intercycle fluxes increased time, heat fluxes, and shedding time.temperatures At T = −2 °C (Figurethe shedding 12). Similar times to between the conventional the lowest IPS, and decreasing the highest heat heat fluxes increased ◦ flux settings variedshedding significantly time. At T less= − 2coCmpared the shedding to lower times temperatures. between the Furthermore, lowest and the highest heat while an increaseflux in settings intercycle varied time significantly seemed to lesslead compared to increased to lower ice shedding temperatures. times Furthermore,for T while ◦ = −5 °C (in accordancean increase with in the intercycle conventional time seemed IPS—although to lead to with increased a smaller ice shedding impact), timesfor T for T = −5 C − ◦ = −2 °C it led to(in decreased accordance shedding with the times. conventional We were IPS—althoughnot able to fully with explain a smaller this behavior impact), for T = 2 C without furtherit ledtests, to but decreased these effects shedding may times. be related We were to notmeasurement able to fully uncertainties explain this behavior(as without the values varyfurther in a range tests, of but ± 1 these s), the effects differ mayence bein ice related shapes, to measurement or the low adhesion uncertainties forces (as the values ± of glaze ice cases.vary The in a continuous range of 1power s), the requirement difference in of ice the shapes, parting or thestrip low scaled adhesion linearly forces of glaze ice cases. The continuous power requirement of the parting strip scaled linearly with ambient with ambient temperature as Pparting strip = [7.1, 20.2, 45.6 W] for temperatures T = [−2, −5, temperature as P = [7.1, 20.2, 45.6 W] for temperatures T = [−2, −5, −10 ◦C], −10 °C], respectively. The valuesparting for the strip parting strip were obtained in a similar approach respectively. The values for the parting strip were obtained in a similar approach as the as the minimum heat fluxes for anti-icing of the IPS. minimum heat fluxes for anti-icing of the IPS.

Figure 12. Shedding times of the parting strip de-icing ice protection system for three different Figure 12. Shedding times of the parting strip de-icing ice protection system for three different temperatures: T = −2◦ (a), temperatures: T = −2° (a), T = −5° (b), and T = −10° (c). T = −5◦ (b), and T = −10◦ (c). 3.5. Parting Strip vs. Conventional De-Icing 3.5. Parting Strip vs. Conventional De-Icing To showcase the effect of the parting strip on ice shedding, a comparison between To showcase the effect of the parting strip on ice shedding, a comparison between the the conventional system, the parting strip system with deactivated parting strip, and ac- conventional system, the parting strip system with deactivated parting strip, and activated tivated parting strip were conducted (Figure 13). The tests were performed at T = −2 °C parting strip were conducted (Figure 13). The tests were performed at T = −2 ◦C and and tintercycle = 4 min at relatively similar heat fluxes q = [10.5 … 12.5 kW/m2]. The results2 tintercycle = 4 min at relatively similar heat fluxes q = [10.5 ... 12.5 kW/m ]. The results showed that thereshowed is little that difference there is little between difference the conventional between the system conventional and the system deactivated and the deactivated parting strip. Theparting latter strip. shed The ice latterslightly shed later ice compared slightly later to the compared conventional to the system conventional even system even though it usedthough almost it20% used higher almost heat 20% loads. higher This heat was loads. likely This related was to likely the unheated related to gap the unheated gap on the parting onstrip the system parting (as strip the parting system st (asrip the itself parting was deactivated) strip itself waswhich deactivated) delayed the which delayed the melting process. The ice/surface interface near the gap needed longer to heat which resulted in the increased shedding time. Also, that area might have been acting as an anchor point of ice, likely reducing aerodynamic shedding efficiency. In contrast, shedding times for the system with activated parting strip were substan- tially lower compared to the conventional system. The parting strip system shed ice more than three times faster than the conventional system. As discussed before, this can be explained by significantly increasing the shedding forces related to the aerodynamic drag of the ice shapes.

3.6. Airspeed The influence of the airspeed was tested at T = −5 ◦C on the parting strip system for three airspeeds v = [10, 18, 25 m/s]. The water flow rate of the droplet spray nozzles was kept constant, meaning that the LWC was coupled to the airspeed. The corresponding values for each airspeed are LWC = [0.96, 0.69, 0.44 g/m3]. The results (Figure 14) showed a clear trend of a decrease in de-icing time with increasing velocity. This is most likely Aerospace 2021, 8, x FOR PEER REVIEW 10 of 16

melting process. The ice/surface interface near the gap needed longer to heat which re- sulted in the increased shedding time. Also, that area might have been acting as an anchor point of ice, likely reducing aerodynamic shedding efficiency. In contrast, shedding times for the system with activated parting strip were substan- tially lower compared to the conventional system. The parting strip system shed ice more than three times faster than the conventional system. As discussed before, this can be ex- plained by significantly increasing the shedding forces related to the aerodynamic drag of the ice shapes.

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related to the substantial increase in aerodynamic shedding forces at higher velocities. The aerodynamic forces are kinetic forces and are thus quadratically related to the airspeed [15]. Thismelting effect process. is dominating The ice/surface over secondary interface effectsnear the that gap would needed result longer in a to tendency heat which to de- re- creasesulted shedding in the increased times forshedding lower airspeeds:time. Also, first,that area the increasemight have in LWC been leadsacting to aslarger an anchor ice accretionspoint of ice, that likely may reducing lead to a aerodynamic decrease shedding shedding times efficiency. due to increased drag. Second, the

lowerIn airspeeds contrast, lead shedding to lower times heat for convection the system andwith thus activated more parting efficient strip heating were from substan- the IPS—althoughFiguretially lower 13. Comparison comparedthis effect of to shedding mightthe conventional betimes offset between by sy thestem. conven higher Thetional parting LWC (Conv.) and strip and larger system parting ice shed thicknesses.strip ice (PS) more IPS, Bothwith theseactivated effects (PS on) were and likely deactivated subdued parting by the strip decrease (PS off). in aerodynamic forces. In addition, than three times faster than the conventional system. As discussed before, this can be ex- we visually observed that more melting seemed to occur for the lower airspeeds compared plained by significantly increasing the shedding forces related to the aerodynamic drag of to3.6. the Airspeed higher airspeeds. the ice shapes. The influence of the airspeed was tested at T = −5 °C on the parting strip system for three airspeeds v = [10, 18, 25 m/s]. The water flow rate of the droplet spray nozzles was kept constant, meaning that the LWC was coupled to the airspeed. The corresponding values for each airspeed are LWC = [0.96, 0.69, 0.44 g/m³]. The results (Figure 14) showed a clear trend of a decrease in de-icing time with increasing velocity. This is most likely related to the substantial increase in aerodynamic shedding forces at higher velocities. The aerodynamic forces are kinetic forces and are thus quadratically related to the airspeed [15]. This effect is dominating over secondary effects that would result in a tendency to decrease shedding times for lower airspeeds: first, the increase in LWC leads to larger ice accretions that may lead to a decrease shedding times due to increased drag. Second, the lower airspeeds lead to lower heat convection and thus more efficient heating from the IPS—although this effect might be offset by the higher LWC and larger ice thicknesses.

Both these effects were likely subdued by the decrease in aerodynamic forces. In addition, FigureweFigure visually 13. 13.Comparison Comparison observed of of that shedding shedding more times timesmelting between between seemed conventional conven to tionaloccur (Conv.) (Conv.) for the and and lower parting parting airspeeds strip strip (PS) (PS) com- IPS, IPS, with activated (PS on) and deactivated parting strip (PS off). withpared activated to the (PShigher on) andairspeeds. deactivated parting strip (PS off). 3.6. Airspeed The influence of the airspeed was tested at T = −5 °C on the parting strip system for three airspeeds v = [10, 18, 25 m/s]. The water flow rate of the droplet spray nozzles was kept constant, meaning that the LWC was coupled to the airspeed. The corresponding values for each airspeed are LWC = [0.96, 0.69, 0.44 g/m³]. The results (Figure 14) showed a clear trend of a decrease in de-icing time with increasing velocity. This is most likely related to the substantial increase in aerodynamic shedding forces at higher velocities. The aerodynamic forces are kinetic forces and are thus quadratically related to the airspeed [15]. This effect is dominating over secondary effects that would result in a tendency to decrease shedding times for lower airspeeds: first, the increase in LWC leads to larger ice accretions that may lead to a decrease shedding times due to increased drag. Second, the

lower airspeeds lead to lower heat convection and thus more efficient heating from the FigureFigureIPS—although 14. 14.Comparison Comparison this effect of of shedding shedding might times timesbe offset of of the the by parting pa therting higher strip strip IPS IPS LWC at at different different and larger velocities. velocities. ice thicknesses. Both these effects were likely subdued by the decrease in aerodynamic forces. In addition, 3.7. Angle of Attack we visually observed that more melting seemed to occur for the lower airspeeds com- paredThree to the test higher series airspeeds. were conducted to investigate the effect of the angle of attack (AOA) during icing on de-icing efficiency. For the parting strip system, three AOAs, with α = [0, 4, 8◦] were tested each at T = [−2, −5 ◦C]. The conventional de-icing system was tested at two AOAs, with α = [0, 4◦] and T = −10 ◦C. All three results are shown in Figure 15. For the parting strip system, a higher AOA resulted in about 20 to 30% reduced de-icing times. This might be related to an increased convective heat transfer over the parting strip, which resulted from the local wind speed increase at higher AOAs. This was likely to accelerate the transversal ice melting and to lead to a faster and enlarged water layer formation at the ice-airfoil interface. This would allow a faster redistribution of external pressure in the liquid layer, which would induce a lift force over the ice, followed by shedding, as described in [29]. The air temperature seemed to play only a minor role in this mechanism.

Figure 14. Comparison of shedding times of the parting strip IPS at different velocities.

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3.7. Angle of Attack Three test series were conducted to investigate the effect of the angle of attack (AOA) during icing on de-icing efficiency. For the parting strip system, three AOAs, with α = [0, 4, 8°] were tested each at T = [−2, −5 °C]. The conventional de-icing system was tested at two AOAs, with α = [0, 4°] and T = −10 °C. All three results are shown in Figure 15. For the parting strip system, a higher AOA resulted in about 20 to 30% reduced de-icing times. This might be related to an increased convective heat transfer over the parting strip, which resulted from the local wind speed increase at higher AOAs. This was likely to accelerate the transversal ice melting and to lead to a faster and enlarged water layer formation at the ice-airfoil interface. This would allow a faster redistribution of external pressure in the liquid layer, which would induce a lift force over the ice, followed by shedding, as de- scribed in [29]. The air temperature seemed to play only a minor role in this mechanism. For the conventional system, a de-icing time increase of about 10% occurred. This was in line with previous experiences that showed an increase in the de-icing time at Aerospace 2021, 8, 83 higher AOAs. This effect was likely linked to the fact that at higher AOAs a larger 11area of 15 of the airfoil is iced and that the increased aerodynamic forces are likely contributing less to shedding on the fully iced leading-edge.

FigureFigure 15. 15.Comparison Comparison of of shedding shedding timestimes betweenbetween conventionalconventional (Conv.)(Conv.)and and partingparting stripstrip (PS)(PS) IPSIPS forfor different different angles angles of of attack. attack.

4. DiscussionFor the conventional system, a de-icing time increase of about 10% occurred. This was in4.1. line General with previous experiences that showed an increase in the de-icing time at higher AOAs.The This main effect objective was likely of this linked study to was the factto compare that at higherthree different AOAs a IPS larger methods: area of anti- the airfoilicing, isconventional iced and that de-icing, the increased and parting aerodynamic strip de-icing. forces For are de-icing, likelycontributing several variations less to of shedding on the fully iced leading-edge. ice accretion time and heat fluxes were tested. In order to identify the most energy-effi- 4.cient Discussion IPS, a time-averaged energy consumption 𝑞 is calculated for each case: 4.1. General 𝑞de-icing ∙𝑡shedding + 𝑞parting strip ∙𝑡intercycle J W 𝑞 = = 2 = 2 The main objective of this𝑡 studyintercycle was+ 𝑡shedding to compare three differents·m IPS methods:m anti- icing, conventional de-icing, and parting strip de-icing. For de-icing, several variations of This value calculates the total energy that has been spent on IPS in relation to the ice accretion time and heat fluxes were tested. In order to identify the most energy-efficient total cycle time. The unit of 𝑞 is the same unit as for heat fluxes and can thus be directly IPS, a time-averaged energy consumption q is calculated for each case: compared to anti-icing heat loads.     The comparisonq − is· tperformed+ q partingfor each strip ambi·t ent temperatureJ separatelyW (Figure 16). = de icing shedding intercycle = = Heat flux levelsq were indicated with “low”, “mid”, and “high” 2and their 2numeric value tintercycle + tshedding s·m m can be identified from Figures 11 and 12. The first observation was that anti-icing required significantlyThis value more calculates energy thethan total all other energy case thats. There has been was spent a strong on IPStemperature in relation depend- to the totalency cycle on this time. effect, The which unit of impliedq is the that same de-icing unit as methods for heat became fluxes and more can efficient thus be compared directly comparedto anti-icing to anti-icingfor lower heattemperatures. loads. The reason for this was that anti-icing systems need The comparison is performed for each ambient temperature separately (Figure 16). Heat flux levels were indicated with “low”, “mid”, and “high” and their numeric value can be identified from Figures 11 and 12. The first observation was that anti-icing required significantly more energy than all other cases. There was a strong temperature dependency on this effect, which implied that de-icing methods became more efficient compared to anti-icing for lower temperatures. The reason for this was that anti-icing systems need to continuously provide enough heat to compensate for the temperature difference between ambient and freezing point, whereas de-icing systems do this periodically. The second conclusion was that parting strip de-icing was more energy-efficient than conventional de-icing, for any given intercycle time. The advantage of the parting strip was temperature-dependent, requiring approximately half of the energy compared to the conventional IPS for T = −2 ◦C and −5 ◦C. For the lowest temperature T = −10 ◦C, the advantage decreased. The reason for this was not clear. It could be related to the higher ice adhesion forces at a lower temperature or to insufficient heating of the parting strip and a too-small gap in the ice. Third, a longer ice accretion time led to lower energy requirements for both conven- tional and parting strip IPS. The system was more efficient since the ratio of ice accretion time to active IPS time was larger, resulting in a larger denominator in q. Aerospace 2021, 8, x FOR PEER REVIEW 12 of 16

Aerospace 2021, 8, 83 to continuously provide enough heat to compensate for the temperature difference be- 12 of 15 tween ambient and freezing point, whereas de-icing systems do this periodically.

FigureFigure 16. 16. OverviewOverview of of the the time-averaged time-averaged total total energy energy consumption consumption for for all all IPS IPS runs. runs.

The secondLast, conclusion no clear was trend that could parting be observed strip de-icing for the was influence more energy-efficient of the level of than the heat flux conventionalon the de-icing, time-averaged for any given energy intercycle consumption. time. The It remains advantage unclear of the if a parting high heat strip flux over a shorter time is more energy-efficient compared to a lower heat flux over a longer time. This was temperature-dependent, requiring approximately half of the energy compared to the was explained by the formulation of q where the deicing heat flux is linked to the shedding conventional IPS for T = −2 °C and −5 °C. For the lowest temperature T = −10 °C, the ad- time. Their inverse proportionality made them partly compensate for their behavior. vantage decreased. The reason for this was not clear. It could be related to the higher ice The outcome of this study strongly indicated that the most energy-efficient method adhesion forces at a lower temperature or to insufficient heating of the parting strip and of IPS is a parting strip de-icing system. However, there were several limitations to this a too-small gap in the ice. study that should be noted. An assessment of IPS efficiency cannot be based solely on its Third, a longer ice accretion time led to lower energy requirements for both conven- energy consumption. Secondary effects, that add to the overall power consumption of the tional and parting strip IPS. The system was more efficient since the ratio of ice accretion UAV need to be considered as well. This includes aerodynamic performance degradations time to active IPS time was larger, resulting in a larger denominator in 𝑞. by intercycle ice and runback icing. These ice forms introduce aerodynamic penalties by Last, no clear trend could be observed for the influence of the level of the heat flux decreasing lift and increasing drag [30,31]. The UAV needs to compensate for these, by on the time-averaged energy consumption. It remains unclear if a high heat flux over a increasing thrust and AOA. This will require additional power consumption that needs to shorter timebe consideredis more energy-efficient for the overall compared assessment to of a IPSlower efficiency. heat flux over a longer time. 𝑞 This was explainedRunback by the icing formulation occurred during of where both,anti-icing the deicing and heat de-icing, flux is and linked can to be the observed in sheddingFigures time. Their7 and inverse8. In addition proportionality to the aerodynamic made them penalties, partly compensate runback icing for introduces their be- the risk havior. of freezing on downstream control surfaces. This can block their movement and is a severe The outcomerisk for the of UAVthis study [32]. Control strongly of indicated runback icingthat withthe most an IPS energy-efficient can be achieved method for example by of IPS is amultiple parting heatingstrip de-icing zones thatsystem. are operatedHowever, sequentially. there were several limitations to this study that shouldDe-icing be noted. also introducesAn assessment the risk of IPS of shedefficiency ice hitting cannot critical be based components solely on downstream. its energy consumption.This is a risk Secondary for equipment effects, like that antennas, add to the sensors, overall propellers,power consumption or engine of inlets the that are UAV needlocated to be considered downstream as well. of protected This includes surfaces aerodynamic [13]. This risk performance is non-existent degradations for anti-icing IPS by intercycleand ice needs and therefore runback carefulicing. These evaluation ice forms for de-icing introduce IPS. aerodynamic penalties by decreasing liftThere and increasing are several drag limitations [30,31]. to The the UAV experimental needs to methods compensate that addfor these, uncertainty by to this increasingstudy. thrust First, and theAOA. calculation This will of require the heat additional fluxes was power based consumption on the electric that power needs provided to to be consideredthe heating for the zones. overall In reality,assessment only of a part IPS efficiency. of this power is effectively used by the IPS. The largest loss occurs due to heat conduction into the wing. Furthermore, losses due to the electric system were not accounted for. We recommend that for future tests the actual heat flux generated by the heating panels should be measured. Second, the ice shedding times

Aerospace 2021, 8, 83 13 of 15

were observed manually. This added subjectivity to the test (based on the experimenter’s reaction time) and limited the accuracy of the time measurement. Video data were collected during these tests, but to process this data takes substantial work. High-speed flow visualizations of the de-icing experiments were recorded at 60 and 120 frames per second from 3 viewpoints. The video-data results could highly increase the shedding time measurement accuracy and give more information about the ice shedding mechanism. Filmed in high-resolution, these visual observations show the liquid layer for- mation under the ice. This could give more information about the detachment mechanism and could be used for a qualitative runback ice investigation.

4.2. Measurement Uncertainties As described in the methodology section, the uncertainty estimations of the imposed experimental conditions (temperature, wind speed, de-icing heat flux, etc.) were low, which ensured good test repeatability. For these parameters, the instrumentation preci- sion was the predominant source of error, the errors increasing in significance for lower measurement values. For measured quantities (anti-icing heat flux, ice thickness and ice shedding time) the estimated relative uncertainties were between 2 to 5 times higher than for the imposed test parameters. For these quantities, the measured values variation was the predominant source of error. This could have been caused by one or both of the following: decreased measurement precision due to human observation/reading of instrumentation or variations of the physical phenomenon between test repetitions, such as slightly different ice accretion shapes, ice cracking during de-icing tests etc. For measured quantities, it is generally observed that for smaller measured values the human reading error is more significant. For example, considering the conventional de- icing tests (Figure 12), at temperature −5 ◦C and −10 ◦C the maximum absolute uncertainty is ~9 s, while the reported ice shedding times ranges differ from a maximum 45 s (at −5 ◦C) to 76 s (same test conditions at −10 ◦C). In the first case, the relative uncertainty is ~28%, while for the second it is only ~12%. Another example is the relative uncertainty variation in determining the anti-icing heat fluxes (Figure9). An absolute uncertainty of ~0.8 kW/m 2 represents ~10% of relative error for the 7.44 kW/m2 measurement, but ~45% for the 1.5 kW/m2 measurement. The same observation applies to the ice thickness measurements, where the absolute uncertainties are ~0.5 mm but the relative uncertainties vary between ~5 and 25%. Regarding the de-icing tests, the most important uncertainty factor observed was the standard deviation of the test sample, compared to which the other sources of error were almost negligible. The spread of ice shedding times around the mean value was more pronounced at higher testing temperatures (higher standard deviations were observed at −2 ◦C temperatures than at −10 ◦C for both de-icing configurations). An explanation for this might be the formation of glaze ice, which has a higher shape variation between two repetitive runs, therefore it might induce an increased random variation between tests. Another observation is that estimated uncertainties are significantly higher for the hybrid de-icing configuration (neglecting one extreme case, ~42% relative uncertainty, while for conventional de-icing the maximum reported was ~28%). This is partially due to the decreased absolute values in ice shedding time, but most of the uncertainty is due to the standard deviation of the samples. The variations between test repetitions might be related to the parting strip operation in anti-icing mode. This generates the existence of a water film on the airfoil’s surface, and a wetter ice accretion than the conventional configuration. Water presence might increase the ice shape’s variability and change the ice adhesion properties between two test repetitions. Although the measurements’ rel- ative uncertainties are high in some cases, affecting the absolute values, the data aspect (generally non-overlapping of error margins between experimental conditions) allows for the observation of general trends. A few sensitive cases are the ice shedding time measurements of the hybrid configuration for −2 ◦C, at medium heat flux (Figure 12). As Aerospace 2021, 8, 83 14 of 15

the uncertainty margins overlap, little can be said about the differences between these tests. To improve measurement accuracy, one should increase the number of repetitions of a testing configuration. By doing so, the obtained mean would be closer to the true value, and the Student’s t-factor would reduce, therefore reducing the random uncertainty.

5. Conclusions Atmospheric icing imposes limitations on UAVs that can be overcome with IPS. An electrothermal IPS was tested at the icing wind tunnel facilities of VTT. The main objective of this study was to identify which IPS method was the most energy-efficient and to investigate the effect of angle of attack and airspeed on de-icing. The results suggest that anti-icing was the least energy-efficient method of IPS. De-icing has proven to require substantially lower heat loads at all temperatures. A conventional IPS, with a periodically heated leading-edge, and a parting strip IPS, with a continuously heated small area, were tested for de-icing. De-icing with the parting strip has shown to require up to 50% less energy compared to a conventional de-icing system. This study showed that the energy-efficiency of an IPS is determined by the IPS method chosen. An efficient IPS needs, therefore, to be carefully engineered and controlled. There are a large number of parameters that influence IPS efficiency, which need to be balanced and adjusted depending on the icing conditions. This experimental work offered additional insights into the interrelation of these parameters and can be used for comparison with numerical methods. Under the assumption that the ice shedding mechanism for a de-icing case results from a force imbalance between the aerodynamical and viscous forces at the substrate surface—future work will be to couple these experiments with phase change and aero- dynamical simulations, the latter validated against simplified force measurements. This would increase the understanding of the de-icing mechanisms and help to further improve the electrothermal IPS efficiency.

Author Contributions: Conceptualization, R.H., T.A.J. and K.T.B.; methodology, R.H. and A.E.; software, M.C.N. and B.N.S.; validation, R.H.; formal analysis, R.H. and A.E.; investigation, R.H., A.E., M.C.N. and K.T.B.; resources, T.A.J. and K.T.B.; data curation, B.N.S.; writing—original draft preparation, R.H.; writing—review and editing, R.H., A.E., J.v.B. and T.A.J.; visualization, R.H. and A.E.; supervision, T.A.J.; project administration, R.H.; funding acquisition, A.E., J.v.B., T.A.J., K.T.B. All authors have read and agreed to the published version of the manuscript. Funding: This project has received funding from the Research Council of Norway under grant number 237906, Centre for Integrated Remote Sensing and Forecasting for Arctic Operations (CIRFA). The research was also funded by grant number 223254, Centre for autonomous marine operations and systems (NTNU AMOS). Further funding was received from the Norwegian Research Council FORNY, grant number 284649, from the Regionalt Forskningsfond Midt-Norge, grant number 285248, from the F.R.S.-FNRS mobility funding—grant number 35699386, and from the VKI Alumni Association Research Travel grant. A. Enache is supported by the Belgian F.R.S-FNRS with an FRIA fellowship—grant number 35484427. Institutional Review Board Statement: Not applicable. Data Availability Statement: Data available upon request. Acknowledgments: We thank Artur Piotr Zolich and Kim Lynge Sørensen from UBIQ Aerospace for supplying the D·ICE system and their general contribution to this work. Also, we thank Tuomas Jokela, Mikko Tiihonen, and Timo Karlsson from VTT—their support during the wind tunnel tests was an integral part of this work. Conflicts of Interest: The authors declare no conflict of interest.

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