EFFECT OF AIRFOIL GEOMETRY ON PERFORMANCE...

AIAA-2003-0728

American Institute of Aeronautics and Astronautics1

EFFECT OF AIRFOIL GEOMETRY ON PERFORMANCE WITHSIMULATED INTERCYCLE ICE ACCRETIONS

Andy P. Broeren* and Michael B. Bragg†

University of Illinois at Urbana-Champaign, Urbana, IL 61801

* Research Scientist, Dept. of Aeronautical and Astronautical Eng., Univ. of Illinois, Member AIAA.† Professor and Head, Dept. of Aeronautical and Astronautical Eng., Univ. of Illinois, Associate Fellow, AIAA.

Copyright © 2003 by Andy P. Broeren and Michael B. Bragg. Published by the American Institute of Aeronauticsand Astronautics, Inc., with permission.

Abstract

This paper presents the results of an experimentalstudy designed to evaluate the performance effects ofintercycle ice accretions on airfoils with differentgeometries. The intercycle ice accretions weresimulated using combinations of various size gritroughness. These simulations were tested on threeairfoils: NACA 23012, NACA 3415 and NLF 0414 at aReynolds number of 1.8×106 and a Mach number of0.18. Results from the NACA 23012 airfoil testsclosely matched those from a previous study, validatingthe ice-shape simulation method. This also showed thata simple geometric (chord-based) scaling of the ice wasappropriate. The simulated ice effect, in terms ofmaximum lift performance, was most severe for theNACA 23012 airfoil. The maximum lift coefficientswere in the range of 0.65 to 0.80 for the icedconfiguration compared to a clean value of 1.47 for theNACA 23012 airfoil at this Reynolds number. Incontrast, the maximum lift coefficients for the NLF0414 airfoil with the same ice simulations were in therange of 0.90 to 1.05, compared to a clean value of1.34. The results for the NACA 3415 with thesimulated intercycle ice shapes were between the othertwo airfoils.

Nomenclature

α Airfoil angle of attackαstall Stalling angle of attack, coincident with Cl,maxc Airfoil chord lengthCd Drag coefficientCl Lift coefficientCl,max Maximum lift coefficient, coincident with αstallCm Quarter-chord pitching-moment coefficientCp Pressure coefficientk Roughness height or thicknessMa Freestream Mach number

Re Freestream Reynolds number, based on chordt Airfoil maximum thicknessx Chordwise position along airfoily Normal position from airfoil chord line

Introduction

The cyclic operation of typical pneumatic aircraftdeicing systems leads to the formation of residual andintercycle ice accretions. For example, pneumaticboots are usually inflated and deflated at either one-minute or three-minute intervals, depending upon theseverity of icing. The ice accretion present on thedeicer surface just prior to its initial activation is the“preactivation” ice. After the system has been cycled asufficient number of times, the periodic activation andice accretion cycle reaches steady state. After steadystate has been reached, “intercycle” ice refers to the iceshape as it exists immediately before subsequentactivations of the deicer. This is not to be confusedwith “residual” ice which refers to any ice that remainson the surface immediately after the deicer operation.This paper addresses the aerodynamic performancepenalties associated with intercycle ice accretions forthree different airfoil geometries.

The characteristics of residual and intercycle iceaccretions have been the subject of several previousinvestigations. Shin and Bond1 analyzed the iceaccretions for several different deicing systems installedon a NACA 0012 airfoil model. The reported resultswere mainly for one minute cycling times and showedthat the deicers generally cleaned the leading edge,leaving little residual ice. The height of the intercycleice roughness, normalized by chord, varied fromapproximately k/c = 0.002 to 0.010, depending upon theicing condition (i.e., glaze or rime) and the type ofdeicer. Shin and Bond1 concluded that the intercycleice would have an effect on airfoil and wingperformance and that uniformly distributed roughness


may not be an appropriate simulation of the actualintercycle ice. No aerodynamic measurements wereperformed during the study.

Aerodynamic performance effects of residual andintercycle ice were included in some of the previousresearch. Albright et al.2 measured the drag coefficientbefore and after the operation of pneumatic deicer on aNACA 651-215 airfoil. The general results showed thatthe intercycle ice (before deicer operation) resulted in ahigher drag coefficient than the residual ice (after deiceroperation), both of which were higher than for the cleanairfoil. Similar research was carried out by Bowden3

for a NACA 0011 airfoil with a deicing boot. Bowden3

also showed how the lift coefficient decreased as icewas accreted between deicer operations and then howthe lift increased when the boot was operated and theice shed. While these results provided importantinsight into the performance effects of residual andintercycle ice accretions, a major shortcoming was thatthe data were acquired at fixed angle of attack.Therefore, the airfoil stall characteristics with these iceaccretions was not documented.

The effect of intercycle ice accretions on airfoilstalling characteristics was investigated as part of alarger study by Jackson and Bragg.4 Intercycle ice-shape tracings were recorded during tests on a 48-inchchord NLF 0414 airfoil model for one icing cloudcondition. Two-dimensional (i.e., no spanwisevariation) ice-shape simulations were produced fromice tracings and attached to the leading edge of an 18-inch chord NLF 0414 airfoil model. The performancedegradation in maximum lift was on the order of 30%.However, the effect of the three-dimensionalcharacteristics of the intercycle ice accretions was notdocumented.

A more recent investigation using high-fidelitysimulations of intercycle ice accretions showedsignificant airfoil performance degradations. Broeren,Addy and Bragg5 generated intercycle ice accretions ona 36-inch chord NACA 23012 airfoil model equippedwith pneumatic deicing boots. Molds were made ofselected intercycle ice accretions that were later used toproduce castings that were attached to the leading edgeof a 36-inch chord NACA 23012 airfoil model used foraerodynamic testing. The aerodynamic testing wasperformed in a pressure tunnel where the Reynolds wasvaried from 2.0×106 to 10.5×106 and the Mach numberwas varied from 0.10 to 0.28. Typical results showedthat the intercycle ice accretions resulted in a 60%decrease in maximum lift. The authors suggested thatthis large performance penalty was related to the ridge-like features of the intercycle accretions and thesensitivity of the NACA 23012 airfoil to this type of iceshape.

Studies by Lee6 and Lee et al.7-9 have shown thatthe clean airfoil geometry (i.e., pressure distribution)can influence the aerodynamic severity of a spanwise-ridge ice accretion. These studies were carried outusing a forward-facing quarter-round shape withnormalized height (k/c) of 0.0139. The quarter-roundwas uniform in size and shape across the model spanand was positioned at several different chordwiselocations. Two of the airfoils tested with this simulatedice ridge were the NACA 23012m and the NLF 0414.The NACA 23012m was a slightly modified version ofthe standard NACA 23012 and it had a 25% chordsimple flap. The NLF 0414 airfoil also had a 25%chord simple flap. Some key results are summarized inReference 8. The lowest Cl,max on the NACA 23012airfoil was 0.25 with the quarter-round located near x/c= 0.12. In contrast, the lowest Cl,max on the NLF 0414was 0.68 and this did not vary significantly with ice-shape location between x/c = 0.02 and x/c = 0.20. Thereason given for this difference was that the largeleading-edge suction pressures on the clean NACA23012 airfoil were prevented from forming in the icedcase, thus resulting in the large lift reductions. Theclean NLF-0414 had a relatively uniform chordwisepressure loading and this was not as significantlyaffected by the ice shape, thus resulting in the smallerlift reductions.

The purpose of this paper is to demonstrate howthe performance of three different airfoils was affectedby the same intercycle ice-accretion simulation. Theairfoils considered were the NACA 23012, NACA 3415and the NLF 0414. These have pressure distributionsand geometries that are quite different. The intercycleice shapes used in this investigation were adapted fromthose presented in Reference 5. The ice-shapesimulations were built up on the airfoil models usingvarious sizes of grit roughness. Performance data werethen acquired over a large angle of attack range,including stall at a Reynolds number of 1.8×106 and aMach number of 0.18.

Experimental Arrangement

All of the aerodynamic testing was carried out at theUniversity of Illinois Subsonic AerodynamicsLaboratory using the low-speed, low-turbulence windtunnel. This wind tunnel was of open-return type witha 3-ft by 4-ft rectangular working section and themaximum speed was approximately 235 ft/sec. Theairfoil models all had a 1.5-ft chord and spanned the 3-ft height of the test section. The NACA 23012 airfoilwas a single element model, while the NACA 3415 andNLF 0414 airfoils each had a 25% chord simple flap.The airfoil cross-sections are shown in Fig. 1. The flap


geometries have been eliminated from the figure forclarity. The NACA 3415 airfoil was the thickest (t/c =0.15) followed by the NLF 0414 (t/c = 0.14) and theNACA 23012 (t/c = 0.12).

The airfoil models were supported by a three-component force balance located below the test-section.The schematic drawing in Fig. 2 shows thisarrangement for one of the flapped models. The flapposition was controlled by a two-member linkagesystem driven by a linear traverse. The traverse wasmounted to the metric force plate of the balance. Forthe present series of tests, the flap angle was fixed atzero degrees and the flap gap was sealed on the lowersurface. All of the models had a dense distribution ofpressure taps located in a main chordwise row near themidspan location and a secondary spanwise row. Alsoshown in Fig. 2 is the traversable wake rake used toobtain the airfoil drag. Both the wake pressures andmodel surface pressures were measured with anelectronically scanned pressure (ESP) system. Moredetails about this experimental arrangement can befound in Lee.6

Fig. 1 The airfoils tested in this study (flapgeometry omitted for clarity).

Fig. 2 Schematic drawing of experimentalapparatus, after Lee.6

The lift coefficient (Cl) and pitching-momentcoefficient (Cm) taken about the quarter-chord werederived from both the force balance and the surface-pressure measurements. The agreement in the resultsfrom these two methods was very good. In this paperonly the values from the pressure measurements areshown for simplicity. The drag coefficient (Cd) wascalculated from the wake pressures using standardmomentum-deficit methods. All of these aerodynamiccoefficients and the angle of attack were corrected forwall interference effects using the methods of Rae andPope.10 The experimental uncertainty in thesecoefficients was also estimated using the methods ofKline and McClintock11 and Coleman and Steele12 for20:1 odds. Table 1 lists these uncertainties for a set ofpressure-derived coefficients, except for the angle ofattack, which was obtained directly from the forcebalance. The values were determined by Lee6 and Leeand Bragg9 for free-steam conditions of Re = 1.8×106,Ma = 0.18. The relative uncertainty in Cm seems largefor this example owing to the small reference value.For cases where the Cm values were larger, the absoluteuncertainty would be similar, therefore resulting in alower relative uncertainty.

Table 1 Estimated Experimental UncertaintiesAerodynamic

QuantityReference

ValueAbsolute

UncertaintyRelative

Uncertaintyα 5.00 ±0.02 ±0.40%Cp -0.712 ±0.0037 ±0.52%Cl 0.633 ±0.00211 ±0.33%Cm -0.0089 ±0.000349 ±3.90%Cd 0.0102 ±0.000143 ±1.40%

The intercycle ice-accretion simulations tested onthe three airfoils were adapted from those recorded andpresented in References 5 and 13. In that work, fouraccretions were selected for testing in the NASALangley LTPT. They were designated by the numbers290, 296, 312 and 322, after the icing run number.Tracings for each shape are shown in Fig. 3. The ice-shape castings used for the LTPT experiments inreferences 5 and 13 represented the highest-fidelitysimulation since the ice accretions had very irregularroughness sizes and spanwise variation. Despite this, acommon characteristic among the ice shapes wereridge-like features that were distinct formations in theroughness. Both the ice accretion and aerodynamictests were carried out using a 36-inch chord NACA23012 airfoil. Therefore, the simulations were scaleddown by a factor of 2 when simulated on the 18-inchchord models used in the present investigation. Therelative chordwise locations of ice-shape features (such

NACA 23012NACA 3415NLF 0414


as characteristic ridges) were also preserved for theseexperiments. For example, an ice feature located at x/c= 0.04 would be maintained for each airfoil, eventhough the distance measured along the surface wouldbe slightly different. The authors concede that thecharacter of the ice accretions may be different ifaccreted on each of the three airfoils separately.However, this did not compromise the main objectiveof this study which was to determine the effect of airfoilgeometry on the performance degradation. In fact, todetermine the effect of airfoil geometry on theaerodynamics with intercycle ice, the same icesimulation must be tested on each airfoil.

Fig. 3 Tracings of the four intercycle ice accretionsthat were simulated in this study, after Ref. 5, 13.

The intercycle ice simulations for the presentexperiments were constructed from various sizes ofloose grit roughness. This material was useful forrepresenting the surface irregularities of the actual iceaccretions. These roughnesses were applied to themodels using a substrate of double-sided tape. Thechordwise distribution of ice thickness was controlledby using roughness of different sizes. Ridge-likefeatures were built-up at the appropriate chordwiselocation by layering the roughness. A spray adhesivewas used to hold the roughness in place. Spanwisevariation was also incorporated into the simulations, butwith less fidelity as the chordwise variations. Figure 4

x/c

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shows a comparison of an actual ice shape (on a 36-inch chord model) with the roughness simulation (on an18-inch chord model). The figure also shows how theice-shape simulation was place on either side of thepressure tap row so that the approximate pressuredistribution could be measured. The lift and pitchingmoment determined from integration of the pressuredistribution agreed very well with the force-balancedata.

In addition to the intercycle ice simulations, theairfoil models were also tested with standard roughnessin the form of 80- and 150-grit paper-backed garnetsandpaper. The purpose of these tests was to provide avery repeatable form of simulated ice roughness thatcould be identically duplicated on each of the airfoilmodels. As shown in Table 2, these grit sizesapproximately represented the chord-length scaledequivalent of the 40- and 80-grit sizes that were testedin the LTPT.5,13 The roughness heights listed in thetable do not include the thickness of the paper backingthat was approximately one and a half times as large asthe roughness itself. Also, 0.003-inch thick double-sided tape was used to attach the sandpaper to themodel. The surface extent of the sandpaper roughnesswas x/c = 0.010 on the lower surface to x/c = 0.07 onthe upper surface and was cut out around the pressureorifices.

Fig. 4 Comparison between ice shape 290 (top) withthe roughness simulation.

Table 2 Comparison of SandpaperRoughness Heights

SandpaperGrit

Number

RoughnessHeight*(k, in.)

NormalizedHeight (k/c)for c = 36 in.

NormalizedHeight (k/c)for c = 18 in.

40 0.0205 0.00057 0.0011480 0.0083 0.00023 0.00046

150 0.0041 0.00011 0.00023*based on nominal size of commercial carborundum10

Results

Clean Airfoil ResultsThe three airfoils tested in this study were selected

because their clean aerodynamic characteristics weresubstantially different. This is illustrated in theperformance plot of Fig. 5 for data acquired at Re =1.8×106 and Ma = 0.18. The lift data show that theNACA 23012 airfoil had the highest Cl,max of about1.47. This was followed by the NACA 3415 (Cl,max =1.37) and the NLF 0414 (Cl,max = 1.34). The lift curvefor the NLF 0414 airfoil exhibited a distinct change inslope near α = 2 deg. This resulted from trailing-edgeseparation aft of about x/c = 0.70 on the airfoil uppersurface. The pitching-moment data show that theNACA 23012 had the lowest values, followed by theNACA 3415 and then the NLF 0414 airfoil. Thisindicated that the NACA 23012 had the least amount ofpositive camber. The NLF 0414 had the lowest drag inthe range of –3 < α < 3 deg., since it was a laminarflow airfoil. For angles of attack greater than 3 deg.there was significant flow separation at this Reynoldsnumber aft of about x/c = 0.70 on the upper surface,which resulted in higher drag than the two NACAairfoils.

The differences in these airfoils are even moreevident in the pressures distributions. An example isshown in Fig. 6 for a similar, nominal lift coefficient of0.6. This corresponded to different angles of attack foreach airfoil as indicated by Fig. 5. The pressuredistributions for the NACA 3415 and NLF 0414 airfoilsshow the expected discontinuities near x/c = 0.75 due tothe flap gap. The NACA 23012 airfoil had a very largesuction peak (with Cp,min = –1.6 ) centered near x/c =0.06. There was a severe pressure recovery (with verylarge adverse pressure gradient) from x/c = 0.08 to 0.22.The pressure recovery was more gradual downstream ofthis location and extended to the trailing edge. TheNACA 3415 airfoil had a pressure distribution that wasquite different. A large suction peak was not present onthis airfoil at this angle of attack, with a Cp,min value of–1.2 located near x/c = 0.18. The pressure recoveryhere was more gradual since a large suction peak was


not present. The pressure gradient was nearly constantfrom x/c = 0.25 to the trailing edge. The NLF 0414airfoil had a nearly constant suction pressure betweenx/c = 0.04 to x/c = 0.72. Downstream of this location,there was a very severe adverse gradient that led to thetrailing-edge flow separation describe above.

Fig. 5 Comparison of the clean airfoil performanceat Re = 1.8××××106, Ma = 0.18.

Fig. 6 Comparison of clean airfoil pressuredistributions at approximately matched Cl = 0.6.

The NACA 23012 airfoil was tested in the cleanconfiguration by Broeren et al.5,13 in the Low-Turbulence Pressure Tunnel (LTPT) at NASA Langleyand at the University of Illinois for the present tests.These results are plotted in Fig. 7 for closely matchReynolds and Mach number conditions. The data showgood agreement in the lift-curve until the maximum liftregion where the LTPT data had a slightly higher Cl,maxvalue. This discrepancy is small and may even havebeen attributable to the small difference in Reynoldsnumber. The present pitching-moment data wereslightly more nonlinear. Agreement in the drag valuesare good except at certain odd angles of attack (e.g., -1and 5 deg). Comparison here is difficult because therewere no wake-survey drag data available in the LTPTdata set for odd numbered angles of attack. Despitethese minor discrepancies, the cross-facilitycomparisons are reasonable.

Iced-Airfoil Results for the NACA 23012 AirfoilThe effect of the intercycle ice shapes on the

performance of the NACA 23012 airfoil wasthoroughly discussed in References 5 and 13. Some ofthat data are shown again here in order to validate theaccuracy of the built-up roughness simulations used forthe present series of tests. The LTPT data wereacquired using castings of the ice shapes and this wasconsidered to be the most reliable method ofsimulation. The performance data are summarized inFig. 8, for Re = 2.0×106 and Ma = 0.10 (except for theclean data at Re = 2.0×106 and Ma = 0.21). Theintercycle ice shapes resulted in large performance

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Fig. 7 Comparison of the present NACA 23012airfoil performance data with LTPT results fromBroeren et al.5,13

penalties, especially in terms of maximum liftdegradation. Three of the four intercycle shapes causedCl,max values in the range of 0.65 to 0.78 and stall anglesin the range of 8 to 10 deg. The remaining ice shape322 had a slightly higher Cl,max value of about 0.90.The intercycle shapes produced a stronger angle ofattack dependence in the pitching moment. Theminimum drag values for three of the four shapes was athree-fold increase from the clean case. The airfoil withice shape 322 had a lower drag than the others, likelybecause this ice shape was smaller and smoother.

Fig. 8 Effect of intercycle ice-casting simulations onNACA 23012 airfoil performance. LTPT data afterBroeren et al.5,13 (Clean data at Re = 2.0××××106, Ma =0.21; iced data at Re = 2.0××××106, Ma = 0.10)

These ice shapes were simulated at half-scale andwere tested at Illinois. These data are shown in Fig. 9for Re = 1.8×106 and Ma = 0.18. The performance datahere were very similar to the results in Fig. 8. Themaximum lift coefficient for three of the four ice shapeswas in the range of 0.65 to 0.80, with ice shape 322resulting in a Cl,max of 0.90. The stalling angles ofattack were reduced to 7 to 9 deg., except for ice shape322 which had an αstall closer to 10 deg. An iced-airfoilCl,max value of 0.70, amounted to a 52% reduction fromthe clean value of 1.47. The pitching moment data in

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CleanIce Shape 290Ice Shape 296Ice Shape 312Ice Shape 322

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Fig. 9 mimic the angle of attack dependence exhibitedin Fig. 8. The most variation in the results from thehalf-scale intercycle ice simulations was in the dragdata. The drag values for the airfoil with the ice-shape290 simulation tested at Illinois was higher than the ice-shape casting tested at LTPT. On the other hand, thedrag values for ice shape 312 were slightly lower forthe half-scale simulation. However, given the randomnature of the ice-shape features and the spanwise

Fig. 9 Effect of intercycle ice simulations on NACA23012 airfoil performance, data from the presentstudy, at Re = 1.8××××106, Ma = 0.18.

variation in drag, this comparison in the drag valuescould be considered quite good.

A detailed comparison of the performance data isshown in Fig. 10 for two of the four intercycle iceshapes. Here again the lift data show that the half-scalesimulations were very effective in reproducing the lift-performance degradations caused by the high-fidelitycastings tested at LTPT. Agreement in the pitching-moment data was not as good, but the general trends

Fig. 10 Comparison of present and LTPTperformance data for the NACA 23012 withsimulated intercycle ice shapes. Present data at Re =1.8××××106, Ma = 0.18 and LTPT data at Re = 2.0××××106,Ma = 0.10, after Broeren et al.5,13

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Ice Shape 296, PresentIce Shape 296, LTPTIce Shape 322, PresentIce Shape 322, LTPT

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were certainly represented in the present data. Theagreement in the drag values was reasonable. Thesedata, taken as a whole indicate that the methods used tosimulate the intercycle shapes on the 18-inch chordmodels were valid. The data also imply that a simplegeometric scaling is appropriate for ice features of thistype and size.

In addition to the intercycle ice-shape simulationtests, standard roughness was also applied to the airfoilleading edge. For tests on the 36-inch chord LTPTmodel, 40- and 80-grit sandpaper was used. Thegeometrically scaled equivalent sized sandpaper for the18-inch chord model used for the present tests wasnominally 80- and 150-grit. The performance effects ofthis uniform roughness are summarized in Fig. 11 forthe NACA 23012 airfoil at Re = 1.8×106 and Ma =0.18. The data show that the lift degradation caused bythe uniform roughness was very significant with Cl,maxvalues near 1.10. However, these were substantiallyhigher maximum lift values than for the airfoil with theintercycle ice simulations. Also noteworthy is the factthat there was very little difference in the liftperformance between the 80- and 150-grit sandpaper,despite the almost two-fold difference in roughnessheight. The effect of the uniform roughness on thepitching moment was similar to that caused by the ice-shape simulations. The drag values were increasedfrom the clean case by less than a factor of two in thelinear range of the lift curve.

The geometric size scaling of the sandpaperroughness height was also validated by comparisonwith the LTPT data in References 5 and 13. Theseresults are given in Fig. 12 for the 40/80-grit case. Themaximum lift values from both tests were nearlyidentical, while the stalling angle of attack was onedegree lower for the present data. This angle of attackdiscrepancy is small and could be less than one degreeif the data were acquired in smaller increments.Agreement in the pitching moment variation with angleof attack was not quite as good between the two tests.Similarly, agreement in the measured drag values wasvery good, except over the range of –2 < α < 4 deg.Despite these small discrepancies the data show that thegeometric scaling of the roughness height was alsoappropriate for this case.

Iced-Airfoil Results for the NLF 0414 and NACA 3415The intercycle ice-shape simulations were tested on

the NLF 0414 and NACA 3415 airfoils to gauge theirsensitivity to this class of ice shape. Significantdegradations in performance were also observed. Theresults for the NLF 0414 airfoil are shown in Fig. 13.The stall behavior with the ice simulations was similarto the NACA 23012 data in that there was a large range

of Cl,max from 0.90 to 1.05. The same ice shape 322also resulted in the highest Cl,max. These iced-airfoil liftcoefficients were significantly higher than the range forthe ice simulations on the NACA 23012 airfoil whichwas 0.65 to 0.90. For example, an iced airfoil Cl,maxvalue of 0.95, constituted a 29% reduction from theclean value of 1.34. This is consistent with thatreported by Jackson and Bragg4 for other intercycle icesimulations tested on the NLF 0414 airfoil. This valueis nearly half that of the 52% reduction in maximum liftobserved for the NACA 23012 airfoil.

Fig. 11 Effect of 80- and 150-grit sandpaperroughness on NACA 23012 airfoil performance,data from the present study at Re = 1.8××××106, Ma =0.18.

α (deg.)-10 -8 -6 -4 -2 0 2 4 6 8 10 12 14 16 18

-0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

1.2

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1.6

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-0.08

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0.14

Clean80-grit sandpaper150-grit sandpaperIce Shape 290

Cl Cm

α (deg.)-10 -8 -6 -4 -2 0 2 4 6 8 10 12 14 16 18

0.00

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

Cd


Fig. 12 Comparison of present and LTPTperformance data for the NACA 23012 withsandpaper roughness. Present data at Re = 1.8××××106,Ma = 0.18 and LTPT data at Re = 2.0××××106, Ma =0.10, after Broeren et al.5,13

A key difference in lift performance with the icesimulations between the two airfoils was observed forlift coefficients in the range of 0.0 to 0.6. In this range,the simulated ice had a more severe effect for the NLF0414 airfoil than for the NACA 23012, as there werelarger differences between the clean and iced liftcoefficients. The effect of the simulated ice on the NLF0414 airfoil pitching moment and drag were alsosimilar to the effects on the NACA 23012 airfoilperformance.

Fig. 13 Effect of intercycle ice simulations on NLF0414 airfoil performance at Re = 1.8××××106, Ma = 0.18.

The performance of the NACA 3415 airfoil withthe intercycle ice simulations was similar to the NLF0414 airfoil as depicted in Fig. 14. In this case, therewas less variation in maximum lift coefficient with thedifferent ice shapes. The Cl,max values ranged fromabout 0.85 to 0.95. An iced-airfoil Cl,max of 0.90constituted a 34% reduction from the clean value of1.37. This reduction is more similar to the NLF 0414airfoil performance than for the NACA 23012 with thesame simulated ice shapes. The lift penalties caused by

α (deg.)-10 -8 -6 -4 -2 0 2 4 6 8 10 12 14 16 18

-0.8

-0.6

-0.4

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0.0

0.2

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1.0

1.2

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0.12

80-grit sandpaper, Present40-grit sandpaper, LTPT

Cl Cm

α (deg.)-10 -8 -6 -4 -2 0 2 4 6 8 10 12 14 16 18

0.00

0.01

0.02

0.03

0.04

0.05

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0.08

Cd

α (deg.)-10 -8 -6 -4 -2 0 2 4 6 8 10 12 14 16 18

-0.8

-0.6

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-0.2

0.0

0.2

0.4

0.6

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1.0

1.2

1.4

1.6

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0.00

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0.14


Cl Cm

α (deg.)-10 -8 -6 -4 -2 0 2 4 6 8 10 12 14 16 18

0.00

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

Cd


Fig. 14 Effect of intercycle ice simulations on NACA3415 airfoil performance at Re = 1.8××××106, Ma = 0.18.

the ice shapes for the NACA 3415 airfoil were verysimilar to the NLF 0414 airfoil in the range of Cl = 0.0to 0.6. The effects on pitching moment and drag werealso similar to the other airfoils.

The NLF 0414 and NACA 3415 airfoils were alsotested with the uniform roughness applied to the leadingedge of the airfoils. Results for the NLF 0414 airfoilare shown in Fig. 15. As with the intercycle icesimulations, the sandpaper roughness caused

Fig. 15 Effect of 80- and 150-grit sandpaperroughness on NLF 0414 airfoil performance at Re =1.8××××106, Ma = 0.18.

significantly smaller reductions in maximum lift fromthe clean value for this airfoil as compared to theNACA 23012 airfoil. However, the roughness alteredthe stalling characteristics such that there was asignificant drop in lift beyond Cl,max. This change install behavior from the clean case with the roughnesspresent was not observed for the other two airfoils. Theperformance effects of the various intercycle ice shapesand standard roughness on the three airfoils is discussedin more detail below.

α (deg.)-10 -8 -6 -4 -2 0 2 4 6 8 10 12 14 16 18

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Cl Cm

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Cd

α (deg.)-10 -8 -6 -4 -2 0 2 4 6 8 10 12 14 16 18

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Clean80-grit sandpaper150-grit sandpaperIce Shape 290

Cl Cm

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0.00

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Discussion

Airfoil and Ice-Shape Geometry EffectsThe effect of the intercycle ice simulations on

maximum lift is summarized in Fig. 16 for the threeairfoils tested. The chart reiterates the preceding resultsthat these ice shapes had the most severe effect, interms of maximum lift degradation, for the NACA23012 airfoil. On the other hand, the maximum liftvalues for the NLF 0414 airfoil were least affected bythe intercycle ice shapes. The NACA 3415 airfoilresults were similar to the NLF 0414 data for ice shapes

Fig. 16 Summary of intercycle ice simulations andsandpaper roughness effect on maximum lift for thethree airfoils tested.

290 and 296. For the remaining two ice shapes, theNACA 3415 Cl,max values were between the NACA23012 and NLF 0414 results. The uniform roughnessresults were somewhat more mixed, given that theNACA 3415 airfoil had the lowest Cl,max values for eachcase. These values were very comparable to the NACA23012 airfoil results and both were substantially lessthan the NLF 0414 airfoil with standard roughness.

The large reductions in maximum lift caused by theintercycle ice accretions on the NACA 23012 waslikely related to the pressure distribution on the cleanairfoil. Since the clean NACA 23012 airfoil had largesuction pressures near the leading edge, even formoderate lift coefficients, the airfoil was particularlysensitive to protuberances in this region. This effect isillustrated in Fig. 17 which compares the clean and icedpressure distributions on the NACA 23012 airfoil. Thedata are for α ≈ 8.3 deg. which was close to maximumlift for the airfoil with ice shape 296. The plot showshow the ice shape reduced the suction pressures on theupper surface over the first 20% of the chord. Therewas also significant deviation of the lower-surfacepressures. This is contrasted with the effect of theuniformly distributed roughness shown in Fig. 18. Thedata are for α ≈ 11.4 deg. which was close to themaximum lift for the NACA 23012 airfoil with the 80-grit sandpaper over the leading edge. The pressuredistribution shows that there was significant deviationsonly in the region of the minimum pressure peak.

This contrast between the effect of the intercycleice shapes versus the uniform roughness was observedfor all of the airfoils tested. A side-by-side comparisonof the intercycle ice accretions and the sandpaperroughness reveals that their geometries were quitedifferent. The sandpaper had a very uniform array ofroughness, whereas the intercycle ice accretionscontained ridge-like features that were not uniform,especially in the spanwise direction. Another factorwas that even the larger uniform roughness (80-grit)was considerably smaller in size than the actualaccretions. For example, the nominal height (ignoringthe larger ridge-like features) of ice shape 296 was onthe order of k/c = 0.0050 (cf. Fig. 3). This was morethan a factor of ten larger than the height of the 80-gritsandpaper at k/c = 0.00046 (cf. Table 2). These resultstend to support the conclusions of Shin and Bond1 thatuniform roughness may not be an adequate method forsimulating intercycle ice. However, the present studydid not consider uniform roughness that wasappropriately sized to the nominal height of theintercycle accretions.

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6


Cl,max

Clean

80-grit Sandpaper

Ice Shape 290

Ice Shape 296

Ice Shape 312

Ice Shape 322

150-grit Sandpaper


Fig. 17 Comparison of clean and iced pressuredistributions for the NACA 23012 airfoil at matchedangle of attack near stall in the iced configuration.

Fig. 18 Comparison of clean and sandpaperroughness pressure distributions for the NACA23012 airfoil at matched angle of attack near stall inthe sandpaper roughness configuration.

The ridge-like features of the intercycle ice shapeslikely played a strong role in the resulting performancedegradation. The sensitivity of the NACA 23012 airfoilto ridge-type protuberances was investigated by Lee6

and Lee et al.7-9 using forward-facing quarter-roundshaving heights of k/c = 0.0083 to 0.0139. The authors

found that the maximum lift decreased significantly asthe quarter-round was located from the leadingdownstream to about x/c = 0.12. Broeren et al.5 notedthat the ridge-like features present in the intercycle iceaccretions were found to vary in height from k/c =0.0091 to 0.0138 and the location on the airfoil surfacevaried from x/c = 0.00 to 0.06. A detailed comparisonwith the data of Lee6 and Lee et al.6-8 showed similartrends, but that the intercycle ice accretions had a lesssevere effect on maximum lift. This was attributed tothe spanwise breaks or gaps in the intercycle accretionsthat were not present in the quarter-round testing.

It was noted above that the intercycle icesimulations resulted in more severe lift penalties for theNACA 3415 and NLF 0414 airfoils in the range of Cl =0.0 to 0.6. This effect is illustrated in Figs. 19 and 20.The pressure distributions are plotted for the NACA23012 and the NLF 0414 airfoils at a clean Cl ≈ 0.5 andwith the ice shape 296 simulation at the same angle ofattack. For the NACA 23012 airfoil (Fig. 19), the icesimulation did not significantly alter the clean pressuredistribution. The iced-airfoil lift coefficient of 0.47 wasslightly reduced from the clean value of 0.54. Therewas a higher suction peak owing to the local flowacceleration around the ice roughness. This wasfollowed by a more severe adverse pressure gradient upto x/c ≈ 0.25, where the clean and iced pressuredistributions were very similar. There was some smalldivergence of the trailing-edge pressure that wasperhaps indicative of boundary-layer separation. In thecase of the NLF 0414 airfoil (Fig. 20), small suctionpeaks caused by the ice shape were evident on both theupper and lower surface. This resulted in significantlylower suction pressures on the upper surface from x/c ≈0.15 to 0.75. In this case the iced-airfoil Cl was 0.35compared to the clean value of 0.48 for the same angleof attack. Very similar results were observed for theNACA 3415 airfoil. Therefore, the lift of airfoilshaving a more uniform pressure loading may have agreater sensitivity to this type of ice accretion thanforward-loaded sections like the NACA 23012 over therange of low to moderate lift coefficients.

It is also noteworthy that this effect on the iced-airfoil pressure distribution continues for higher liftcoefficients leading up to the stall. This trend isillustrated in Fig. 21 for the NACA 3415 airfoil at α ≈ 8deg. This angle of attack is very close to maximum liftfor the airfoil with the ice shape 296 simulation. Thetrend here is analogous to that in Fig. 20 for the NLF0414 airfoil at lower lift coefficient. The suction peakwith the ice shape present is reduced somewhat, but theupper surface suction pressures are lower than in theclean case from x/c ≈ 0.12 to 0.75. Contrast this withthe NACA 23012 results shown in Fig. 17, where the

x/c0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

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Clean, α = 8.3 deg. ( = 1.00)Ice Shape 296, α = 8.2 deg. ( = 0.69)

Cp

ClCl

x/c0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

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Clean, α = 11.4 deg. ( = 1.26)80-grit sandpaper, α = 11.3 deg. ( = 1.10)

Cp

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suction pressures were reduced only over the first 20%of the chord. These data confirm that more forward-loaded airfoils are likely to be more sensitive, in termsof maximum lift degradation, to ice accretions in theleading-edge region.

Fig. 19 Comparison of clean and iced pressuredistributions for the NACA 23012 airfoil at matchedangle of attack based on a clean Cl ≈≈≈≈ 0.5.

Fig. 20 Comparison of clean and iced pressuredistributions for the NLF 0414 airfoil at matchedangle of attack based upon a clean Cl ≈≈≈≈ 0.5.

Fig. 21 Comparison of clean and iced pressuredistributions for the NACA 3415 airfoil at matchedangle of attack near stall in the iced configuration.

Comment on Reynolds Number EffectsThe data from the Illinois wind tunnel presented

here were all acquired at Re = 1.8×106 and Ma = 0.18.Broeren et al.5 investigated the effects of Reynoldsnumber and Mach number over a range of Re = 2.0×106to 10.5×106 and Ma = 0.10 to 0.28. They found thatthere was only a small (less than 0.05 in Cl) increase inmaximum lift coefficient between Re = 2.0×106 and Re= 3.5×106 for the NACA 23012 airfoil with each of thefour intercycle ice accretions. The 40- and 80-gritsandpaper cases had about 0.10 increase in Cl. Therewas virtually no change in Cl,max for Reynolds numberslarger than 3.5×106. Similar results were reported byAddy and Chung14 for tests of glaze-ice simulations onan NLF 0414 airfoil. Also, a large glaze-ice simulationwas tested on a multi-element super-critical airfoil byMorgan et al.15 Performance measurements werecarried out on the model in the cruise configurationover a Reynolds number range of 3.0×106 to 12.0×106

and only very minor changes in maximum lift wereobserved for the iced-airfoil case. Therefore, the resultsof the present study, should be applicable for higherReynolds numbers. The caveat is that the cleanmaximum lift performance of the airfoils considered inthis study is Reynolds number dependent. For example,Broeren et al.5 reported a clean Cl,max value for theNACA 23012 at Re = 10.5×106 and Ma = 0.21 of 1.80.This is significantly higher than the value of 1.47reported here for Re = 1.8×106 and Ma = 0.18. Giventhat an intercycle iced-airfoil maximum lift coefficient

x/c0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

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x/c0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

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Clean, α = 0 deg. ( = 0.48)Ice Shape 296, α = 0 deg. ( = 0.35)

Cp

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x/c0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

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of 0.70 would be relatively invariant over this Reynoldsnumber range, the performance penalty would be 61%,instead of the 52% value mentioned above. However,clean maximum lift values for these and other airfoilsare more readily obtainable from historical data and/orcomputational methods.

Summary and Conclusions

The purpose of this paper was to evaluate theaerodynamic performance effects of intercycle iceaccretions on the NACA 23012, NACA 3415 and NLF0414 airfoils. These sections have geometries andpressure distributions that are substantially different.The intercycle ice shapes used in this study wereadapted from an actual icing test on a 36-inch chordNACA 23012 airfoil model. The ice was simulatedusing various sizes of grit roughness and geometricallyscaled to the 18-inch chord models used for these tests.Performance data were acquired for each of the airfoilswith these ice simulations over a large angle of attackrange at a Reynolds number of 1.8×106 and a Machnumber of 0.18. Uniform roughness in the form of 80-and 150-grit sandpaper was also applied to the airfoilleading edge and tested.

The results for the ice simulations tested on theNACA 23012 airfoil were validated against previoustesting of a 36-inch chord model that used castings ofthe actual ice shapes. Agreement between these testswas excellent and showed that the ice could besuccessfully scaled down by the ratio of the chordlengths and simulated with relatively simple methods.As shown in previous studies, the performancedegradation resulting from the intercycle icesimulations was severe. For the NACA 23012 airfoil,the maximum lift coefficients were reduced to a rangeof 0.65 to 0.80 from a clean value of 1.47. Performanceeffects, in terms of maximum lift coefficient, were lesssevere for the other two airfoils. The NLF 0414 airfoilmaximum lift coefficient was reduced to a range of 0.90to 1.05 from a clean value of 1.34 with the intercycleice simulations. These same simulations reduced themaximum lift coefficient of the NACA 3415 airfoil to arange of 0.85 to 0.95 from a clean value of 1.37.

The severe reductions in maximum lift for theNACA 23012 airfoil was related to the clean pressuredistribution. This airfoil generated most of its lift fromlarge suction pressures near the leading edge. Thepresence of the ice shape prevented the formation ofthese large suction pressures and hence the lift wassubstantially reduced. In the case of the other twoairfoils, the pressure loading was distributed moreuniformly along the chord and resulting maximum liftpenalties were smaller. The lift penalties at low to

moderate lift coefficients for the NLF 0414 and theNACA 3415 airfoils were more severe than for theNACA 23012. The former two airfoils’ moderatepressure loading was more adversely affected at lowerlift coefficients by the presence of the ice than for thefront-loaded, NACA 23012 airfoil.

Tests conducted with the uniform roughness on theNACA 23012 airfoil were also validated againstprevious testing on a larger-chord model. These resultsalso showed that these roughness heights could simplybe scaled by the ratio of the chord lengths. Theperformance degradation resulting from the standardroughness was substantially less than that caused by theintercycle ice simulations. These effects, in terms ofmaximum lift, were more severe for the NACA 23012and NACA 3415 airfoils, while the NLF 0414 was leastaffected. The sandpaper roughness was more than tentimes smaller than the nominal height of the intercycleice shapes. It is unclear if uniform roughness withsimilar heights would be an adequate ice simulation.Future testing should be conducted to determine whatlevel of simulation provides acceptable aerodynamicresults.

Acknowledgements

This work was supported in part under FAA grantDTFA MB 96-6-023 with Jim Riley as technicalmonitor. The authors wish to acknowledge Sam Lee,Ramesh Arakoni and Chris LaMarre of the Universityof Illinois for assistance in acquiring, processing andanalyzing the data.

References

1. Shin, J., and Bond, T.H., “Surface Roughness Due toResidual Ice in the Use of Low Power DeicingSystems,” NASA TM-105971, AIAA Paper 93-0031,Jan. 1993.

2. Albright, A.E., Kohlman, D.L., Schweikhard, W.G.,and Evanich, P., “Evaluation of a Pneumatic BootDeicing System on a General Aviation Wing Model,”NASA TM-82363, June 1981.

3. Bowden, D.T., “Effect of Pneumatic De-Icers andIce Formations on Aerodynamic Characteristics of anAirfoil,” NACA TN-3564, Feb. 1956.

4. Jackson, D.G., and Bragg, M.B., “AerodynamicPerformance of an NLF Airfoil with Simulated Ice,”AIAA Paper 99-0373, Jan. 1999.


5. Broeren, A.P., Addy, H.E., Jr., and Bragg, M.B.,“Effect of Intercycle Ice Accretions on AirfoilPerformance,” AIAA Paper 2002-0240, Jan. 2002.

6. Lee, S., “Effect of Supercooled Large Droplet Icingon Airfoil Aerodynamics,” Ph.D. Dissertation, Dept. ofAeronautical and Astronautical Engineering, Univ. ofIllinois, Urbana, IL, 2001.

7. Lee, S., Kim, H.S., and Bragg, M.B., “Investigationof Factors that Influence Iced-Airfoil Aerodynamics,”AIAA Paper 2000-0099, Jan. 2000.

8. Lee, S., and Bragg, M.B., “Effect of Simulated-Spanwise Ice Shapes on Airfoils: ExperimentalInvestigation,” AIAA Paper 99-0092, Jan. 1999.

9. Lee, S., and Bragg, M.B., “ExperimentalInvestigation of Simulated Large-Droplet Ice Shapes onAirfoil Aerodynamics,” Journal of Aircraft, Vol. 36,No. 5, Sept.-Oct., 1999, pp. 844-850.

10. Rae. W.H., and Pope, A., Low-Speed Wind TunnelTesting, Wiley, New York, 1984, pp. 349-362, 449.

11. Kline, S.J., and McClintock, F.A., “DescribingUncertainties in Single-Sample Experiments,”Mechanical Engineering, Vol. 75, Jan. 1953, pp. 3-8.

12. Coleman, H.W., and Steele, W.G., Experimentationand Uncertainty Analysis for Engineers, John Wileyand Sons, New York, 1989, pp. 40-118.

13. Broeren, A.P., and Bragg, M.B., “Effect ofResidual and Intercycle Ice Accretions on AirfoilPerformance,” Report. No. DOT/FAA/AR-02/68, May2002.

14. Addy, H.E., Jr., and Chung, J.J., “A Wind TunnelStudy of Icing Effects on a Natural Laminar FlowAirfoil,” AIAA Paper 2000-0095, Jan. 2000.

15. Morgan, H.L., Ferris, J.C., and McGhee, R.J., “AStudy of High-Lift Airfoils in the Langley Low-Turbulence Pressure Tunnel,” NASA TM 89125, July1987.

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