Wind Tunnel Aerodynamic Characteristics of a Transport-Type Airfoil

23 Pages·1997·0.55 MB·English

NASA Technical Paper 3184
Wind Tunnel Aerodynamic Characteristics of a
Transport-Type Airfoil in a Simulated Heavy
Rain Environment
Gaudy M. Bezos, R. Earl Dunham, Jr., Garl L. Gentry, Jr., and W. Edward Melson, Jr.
August 1992
Summary Until the late 1970’s the recognition of weather-
relatedsafetyhazardstoaircraftperformance during
A wind tunnel investigation was conducted with take-o(cid:11) and landing operations included lightning,
a cambered airfoil representative of typical com- icing, hail, low-altitude wind shear, and microburst
mercial transport wing sections in the Langley 14- phenomena. Since 1971, research has been directed
by 22-Foot Subsonic Tunnel to determine the aero- at determining the nature and characteristics of
dynamic penalty associated with a simulated heavy the wind shear/microburst phenomenon. In 1977,
rain encounter. The model was comprised of an the Federal Aviation Administration (FAA) con-
NACA64-210 airfoil section with a chord of2.5 ft, a ducted a study on aircraft accidents and incidents
span of 8ft, and wasmounted on the tunnel center- from 1964 to 1976 in which low-altitude wind shear
line between two large endplates. The rain simula- could have been a contributing factor (ref. 1). The
tion system manifold, which was located 10 chord study, whichidenti(cid:12)ed 25cases(23approach orland-
lengthsupstream ofthe model, produced liquid wa- ing and 2 take-o(cid:11)) involving large aircraft (gross
ter contents ranging from 16 to 46 g/m3. Aero- weightsinexcessof12500 lb),indicatedthat10cases
dynamic measurements in and out of the simulated had occurred in arain environment, 5 of which were
rain environment were obtained for dynamic pres- classi(cid:12)ed asintense orheavy rain encounters. These
sures of 30 and 50 psf and an angle-of-attack range (cid:12)ndings led to the consideration of heavy rain, i.e.,
(cid:14) (cid:14) (cid:14) (cid:14)
of0 to20 forthecruisecon(cid:12)guration and 4 to20 high-intensity, short-duration rain, asbeingapoten-
for the landing con(cid:12)guration (leading-edge slat and tial weather-related aircraft safety hazard. Speci(cid:12)-
trailing-edge double-slotted (cid:13)ap). Both con(cid:12)gura- cally, a pilot of an aircraft encountering low-altitude
tions experienced signi(cid:12)cant losses in maximum lift wind shear during take-o(cid:11) or landing operations
capability, increases in drag for a given lift condi- would depend upon \dry"aircraftperformance mar-
tion,and aprogressive decreaseintheliftcurveslope gins. Ifthewind shearenvironment isimmersed in a
at both dynamic pressures as the liquid water con- severe convective rainstorm, the actual performance
tent increased. The results obtained on the landing margins may be signi(cid:12)cantly reduced. A determi-
con(cid:12)guration also indicated a progressive decrease nation of the e(cid:11)ect of rain on aircraft performance
in the angle of attack at which maximum lift oc- is required to provide safe piloting procedures for a
curred and an increase in the slope of the pitching- wind shearencounter in a severe rain environment.
moment curve asthe liquid water content increased.
Accompanying the reduction in the stall angle ofat-
The rain environment present in convective
tack was the general (cid:13)attening ofthe lift curve past
stormshasbeen ofinterestto the meteorologicaland
stall as the liquid water contentwas increased. The
aviation communities for many years. The parame-
NACA64-210 data indicated thatthe severity ofthe
tersused tocharacterize rain aretherainfallrateand
raine(cid:11)ectappearstobecon(cid:12)guration-dependent and
the liquid water content. At ground level, rainfall
ismostsevere forhigh-liftcon(cid:12)guration airfoilswith
rate, the rate at which rain falls (usually expressed
leading- and trailing-edge devicescon(cid:12)gured forland-
in eitherin/hr or mm/hr)isgenerally used to char-
ing or take-o(cid:11) operations. Experiments were also
acterize a rain event. For airborne measurements,
conducted toinvestigatethesensitivityoftestresults
the relevant parameter is the liquid water content,
tothe e(cid:11)ects ofwatersurface tension by introducing
which isthe massofliquid watercontained in aunit
a surface-tension reducing agentinto the rain spray.
volume ofairand is usually expressed in grams per
The reduction in the surface tension of water by a cubicmeterofair(g/m3). The relationship between
factorof2did notsigni(cid:12)cantly alter the levelofper-
liquid water contentand rainfall rate isuniquely de-
formance losses for the landing con(cid:12)guration tested.
pendent on the type ofstormand the intensity level
of the storm as detailed in appendix A. Measure-
Introduction mentsmadeabove ground byairplanesinstrumented
foratmospheric research have shown thatconvective
This investigation is part of a broad National stormscan contain localized regionsofhigh-intensity
Aeronautics and Space Administration (NASA) re- rain. Astheselocalizedregionsofhigh-intensity rain
search program to obtain fundamental aerodynamic precipitate toward the ground, gusting winds dis-
information regarding the e(cid:11)ect of heavy rain on perse the liquid water over a larger region that re-
aircraft performance. The aim of the program is sults in lower ground-based rain intensity measure-
to understand the physical phenomena associated ments than actually exist at altitude. The world
withany aerodynamic performance penalty thatmay record ground-level rainfall rate of 73.8 in/hr was
occur during a rain encounter, particularly during recorded in Unionville, Maryland, for a short time
take-o(cid:11) and landing. (approximately 1min)on July 4, 1956, in an intense
1
afternoon thunderstorm (ref. 2). In 1962, Roys and tionsinliftcapability were calculated with empirical
Kessler(ref.3)conducted measurementsofliquidwa- data ofroughness e(cid:11)ectsonairfoillift. Theiranalysis
tercontent in a thunderstorm with an instrumented indicated that a Boeing 747 transport encountering
F-100 airplane. The instrumented airplane recorded a rain cloud with a liquid water content of18 g/m3
an average liquid water content value of approxi- (basedonarainfallrateof39in/hr)wouldexperience
mately 8.4 g/m3 with a peak value of44 g/m3. The a 5-percent increase in drag, a 29-percent reduction
(cid:14)
ground-based radar measurements in those experi- in maximumliftcapability, and a5 reduction in the
ments indicated modest rainfall rates of 1.48 in/hr. angle ofattack for maximum lift. These predictions
Priorto 1987, all ground-based natural rainfall rate constitute asubstantial loss of performance.
measurements were averaged over relatively long
time periods, on the order of minutes and hours
AtthetimeoftheHaines’ and Luers’analysis, no
(ref. 4), which masked the short-duration, high-
experimental data existed for veri(cid:12)cation ofthe pre-
intensityrain characteristicsassociated with convec-
dictions. Hence in 1981, an experimental research
tive storms and wind shear/microburst phenomena.
program wasestablished atLangley Research Center
Thisresultled to thedevelopment ofaground-based
toobtain aheavy rain e(cid:11)ects data base. Small-scale
natural rainfall rate measurement technique by Mel-
wind tunnel model tests were considered to provide
son to acquire data over very short time periods,
the most controlled environment for evaluating the
as short as one sample per second (ref. 5). His re-
e(cid:11)ectsofrain onairfoilperformance. Testtechniques
sultsveri(cid:12)ed theexistenceofhigh-intensity rain: over
and procedureswere developed and exploratory wind
7000 eventsmeasuredabove 4in/hrwith amaximum
tunnel testswereconducted on anNACA0012airfoil
rain eventof 29 in/hrforshort time intervalson the
section(cid:12)ttedwith asimplyhinged, full-span trailing-
orderof 10 to30 sec at ground level.
edge (cid:13)ap in the Langley 14- by 22-Foot Subsonic
Tunnel (ref. 8). The wind tunnel rain simulation
system produced liquid water content levels rang-
The earliest analytical work on the e(cid:11)ect of
3
ing from 13 to 22 g/m . A 15-percent reduction in
rain on aerodynamic performance was conducted by
the maximum lift capability of both the cruise and
Rhode in 1941 (ref. 6). His analysis indicated that
landing con(cid:12)gurations ofthe airfoil model wasmea-
drag increases associated with the momentum im-
sured inthe simulated rain environment independent
parted to a DC-3 aircraft encountering a rain cloud
of the liquid water content level. The exploratory
with a liquid water content of 50 g/m3 would cause
small-scale wind tunnel results con(cid:12)rmed the exis-
an 18-percent reduction in airspeed. Rhode consid-
tence of a performance penalty in a simulated rain
ered such an encounter to beoflittle consequence to
environment.
an aircraft (cid:13)ying at 5000 ft. Because low-visibility
take-o(cid:11)s and landings were not routine in 1941, the
consequences ofa heavy rain encounter during these The objective of the present investigation was
phases of (cid:13)ight were not considered. However, for a threefold: (cid:12)rst, todetermine the severity of the rain
modern day transport such an airspeed loss during e(cid:11)ecton a cambered airfoil representative of typical
take-o(cid:11) or landing would be signi(cid:12)cant. commercialtransportwingsections;second,todeter-
mine the aerodynamic penalty over awider range of
rain intensities;and third,toexplorethe importance
The reconsideration of heavy rain as a potential of surface-tension interactions of water as a scaling
weather-related aircraft safety hazard in 1977 led to parameter. The data presented in this report were
thedevelopment ofabroad experimentaland analyt- obtained with an NACA 64-210 airfoil model with
ical research e(cid:11)ort, spearheaded by NASA to deter- leading- and trailing-edge high-lift devices tested in
mine thee(cid:11)ectofheavy rainon aircraftperformance. cruise and landing con(cid:12)gurations. The tests were
In 1982, Haines and Luers (ref. 7), under contract conducted in the Langley 14- by 22-Foot Subsonic
from Wallops Flight Facility, analytically evaluated Tunnel at dynamic pressures of 30 and 50 psf,
the e(cid:11)ect of rain on aircraft landing performance. which correspond to Reynolds numbers of 2:6(cid:2)106
Their study re(cid:12)ned the work of Rhode by estimat- and 3:3(cid:2)106 based on airfoil chord. The rain inten-
ing the e(cid:11)ects of rain on a modern day transport. sity wasvariedtoproduce liquid watercontentlevels
Their analysis not only included the calculation of ranging from16to46 g/m3. The test resultsarede-
the impact momentum of the raindrops, butalso es- scribed in terms of lift, drag, and pitching-moment
timatedthe increaseinskin-friction drag byequating characteristicsin and outofthe simulated rain envi-
the two-phase (cid:13)ow phenomenon overthe airfoil sur- ronment and represent the baseline data to be used
face to an equivalent sandgrain roughness. Reduc- in theevaluation offull-scalemodelingofraine(cid:11)ects.
2
Symbols Apparatus and Procedure
b airfoil metric section span, 1 ft Wind Tunneland ModelSupport
c airfoil chord, 2.5 ft
The present investigation was conducted in the
Langley 14- by 22-Foot Subsonic Tunnel, which is
c section drag coe(cid:14)cient, Drag/qbc
d
a closed-circuit atmospheric wind tunnel allowing
cl section lift coe(cid:14)cient, Lift/qbc open and closed test section operation (ref. 9). For
this investigation, the tests were conducted in the
c section pitching-moment coe(cid:14)cient,
m
closed test section with dimensions of 14.5 ft high
Pitching moment/qbc2
by 21.75 ftwide by 50 ft long. A photograph of the
D drop diameter, mm testsetup isshown in (cid:12)gure1. The model hardware,
aligned laterally with the tunnel centerline, was lo-
D arithmetic mean drop diameter, mm
1 cated in the aft bay of the test section. The rain
simulation system manifold, which was located ap-
D volumetric mean drop diameter, mm
2
proximately 10 wing chord lengths upstream of the
H height of rain spray atmodel model location, directed the rain spray horizontally
location, ft atthe model.
ID innerdiameter of hypodermic-type
nozzle, in. Model
K conversion constant Themodelused inthisinvestigation had arectan-
gularplanform and wassupported between twoend-
LWC liquid watercontent, g/m3 (see
plates in an attempt to represent a two-dimensional
(cid:12)g. 12and appendix A) test setup ((cid:12)g. 2). The airfoil chord was 2.5 ft
and the span between the endplates was 8 ft. An
N(D) drop size density function
NACA64-210 airfoil section was chosen as being
Q volumetric (cid:13)ow rate, gal/min representative of a cambered, commercial transport
wing section. Themodelwasequipped with leading-
q free-stream dynamic pressure, psf
and trailing-edge high-lift devices. Details of the
R rainfall rate, in/hr cruiseand landing con(cid:12)gurations thatweretestedare
shown in (cid:12)gure 3and in tables1(a)and (b). Forthe
Re free-stream Reynolds number,
landing con(cid:12)guration, the high-lift devices consisted
(cid:26) Vc=(cid:22) (cid:14)
a a of a leading-edge slat de(cid:13)ected 57 and a trailing-
(cid:14)
V free-stream velocity, ft/sec edge double-slotted (cid:13)ap de(cid:13)ected 35.75 . For the
cruise con(cid:12)guration, the model leading-edge slatwas
W width ofrain spray at model stowedand thetrailing-edge (cid:13)ap systemwasreplaced
location, ft with acruise(cid:13)ap sectioninstalled (cid:13)ush totheaftend
of the main wing section. The stowed leading-edge
We Weber number, 0:00328(cid:26)wV2D2=(cid:27)w;a slatproduced an aft-facing step of0.059 in. thatwas
x chordwise location ofmodel notfaired to the main wing section contour.
geometry, ft
The model consisted ofa 1-ftspan metric center
z ordinate location corresponding section mounted between two nonmetric outer pan-
to x, ft els. The center section was mounted on an internal,
three-component, strain-gauge balance and was sep-
(cid:11) angle of attack, deg
arated fromthe outboard panels by small gaps that
(cid:22) viscosity of air, slugs/ft-sec were sealed with athin, (cid:13)exible layerofrubber(den-
a
tal dam) to approximate two-dimensional (cid:13)ow and
(cid:22)w viscosity of water, slugs/ft-sec eliminate three-dimensional e(cid:11)ects caused by leak-
age. The sealing technique did not degrade the bal-
(cid:26)a density of air, slugs/ft3
ance performance. Aerodynamic data were obtained
(cid:26) density of water, slugs/ft3 with the boundary-layer transition (cid:12)xed with No. 80
w
grit along the 5-percent chordline on the upper and
(cid:27)w;a surface tension between waterand lowersurfacesofthe wingand each high-liftelement
air, slugs/sec2 in accordance toreference 10.
3
Rain Simulation System The rain simulation system hardware was de-
signed to meet the volumetric (cid:13)ow requirements of
The simulation of natural rain in a wind tunnel thefan jetnozzle. Aschematicofthehardwaresetup
environment should be able to simultaneously pro- isshown in (cid:12)gure5. The rain simulation systemcon-
duce a natural rain drop size distribution, vary the sisted ofa20-gallon watertank accumulator, aman-
rain intensity level, and provide uniform rain spray ifold, and three sets of nozzles (5-tube, 7-tube, and
coverage at the model location with minimal in(cid:13)u- fan jetnozzleset). Aremotelycontrolled airpressure
ence on the tunnel free-streamconditions. The wind valve regulated the water supply to the tank, which
tunnel rain simulation technique developed during wasconnected to the manifold. The airsupply pres-
theexploratory small-scale tests(ref. 8)identi(cid:12)edan sure wasvaried to control the volume of water pass-
inherent di(cid:14)culty in producing large size drops typ- ingthrough themanifold and exitingoutthenozzles.
ical ofnatural rain while atthe same time achieving The operating pressure waslimitedtoamaximumof
the desired rain intensity and drop size distribution. 100 psig. Thevolumetric (cid:13)owrateand theairsupply
Whenwaterisinjectedintoahigh-velocity airstream pressure were both measured and recorded.
atavelocitysubstantially lessthan the airstreamve-
The rain simulation system manifold was fabri-
locity,thelargerdropsthatformbreakupalmostim-
cated from streamlined steel tubing having a chord
mediately intomuch smaller dropsasdetailed in ref-
length of 3.5 in. and a (cid:12)neness ratio (chord length
erence 11. Although this di(cid:14)culty can be alleviated
to thickness) of 2.2 to minimize the interference ef-
byincreasingthe waterinjection pressuresothatthe
fect on tunnel free-streamconditions. The manifold
initial drop velocity approaches the airstreamveloc-
was located 25 ft upstream of the model (approxi-
ity, the resulting rain intensity tendstobe toohigh.
mately 10 wing chord lengths) to allow time for the
The exploratory small-scale tests indicated that the
stabilization of the accelerating water droplets and
drop size distribution and the rain intensity levels
the dispersion of the manifold disturbances on tun-
were a function of the nozzle design, waterinjection
nelfree-streamconditions. Themanifold wasaligned
pressure, and airstream velocity.
approximately 6 in. above the chord plane of the
model to account for gravity e(cid:11)ects on the water
Anextensiveexperimentalresearch e(cid:11)ortwascar- droplets. Comparisons of model aerodynamic data
ried out by the Jet Propulsion Laboratory (JPL)to in and out of the simulated rain environment were
develop a nozzle design that would simulate a range
measured with the spray manifold in position at all
of rain intensity levels and a drop size distribution times.
that would include drops 2 mm in size and larger
Themanifold hardwareshown in (cid:12)gure 6wasini-
(ref. 12). The JPL-designed nozzle consisted of ase-
tially designed to immerse the NACA 0012 airfoil
riesof0.063-in-diameter hypodermic tubesarranged
model in a simulated rain(cid:12)eld (ref. 8). The man-
circumferentially around a plenum ((cid:12)g. 4(a)). The
ifold was modi(cid:12)ed for the present investigation to
hypodermic nozzle design provided the (cid:13)exibility to
ensure coverage of the NACA 64-210 model. Two
independently vary the rain intensitywhileretaining
vertical posts spaced 1 ft apart at the center of the
control over the drop size. The rain intensity pro-
horizontal bar of the manifold were added as shown
duced by the hypodermic nozzle design was a func-
in (cid:12)gure 7. The vertical and horizontal spacing of
tionoftheairsupply pressure, dynamic pressure,and
the three sets of nozzles was determined by trial
number of tubes in the particular nozzle con(cid:12)gura-
and error. The optimum nozzle spacing (the same
tion. The JPL research e(cid:11)ort led to the selection of
for all three nozzle sets) is shown in (cid:12)gure 7 with
twonozzlecon(cid:12)gurations, a5-and 7-tube (B1N5and
a total of four nozzles, two nozzles on each verti-
B1N7, respectively), as shown in (cid:12)gure 4(b). Also
cal post. Each nozzle is spaced 1 ft apart from the
shown in (cid:12)gure 4(b) is a commercially available fan
other both horizontally and vertically to completely
jetnozzle (1570), previously used in the exploratory
immerse the instrumented 1-ft metric center section
wind tunnel tests on the NACA 0012 airfoil model.
of the NACA 64-210 airfoil model. The simulated
This nozzle had an elliptical cross section and pro-
rain(cid:12)eld was centered about the model chordline for
duced the highest volumetric (cid:13)ow rate of the three.
atotalheightofapproximately 4ftand extended ap-
The 5- and 7-tube hypodermic nozzles, along with
proximately 1.5 ft on eithersideofthe metric center
thefan jetnozzle, were each used separately(nomix-
section for all the nozzle con(cid:12)gurations tested.
ing of nozzle type) in the present investigation to
determine the severity of the performance degrada-
Rain Simulation System Calibration
tionasafunction ofrain intensity. The5-tube hypo-
dermic nozzleproduced the lowestrain intensity and As was previously mentioned, the parameters
thefan jetnozzleproduced thehighestrain intensity. used to characterize rain are the rainfall rate (R)
4
and the liquid water content (LWC). The relation- droplets produced by the hypodermic-type nozzle
ship between R and LWC is uniquely dependent on weremovingatapproximately 92-percentfree-stream
thetypeofstormand the intensity levelofthe storm velocity, butatthehigherdynamic pressureof30psf,
(refs. 2 and 13). A detailed description of this re- the drop velocity was approximately 89 percent of
lationship can be found in appendix A. The wind the free-stream velocity. It is interesting to note
tunnel rain system simulates a thunderstorm-type that the measured drop velocities were independent
rain that is de(cid:12)ned as being a high-intensity, short- of the drop size at the model location. In summary,
duration rain. The wind tunnel simulated rain(cid:12)eld the mean drop size decreased with increasing tunnel
wasquanti(cid:12)ed in termsofdrop sizedistribution,drop testspeed and the drop velocitywasmeasured to be
velocity, and LWC. about 90 percent of the free-stream velocity for all
nozzle con(cid:12)gurations.
Aspartofthe nozzledevelopment research e(cid:11)ort,
JPL developed ashadowgraph technique tomeasure In a wind tunnel environment, LWC is expressed
the drop size and drop velocity distributions pro- as a function of the rain(cid:12)eld area, volumetric (cid:13)ow
duced in a wind tunnel environment. The shadow- rate, and free-stream velocity as shown by the re-
graph technique used a pulsed ruby laser as shown lationship in (cid:12)gure 12. A (cid:13)owmeter measured the
in (cid:12)gure 8. The photographic optics were arranged volume ofwater (cid:13)owing through the manifold. The
to sample a small region in the central portion of rain(cid:12)eld width and heightwerephotographically ob-
the spray justin frontofthe model location. A typ- tained at the model location with a (cid:13)uorescentdye,
ical shadowgraph photograph is shown in (cid:12)gure 9 an ultraviolet strobe light to enhance the photo-
for a dynamic pressure of 30 psf. The photographic graphic qualities of the rain(cid:12)eld, and a near(cid:12)eld
negatives were digitized on a computerized optical linear-length reference. A photograph of a typical
scanner and analyzed to determine drop population simulated rain(cid:12)eld is shown in (cid:12)gure 13. The rain-
characteristicsof the hypodermic and fan jet nozzle (cid:12)eldappearstobeuniformly distributed atthemodel
types. location. Because of the dynamic nature of water
The drop size distribution data were obtained drops, the boundaries of the rain spray region at
at dynamic pressures of 15, 30, and 50 psf for the any instant in time are not precise straight lines.
hypodermic-type nozzleand 30and 70psfforthefan Therefore, deriving the rain spray by photographic
jetnozzle type as tabulated in table 2(a). Drop size means involves subjectively determining the usable
was a function of nozzle type, water injection pres- rain spray region boundaries. The rain spray area
produced by the three nozzle con(cid:12)gurations is pre-
sure, and tunnel test velocity. Note that the drop
size and drop velocity characteristics measured for sented in table 2(c) as a function of nozzle type,
the JPL-designed hypodermic nozzle are applicable tunnel dynamic pressure, air supply pressure, and
forboth the5-and 7-tube con(cid:12)gurationsbecause the volumetric (cid:13)ow rate.
nozzles have the same tube geometry (i.e., inner di-
ameter, see (cid:12)g. 4(a)). The drop population char- Scalingof Rain E(cid:11)ects
acteristics are tabulated in terms of the arithmetic
Because of the complexity of the two-phase (cid:13)ow
mean drop diameter, the volumetric mean drop di-
environment, the established wind tunnel model to
ameter,and the ratioofthe drop velocitytothefree-
full-scale scaling laws may not be applicable in the
stream velocity. The volumetric mean drop diame-
rain environment. In 1985, Bilanin (ref. 16) ad-
ter is de(cid:12)ned as that drop diameter for which half
dressed the subjectof scaling for model tests ofair-
the total volume ofthe rain spray is in larger drops
foils in simulated rain. Hisanalysisshowed that the
and half in smaller drops. The distribution of the
following variables, relevant to the rain spray, are
drop sizes, shown in (cid:12)gures 10 and 11, indicates a
important in scaling the e(cid:11)ects of rain from model
large di(cid:11)erence between the geometric and volumet-
tests: the density of water, kinematic viscosity of
ricmean drop diametersduetotheexistenceofmany
water, surface tension interactions of water, mean
small drops in the simulated rain(cid:12)eld. The wind
drop spacing, volumetric mean drop diameter, and
tunnel rain simulation technique did not produce
drop velocity. The (cid:12)rst three variables are inher-
a natural rain drop size distribution as detailed in
entproperties ofwateritselfand the lastthree vari-
references13 to 15.
ablesaredependent on the testtechnique used. The
The laser system also was operated in a double- scaling parameters thatare derived from the group-
pulsed mode with about 20 msec between pulses ing ofthe aforementioned variablesare Weber num-
to determine drop velocity. Drop velocity data are ber(We),LWC,and R. TheWeistheratioofinertial
shown in table 2(b) for the hypodermic-type nozzle. forces to surface tension forces and is a function of
At the lower dynamic pressure of 15 psf, the water the density of water, drop velocity, volumetric mean
5
drop diameter, and the surface-tension interactions Test Conditions
betweenwaterand air. The LWC isafunction ofthe
drop size distribution, drop velocity, and density of The location ofthe rain simulation system man-
water. The R is a function of LWC. The sensitivity ifold 10 wing chord lengths upstream of the model
ofthewetairfoilaerodynamic characteristicstoeach and 6 in. above themodel chord plane waschosen to
ofthe aforementioned parameters must be assessed. mitigate any manifold-induced air(cid:13)ow disturbances.
The data of reference 17 indicate the manifold up-
stream of the model location produced a slight in-
This investigation assessed the sensitivity of a
creasein the free-streamturbulencelevel,in addition
cambered, commercial transport-type airfoil equip-
to a slight increase in drag. Comparisons of model
ped with high-lift devices to LWC and We. The
aerodynamic data in and out of the simulated rain
sensitivitytoLWCwasassessedbyvarying thenozzle
environment are with the manifold in position at all
type, waterinjection pressure, volumetric (cid:13)ow rate,
times.
and the tunnel dynamic pressure. The sensitivity
of the wet aerodynamic characteristics to Weber
number, i.e., surface-tension interactions between
During this investigation, no signi(cid:12)cant changes
waterand air, alsowasaddressed. Asurface-tension
in dynamic pressure were measured at the model
reducing agent was added to the water in su(cid:14)cient
location in the simulated rain environment. Con-
quantity (24 ml/gal of water) to reduce the surface
2 sequently, the calibrated dry air tunnel dynamic
tension by afactorof2 (from0.0047965 slug/sec to
0.0021242 slug/sec2), which changed We by a factor pressure was used to nondimensionalize the aero-
dynamic data obtained. All data shown were ob-
of2.
tained with the boundary-layer transition (cid:12)xed as
discussed previously.
Data Accuracy
An internal, three-component, strain-gauge bal-
The tunnel dynamic pressure, angle of attack,
ance was used in this investigation to measure the
and LWC conditions were parametrically varied
aerodynamic forces and moments in and out of the
to determine the performance degradation of the
simulated rain environment. Thisbalance hasan ac-
curacy rating of (cid:6)0.5 percent of full-scale loading. NACA64-210 airfoil model equipped with and with-
outhigh-liftdevices. Aerodynamic measurementsin
The calibration and corresponding error range for
and out of the simulated rain environment were ob-
each component are asfollows:
(cid:14) (cid:14)
tained over an angle-of-attack range from 0 to 20
for dynamic pressures of 30 and 50 psf for the
cruise con(cid:12)guration and an angle-of-attack range
Component Load Error of 4(cid:14) to 20(cid:14) for dynamic pressures of 30 and 50 psf
Normal force, lb . . . . (cid:6)600 (cid:6)3.0 for the landing con(cid:12)guration. The rain inten-
sity was varied to produce LWC values ranging
Axial force, lb . . . . . (cid:6)100 (cid:6)0.5 from16to46 g/m3. The sensitivityofthewetairfoil
Pitchingmoment, lb . . . (cid:6)2000 (cid:6)10 characteristics to water surface-tension interactions
was also investigated forthe landing con(cid:12)guration.
In aerodynamic coe(cid:14)cient form, the correspond-
ingerror range isas follows: Presentation of Results
The results of this investigation have been re-
duced to coe(cid:14)cient form as presented in (cid:12)gures 14
Aerodynamic
through 37 and are listed in tabulated form in ap-
coe(cid:14)cient q = 30psf q = 50psf pendix B. The data were normalized with respect to
c . . . . . . . . (cid:6)0.04 (cid:6)0.024 the dry air dynamic pressure. The pitching-moment
l
data were measured about the quarter-chord of the
cd . . . . . . . . (cid:6)0.0067 (cid:6)0.004 model. The data presented were obtained with the
cm . . . . . . . (cid:6)0.053 (cid:6)0.032 rain simulation system manifold in place. A listing
ofthe data (cid:12)gures is as follows:
6
Figure
Reynoldsnumber e(cid:11)ect . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14, 15
Cruise con(cid:12)guration liftcurve and drag polaratq = 30 psfwith LWC =0, 25, and 39 g/m3 . . . . . 16
Cruise con(cid:12)guration liftcurve and drag polaratq = 50 psfwith LWC =0, 19, and 30 g/m3 . . . . . 17
Cruise con(cid:12)guration drag asa function of angle ofattack atq = 30and 50 psf . . . . . . . . . . . 18
Cruise con(cid:12)guration pitching momentversuslift and angle ofattack at q = 30 and 50 psf . . . . . . 19
(cid:14) (cid:14)
Water (cid:13)ow characteristicson upper surface ofcruise con(cid:12)guration at (cid:11)= 0 and 20 and
q= 15 psfwith LWC = 17 g/m3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23, 24
(cid:14) (cid:14)
Water (cid:13)ow characteristicson lower surface ofcruise con(cid:12)guration at (cid:11)= 4 and 16 and
q= 15 psfwith LWC = 14 g/m3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25, 26
Landing con(cid:12)guration liftcurve and drag polarat q= 30 psfwith LWC = 0, 29, and 46g/m3 . . . . 27
Landing con(cid:12)guration liftcurve and drag polarat q= 50 psfwith LWC = 0, 16, and 36g/m3 . . . . 28
Landing con(cid:12)guration drag as function ofangle of attack at q= 30 and 50 psf . . . . . . . . . . . 29
Landing con(cid:12)guration pitching moment versus liftand angle ofattack at q = 30 and 50psf . . . . . . 30
(cid:14) (cid:14)
Water (cid:13)ow characteristicson upper surface oflanding con(cid:12)guration at(cid:11) =8 and 20 and
q= 30 psfwith LWC = 46 g/m3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32, 33
(cid:14) (cid:14)
Water (cid:13)ow characteristicson lower surface oflanding con(cid:12)guration at(cid:11) =4 and 20 and
q= 30 psfwith LWC = 29 g/m3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34, 35
Water surface tension e(cid:11)ectson landing con(cid:12)guration aerodynamics at q= 30 psf . . . . . . . . . . 36
Water surface tension e(cid:11)ectson landing con(cid:12)guration aerodynamics at q= 50 psf . . . . . . . . . . 37
Results and Discussion
vestigation and are discussed in detail later in this
section.
Reynolds NumberE(cid:11)ect
The drag data at aconstant lift condition do ap-
The dry aerodynamic data presented in (cid:12)g-
pear to be sensitive to LWC and dynamic pressure
ures 14 and 15 show the e(cid:11)ect of Reynolds number
at and beyond stall. For example, the drag data
atdynamic pressures of30 and 50 psfforthe cruise
in (cid:12)gure 16 for q = 30 psf and LWC = 25 g/m3 and
and landingcon(cid:12)gurations. Theresultsindicatethat
q = 50psfand LWC = 19g/m3 in (cid:12)gure 17showin-
Reynolds number had a negligible e(cid:11)ect on lift co-
creasesin drag coe(cid:14)cientataconstantliftcoe(cid:14)cient
e(cid:14)cient versus angle of attack for both model con-
(cid:12)gurations over the range tested. The data forboth ofcl = 1.0 of 37and 71percent, respectively. An in-
crease in drag wasmeasured for both dynamic pres-
con(cid:12)gurationsalsoshowsmall di(cid:11)erencesin thedrag
suresatlow and moderateanglesofattackas shown
polar.
in (cid:12)gure 18. The drag data as afunction ofangle of
attack and dynamic pressure do notappear to be as
Cruise Con(cid:12)guration Aerodynamics
sensitive to increases in LWC asthe lift data.
The e(cid:11)ect of rain on the NACA 64-210 cruise
The e(cid:11)ect of rain on pitching moment for the
con(cid:12)guration for q= 30 and 50 psf is shown in (cid:12)g-
cruise con(cid:12)guration isshown in (cid:12)gures19(a)and (b)
ures16and 17,respectively. Signi(cid:12)cant reductionsin
forq = 30 and 50 psf. Prior to stall ((cid:12)g. 19(b)), the
maximumliftwere measured asLWC wasincreased,
e(cid:11)ect of rain on pitching moment is negligible for
on the order of 8 and 11 percent at q = 30 psf
both dynamic pressures. The change in the slope of
(LWC = 25 and 39 g/m3) and 12and 17percent at
the pitching-moment curve ((cid:12)g. 19(a)) is marginal
q = 50psf (LWC = 19and 30g/m3). A progressive
with increasing LWC for both dynamic pressures,
decrease in the lift curve slope with increasing
although the break occurs earlier with a more pro-
LWC was also observed for the cruise con(cid:12)guration
nounced e(cid:11)ect with increasing LWC at a dynamic
((cid:12)gs. 16and 17). Thise(cid:11)ectmaybeexplained by the
pressure of 50psf.
observed water (cid:13)ow characteristics at low to mod-
erate angles of attack discussed by Hastings et al. The progressive decrease in the NACA 64-210
(ref. 18) and Hansman et al. (ref. 19). These char- cruise con(cid:12)guration lift curve slope with increas-
acteristics were also observed during the present in- ing LWC may be explained by the water (cid:13)ow
7
characteristics observed during the present investi- and the rivulet(cid:12)eld highlights theseparated (cid:13)ow re-
gation. The photographic qualitiesofthe rain spray gions present as shown in (cid:12)gure 24. On the lower
(cid:14)
were enhanced by the addition of a (cid:13)uorescent dye surface, at (cid:11) =4 , the water (cid:12)lm layer appears to
to the water and the use of an ultraviolet strobe extend back to approximately the 40-percent chord-
light in a darkened test section to capture the wa- line beforebreaking up intorivulets((cid:12)g.25). Asthe
ter(cid:13)ow patterns on the upper and lowersurfacesof angleofattack increases,the chordwise extentofthe
the airfoil. The surface blemishes on the upper and water (cid:12)lm increases until it encompasses the entire
(cid:14)
lowersurfacesoftheairfoilmodelareidenti(cid:12)edin (cid:12)g- length of theairfoil. In (cid:12)gure 26 at(cid:11) =16 , theair-
ures 20 and 21 to make the reader aware that these foil has stalled and the water (cid:12)lm layer appears to
blemisheswillshow up in anenhanced formatduring extend back to approximately the 75-percent chord-
the visualization process. linebefore breaking upintorivulets. Fromvisual ob-
servations, the rivulets on the lower surface appear
The pattern of the water (cid:13)ow that develops can
to be pushed upward around the trailing edge and
be qualitatively described in terms of an \ejecta
pooled on the upper surface. The cratering of the
fog" layer, water (cid:12)lm layer, and \rivulet" (cid:13)ow (cid:12)eld
water (cid:12)lm layer by droplet impacts and the break-
as sketched in (cid:12)gure 22(a). As the water droplets
up of the water (cid:12)lm layer into rivulets simultane-
impact the leading edge of the airfoil athigh speed,
ously interact with the turbulent airboundary layer
a layer ofvery (cid:12)ne dropletsisformed in frontof the
resulting in an early de-energization of the airfoil’s
leading edge as a consequence. This phenomenon
boundary layer and hence, constantly changing the
has been de(cid:12)ned as the \ejecta fog" layer. Beneath
e(cid:11)ective camber of the airfoil, which adversely af-
the ejecta fog layer, a water (cid:12)lm layer develops at
fectsthe airfoil’s performance throughout the entire
the wing leading edge and extends back toward the
angle-of-attack range.
trailing edge along the upper and lower surfaces of
theairfoil. Atsome pointthewater(cid:12)lmlayerbreaks In summary, the NACA 64-210 cruise con(cid:12)gura-
up intorivulets,which are thin capillary-like streams tiondatapresentedin(cid:12)gures16through 19indicated
ofwater running chordwise toward the trailing edge the sameperformance trendsasthe cruise con(cid:12)gura-
ofthe airfoil. tion data oftheNACA0012 airfoil model previously
The chordwise extent of the water (cid:12)lm layer cited in reference 8. Both airfoil sections exhibited
has been found to be dependent on the airfoil con- signi(cid:12)cant reductionsinmaximumliftcapability and
(cid:12)guration, surface treatment, and angle of attack increasesindrag foragiven liftcondition in the sim-
ulated rain environment. The most signi(cid:12)cant dif-
(refs. 18and 19). Asthe angle ofattack isincreased,
thechordwiseextentofthewater(cid:12)lmlayerdecreases ference between thecruise results ofthese twoairfoil
on the uppersurface and increases on the lowersur- sectionswas the sensitivity ofthe NACA 64-210 air-
face as shown in (cid:12)gures 23 to 26 forthe cruise con- foil section to LWC. This di(cid:11)erence indicates that
(cid:12)guration. Figures 23 and 24 were taken with an there isa rain e(cid:11)ectsensitivity tocamber. Asprevi-
overhead camera and show the pattern of the wa- ously mentioned, theNACA0012 performance losses
ter (cid:13)ow on the upper surface of the cruise con(cid:12)gu- in the rainenvironment werenotafunction ofLWC.
ration at q = 15 psf and LWC =17 g/m3 for angles
of attack of 0(cid:14) and 20(cid:14), respectively. Photographs Landing Con(cid:12)guration Aerodynamics
in (cid:12)gures 25 and 26 were taken with a camera lo-
The e(cid:11)ects of rain on the NACA 64-210 land-
cated on the tunnel (cid:13)oor beneath the wing model
ing con(cid:12)guration for q = 30 and 50 psf are shown in
and show the pattern ofthe water (cid:13)ow on the lower
(cid:12)gures27 and 28, respectively. The resultsobtained
surface of the cruise con(cid:12)guration atq = 15psf and
LWC = 14g/m3 for angles of attack of 4(cid:14) and 16(cid:14), athtestLaWllCforcotnhdeitliaonndionfg3c6ong(cid:12)/gmu3raatitonthiendhiicgahteertdhyat-
respectively. Note that the spanwise extent of the
namic pressure of 50 psf ((cid:12)g. 28) produced almost
water (cid:12)lm layer developing on the wing surface is
as great a lift loss (18 percent) as the higher LWC
limited to the width of the rain spray produced. condition of 46 g/m3 at the lower dynamic pres-
(cid:14)
Focusing onthe uppersurface(cid:12)rst, at(cid:11) =0 , the sure of 30 psf (22 percent) ((cid:12)g. 27). In addition,
(cid:14)
water(cid:12)lm layer appears to extend back to approxi- the greatest reduction in stall angle of attack (8 )
mately the 50-percent chordline before breaking up was also measured at the higher dynamic pressure
into rivulets ((cid:12)g. 23). The presence of the rivulet of 50 psf and LWC =36 g/m3. Note that accompa-
(cid:12)eld acts as the boundary for attached and sepa- nying the reduction of the stall angle is the general
rated (cid:13)ow. The rivulet (cid:12)eld indicatesattached (cid:13)ow (cid:13)attening of the lift curve past stall with increas-
at the low angles of attack. As the angle of attack ing LWC forboth dynamic pressures. Similar to the
(cid:14)
is increased to 20 , the water (cid:12)lm layer disappears cruise con(cid:12)guration data, the landing con(cid:12)guration
8
data also indicated a progressive decrease in the lift the water patterns on the lower surface of the land-
curve slope with increasing LWC. The same mecha- ingcon(cid:12)guration atq = 30 psfand LWC = 29g/m3
(cid:14) (cid:14)
nism described in the cruise data section appears to foranglesofattack of4 and 20 .
be atwork in this case. (cid:14)
Focusing on the uppersurface at(cid:11)= 8 ((cid:12)g. 32),
Increases in drag were measured for both two interactions can be seen: the formation of a
dynamic pressures at low and moderate angles rivulet(cid:12)eld on the main airfoil section and the water
of attack as shown in (cid:12)gures 27 and 28. The being driven upward from the underside of each
drag data at a constant lift condition do appear high-lift device (leading-edge slat and trailing-edge
to be sensitive to LWC and test velocity. For double-slotted (cid:13)aps) through the gap openings onto
example, the drag data for q= 30 psf ((cid:12)g. 27) the upper surfaces of the adjacent airfoil element.
show increases in drag coe(cid:14)cient at a constant As the angle of attack increases, the presence of
lift coe(cid:14)cient of cl= 2.3 of23 and 40 percent for the water reduces the gap openings, which alters
3
LWC = 29and 46g/m , respectively. Atthe higher the separated air(cid:13)ow regions on the upper surface
dynamic pressure of 50 psf and the same lift coe(cid:14)- of the airfoil and causes a breakdown of the water
cient of 2.3 ((cid:12)g. 28), the drag data show increases (cid:13)ow pattern, which results in regional pooling of
of14 and 15 percentforLWC = 16 and 36 g/m3,re- the water, as shown in (cid:12)gure 33 at (cid:11) =20(cid:14). On
(cid:14)
spectively. Aswaspreviously noted inthecruisecon- the lower surface at (cid:11) =4 ((cid:12)g. 34), the water (cid:12)lm
(cid:12)guration data, the landing con(cid:12)guration drag data layer extends back to approximately the 90-percent
asafunction ofangleofattackand dynamic pressure chordline on themain airfoil section. Asthe angle of
do notappear tobe assensitive toincreasesin LWC attackincreases, the slatand (cid:13)ap mounting brackets
atlowtomoderateanglesofattackasdothe liftdata block some of the water (cid:13)ow as indicated by the
((cid:12)g. 29). nearly dry areas on the main airfoil section and (cid:13)ap
systemaftofthe bracketsshown in (cid:12)gures33and 35.
The e(cid:11)ect of rain on pitching moment for the (cid:14)
At (cid:11) =20 , the water (cid:12)lm layer extends back to
landing con(cid:12)guration is shown in (cid:12)gures 30(a)
and (b) for q = 30 and 50 psf. Prior to stall the trailing edge ofthe main airfoil section ((cid:12)g. 35)
and from visual observations the gap openings of
((cid:12)g.30(a)),thereappears tobeaprogressiveincrease
all the high-lift devices appear to be signi(cid:12)cantly
in the slope of the pitching-moment curve with in-
immersed with water. In (cid:12)gure 35, a sheet of water
creasing LWC for both dynamic pressures. Similar
appearstobecomingfromtheundersideofthewing,
tothecruise con(cid:12)guration data, the landing con(cid:12)gu-
through the (cid:13)ap gap opening, and outward in the
ration data ((cid:12)g. 30) also indicate the break in the
direction of the free-stream (cid:13)ow. The photographic
pitching-moment curve occurs earlier with a more
coverage indicates that the large amount of water
pronounced e(cid:11)ectwith increasingLWCatadynamic
that(cid:13)owed through thegaps played asigni(cid:12)cant role
pressure of 50 psf. Past stall, the rain environment
in the performance lossesexperienced by thelanding
continues to degrade pitching-moment performance
con(cid:12)guration.
atboth dynamic pressures. Theq = 30 psfpitching-
momentdata versusliftcoe(cid:14)cientappeartobemore In summary, although reductions in maximum
sensitive to increases in LWC than the q = 50 psf lift capability and corresponding increases in drag
pitching-moment data ((cid:12)g. 30(b)). were measured for both the cruise and landing con-
(cid:12)gurations of the NACA 64-210 airfoil model, the
Thephotographic coverage ofthewater(cid:13)owchar-
landing con(cid:12)guration was more sensitive to the rain
acteristics on the upper and lower surfaces of the
environment than the cruise con(cid:12)guration. Of par-
landing con(cid:12)guration indicates thatfor low to mod-
ticularsigni(cid:12)cance wastheassociated decreasein the
erateanglesofattack,whereattached (cid:13)owconditions
angleofattack atwhich maximumliftoccurred with
exist,the wateradherestotheairfoilsurface forming
increasingLWC. Accompanying the reduction ofthe
a water(cid:12)lm layerand a rivulet (cid:12)eld. An additional
stall angle ofattackwasthe general (cid:13)attening ofthe
(cid:13)ow complication is the presence ofthe high-liftde-
lift curve slope past stall. The severity of the rain
vices((cid:12)g. 22(b)). The waterpasses through the gap
e(cid:11)ect appears to be dependent on test velocity and
openings between the high-lift devices and the main
LWC.
airfoil section and decreases the air(cid:13)ow through the
gap openings. In (cid:12)gure31, thelanding con(cid:12)guration
Surface Tension E(cid:11)ects
is shown immersed in the simulated rain(cid:12)eld. Fig-
ures 32 and 33 show the patterns of the water (cid:13)ow The wetted aerodynamic characteristics of the
on the upper surface of the landing con(cid:12)guration at landing con(cid:12)guration are shown in (cid:12)gures36 and 37
q = 30psf and LWC =46 g/m3 for angles of attack with and withoutthe surface-tension reducing agent
(cid:14) (cid:14)
of 8 and 20 , respectively. Figures 34 and 35 show added to the water at dynamic pressures of 30
9