NACA Airfoil Characteristics

41 Pages·1997·4.02 MB·English

by  Technical Library

REPORT No. 586
AIRFOIL SECTION CHARACTERISTICS AS AFFECTED BY VARIATIONS OF THE
REYNOLDS NUMBER
By EAWIWANN. JACOBaSndALBERTSHFIR~AN
SUMMARY
Reynolds Number. This phenomenon isreferredto as
An invedigation oj a 8y8t&tiy cho8enrepre8enta-- ‘(scaleeffect.”
tivegroup of relutedaizjoih wasmathin theN. A. C.A. Early investigations of scale effect were made in
curiabl.e-d.en.dywind tunnel over a wide range oj the smallatmospheric tunnelsat comparatively low values
Reynolds Number &ending d into tlwylight range. of the Reynolds Number and, for airfok, covered a
Thete8Lsweremudetoprovideinformationjrom whichthe range of the Reynolds Number too limited and too
variution.soj aiq%i.!8ectioncharaeteridicswithc?umgtxin remote from the full-scale range to permit reliable
the Reyno.MsNumber could be injerred and metlwdaoj extrapolations to flight conditions. Attempts were
cdlawingjor thae cariatiorwin practice could be oMer- made to bridge the gap between the two Reynolds
mined. Thixworkixonephiweojan extm”ve andgen.end Number ranges by making full-scale flight tests for
aijoil invixtigationbeingconduciedin thevurialie-den.sdy comparkon with model tests. These investigations of
tunnel and ezL9uisthe previously publidwd Te#earch@ scale effect, however, proved disappohting owing
conwrning airjoil charactenkticsas a$ecied by variuiiom partly to the difficulty of obtaining good Ilight tests
in airjOiJprojih oktennined ai a tingle value of the and to tbe dMculty of reproducing flight conditions
ReynoldsNumber: in the model tests and partly to the large unexplored
Theobjectoj thisreporth toprovidemeawjor making Reynolds Number range between the model and flight
availableas seciim characfei%tia at any jree-air value tests with consequent uncertainties regarding the
oj theReynoldsNumberthevariablederwiiy-tunnelairjoil continuity of the characteristics over this range.
datapreviouslyPubltibd. Accordingly, thevariow cor- I?urthennore, the flight tests could not ordinarily
rection-sinvolvedin a%iving moreaccurateairjoi?seetion include a sufficiently large range of the Reynolds
characteristi.athan those hereiojore employed are jint Number to establish the character of the scnle effects
consz”deredatlengthandthecorrection jar turbulenceare for certain of airfoil characteristics over the full-
eqx?m%wd.An appendix ?k heluded that wi3rs the scalerangeof theReynolds Number, whichmay extend
results oj an invedgation oj certain comisteni ‘errors from values aslow asafew hundred thousand to thirty
pre9ent in test resuti8jrom the variabhihwity tunnel. million or more.
The origin and nature“ojscale e$ect8 are diwu88edand These limitations of the early investigations were
theairjoi.1wxzle-e~ectdaiaare anu.?yzed. FinuLly,metho- @t overcome by the N. A. C. A. through the use of
ds aregivenoj dewing jor 8cu.feeffeet8on airjoi.1section the variabledensity tunnel, which was designed to
churacten”8tiixin practice wiihin ordinurylimi?soj a.ccw facilitate aerodynamic investigations over the entire
raq jor t)wapplication oj variuble+iewi-ty-tunnelai~m”l range of Reynolds Numbem between the wind tunnel
datatojlightproblenw. and flight VOh.18S.Several miscellaneous and com-
monly used airfoils were investigated for scale effect
INTRODUCTION
in the variable-density tunnel during the tit years of
When clntnfrom nmodel test nreapplied to a fright its operation. The results indicnted that important
problem, the con&ion that should be mtisiied is that male effects for some airfoils may be expected above
the flows for the two cases be similar. The Reynolds the usualwind-tunnel range and even within the flight
Number, which indicatestheratio of themassforces to range of values of the Reynolds Number. Later,
the viscous forces in aerodynamic applications, is ordi- when the N. A. C. A. full-scale tunnelwas constructed,
narilyusedasthecriterionof The practical airfoil tests therein served to conii.rmthe importrmce
similarity.
necessity for having the flow nbout the model aerody- of scaleeffectsoccurring in thefull-scalerange and also
namically similnrto the flow about thefull-scaleobjeet provided valuable data for the interpretation of the
in flight becomes apparent from the fad that. aero- variabledensity-tunnel results, particularly in con-
dynamic coefficients,asarule,vaqvwith changesin the nectionwith the effectsof the turbulencepresentin the
227
.—..- — —.——- —
2% REPORTNO.586-NATIONAL ADVD30RYCOMMITTEEFORAERONAUTICS
variabledensity tunnel. The interpretation of the Camber shape.
variabledensity-tunnel results has consequently been Sectionswith high-lift devices.
modified to allow for the turbulence on the basis of an The testing program was begun in May 1934 and
“effective Reynolds Number” b“gher than the test extended several times as it became apparent that
Reynolds Number. additional tests would be des~ble, The @al tests
In the meantime, the investigations of airfoiIs in in the variabl+density tunnelweremade in September
the variabledensity tunnel had been turned to an 1935.
extensive study of airfoil characteristic as rdlected TESTSANDMODELS
by airfoil shape. This phase, which resulted in the
development of the well-known N. A. 0 A. airfoils, Descriptions of the variabl~deusity wind tunnel
involved the testing of a large number of related and of the methods of testing aregiven in reference 1.
airfoils, but these tests were largely cordined to one The teats herein reported were made for the most
value of the Reynolds Number within the full-scale part for each airfoil at tank pressuresof 1/4, 1/2, 1, 2,
range. Such a procedure expedited the investigation 4, 8, 15,and 20 atmospheres, covering a range of test
rmdprovided comparable data for the various airfoils Reynolds Numbers from 40,000 to 3,100,000. The
within the full-scale range of the Reynolds Number 1/4- and l/2-atmosphere runs were omitted for many
but, of course, gave no information about scale effects. of the airfoils and, in several cases, only the lift-curve
& previously stated, thefull+xde-tumel resultshad peaks were obtained at the lower Reynolds Numbers.
provided information regarding the application of the Runs atreduced speeds (1/5 and 1/2 thestandardvalue
variabldmsity-tunnel data to flight. Methods were of the dynamic pressure q) at 20 atmospheres were
accordingly developed for correc~” the data and for sometimessubstituted for the testsat 8 and 15 atmos-
presenting them in forms that would facilitate their pheres. Several check tests at 8 and 16 atmospheres
use m applied to ~Wht problems. Flight problems, andresultsfrom someearlierinvestigationshave shown
however, require airfoil data at V~OUS V~U~ of the that the specific manner of varying the Reynolds
Reynolds Number between values as low as a few Number withrespecttospeedordensityisunimportant
hundred thousand in some cases to thirty million or when the effects of compressibility arenegligible. l?or
more in othe~. Obviously the results mailable from all the airfoils, the airin the tunnel was decompressed
the testsof relatedairfoilsatone value of theReynolds and the airfoil repolished before running the higher
Number (effective Reynolds Number= 8,000,000) are Reynolds Number tests. Tares obtained ~t corre-
inadequate for thepurposeunlessthey canbe corrected sponding Reynolds Numbers were used in working up
to other values of theReynolds Number. The present the results.
investigation was therefore undertaken. to study the The airfoilmodels areof metal, usually of duralumin
scale effects for the related airfoil sections primarily and of standard 5- by 30-inch plan form; the sections
with a view to the formulation of general methods for employed (see @. 1), except for the slotted Clark Y,
determiningg scab-effect corrections for any normal me members of N. A. C. A. airfoil families (references
airfoil section so that the standard test rewlts from 2and.3). The slotted ClarkY modelisof 36-inchspan
the vnriabledensity tunnel could be applied to flight and 6-inch chord (with the slot closed) and was made
at any Reynolds Number. I?or most pmctic~ US@it to the ordinatesgiven in reference 4. For this airfoil,
is considered desirable and sufficient to present airfoil the coefficientsaregiven asbasedonthechord and area
testresultsin the form of tabular valuesgiving certain corresponding to the sloixlosed condition. The slat
important aerodynamic characteristics for each a~ail was made of stainlesssteel and fastened to the main
8ectwn. The primary object of this investigation, wing in the position reported (refercuce 4) to resultin
therefore, is to give information about the variation of the highest value of maximum lift coei%cient. This
these important airfoil section characteristics with modelwastestedatamuch earlierdatethantheothers,
Reynolds Number. and the test data are somewhat less accurate, The
In regard to the scope of the experimentalinveAga- mainwingof theN. A. C.A. 23012airfoilwithexternal-
tion, the Reynolds Number range was chosen as the airfoil flap is of 30-inch span and 4.167-inch chord.
largest possible in the vmiable-density tunnel and the The flap is of stainlesssteel and is also of N. A. C. A.
airfoil sections were chosen to cover as far as possible 23012 section having a chord of 20 percent that of the
the range of shapes commonly employed. Accord- main airfoil. It was fastened to the main wing m the
ingly, groups of related airfoils (@g. 1) were tested to opt&mm hinge position reported in reference 6. Dat~
investigate the followi.qg variables related to the for this airfoil combination are given herein for two
airfoil-sectionshape: angular flap settings: –3°, which corresponds to the
Thickness. minimumdrag condition; and 30°, which corresponds
. Camber. to the maximum-lift condition. The coefficients are
Thickness and camber. given as based on the sums of the main wing and flap
Thicknem shape. chords and areas.
AIRFOILSECTIONCHARACTERISTICSASAFFECTEDBY VARIATIONSOFTHE REYNOLDSNUMBER 229
7hickness Canber shape
N.A.CM. NA. C.A.
CU12
~
0012 “’’c====—
00/5
230/2
0018 i’Ra12
4412
Cbmber
430/2
‘“”~
c====—
24’2
64/.2
67/2
High-lif f devices
00/2
6“
23012
.
‘< 75*
=?$
23015
75”
=$
83/8 43012
6
cJuld
Thickness shape
23012
Clark Y{~y’
With Hond/ey-Puge slof
FIGURE L—Alrfiil sectfors emplopwi forthe @e-eff@t lnvedgatIorL. The SEUIIXB. exmpt for the dotted Clerk Y. are mwnbra ofN. A. O. A. alrfoll fmnilleb.
230 REPORTNO.586-NATIONAL ADVISORYCOMMITTEEFORAERONAUTICS
ACCURACY cases are not presented below an effective Reynolds
The accuracy of theexperimentaldataof thisinvesti- Number of 800,000.
RESULTS
gation at the highest Reynolds Number is comparable
with that of the stmdad airfoil test data as discuss.ed Figures2to 24presentthetestresultscorrected after
in reference 2. The systematic errorsof measurement the methods given in reference 1 for approximating
therein mentioned, however, have since been investi- i.niinite-aspect-ratiocharactmistics. Curves are given
gated and the resultsarepresented in tho appendix to (for eachairfoilfor diilerenttestReynolds Numbers) of
thisreport. The systematicerrorsof velocity measure- lift coefficient CLagainst effective angle of attack aoj
ment have hence been eliminated,the errorsassociated and of profile-drag Codicient of Pi~~%
6’Do ~d
with support deflcdion have been largelyremoved, and moment coefficient about the aerodynamic center
the errors associated with model roughness have been c.=-Ca.gainsltiftcoefficient CL. The x and y coordi-
minimized by giving careful attention to the model natesof theaerodynamic centerfrom theairfoilquarter-
surfaces chord point are also given where the data permit.
The remaining systematic errors are mainly those Although not preciselygectionchurucidics, character-
associated with the interpretation of the wind-tunnel isticsso corrected have been used heretofore assection
results rather than the direct errors of measurement. characteristics because of the lack of anything more
These errors are associated, first, with the calculation exact.
of airfoil seciioncharacteristicsfrom the testsof iinite- Further corrections, however, to allow for the effects
rmpecbratio airfoils and, second, with the correction of wind-tunnel turbulence, airfoil-tip shape, and some
of the test results’to zero turbulence or free-air condi- of the limitations of the previous corrections based on
tions. Such errors will be more fully treated in the airfoil theory were developed during the course of this
discussion where the methods of correction, including investigation and, when applied, give results repre-
the interpretation of the resultsasinvolving the effec- senting the most reliable section data now m-ailfible
tive Reynolds Number, areconsidered. from the variabledensi~ wind tunnel. These addi-
The magnitude of the direct experimental errors, tional corrections and their derivation are fully dis-
particularly of the accidental errors, increases as the cussed later in this report. The more e--act gectim
Reynolds Number is reduced. by variation of the characteristics have been distinguished by lower-case
support interference with the Reynolds Number was symbols, e. g., section lift coticient cl, section profle-
not takeninto account in spite of the fact that the test drag coefficient c%, section optimum lift coefficient
resultstend to indicate that the uncorrected part (see czo,,,and section pitching-moment coefficient about the
appendix) of the support interference may cease to be aerodynamic center % .C.. These values are then con-
negligibleatlow testReynolds Numbem. These errors sidered applicable to flight at the effective Reynolds
may be judged by a study of the dissymmetry of the Number, R..
test results for positive and negative angles of attack Table I presents,for various Reynolds Numbemj the
for thesymmetricalairfoilsandby thescatteringof the principal aerodynamic characteristics, in the form of
points representing the experimental data. (See figs. thesefully corrected section characteristics, of the &-
2 to 24.) Such a study indicateathat the resultsbm foils tested. Cross plots of certain of these section
tests at tank pressures at and above 4 atmospheres characteristicsagainstReynolds Number arealsogiven
(effective Reynolds Numbem above 1,700,000) are of for‘We with the discussion. (See fig. 28 and @s. 32
the same order of accuracy as those from the highest to 43.)
DISCUSSION
Reynolds Number tests. The drag and pitching-
moment resultsfor effective Reynolds Numbers below Scale effects, or the variations of aerodynamic coef-
800,000, however, become relatively inaccurate owing ficients with Reynolds Number, have previously been
tolimitationsimposed by thesensitivityof themeasur- considered of primary importance only in relation to
ing equipment. In fact, it appears that the accuracy the interpretation of low-stile testresultsfrom rLtmos-
becomes insufficientto define withcertainty the shapes pheric wind tunnels. It now appears from variable-
of curves representing variations of these quantities density and full-scale-tunnel data thnt important
with angle of attack or lift coetlicient. Hence airfoil variations of the coefficientsmust be recognized within
characteristicsdependent on the shape of such curves, the flight range of values of the Reynolds Number,
e.g., theoptimum lift coefficient and the aerodynamic- particularly in view of the fact that the flight range is
ccnter position, are considered unreliable and in most continually being increased.
AIRFOILSECTIONCHARACTERISTIC-SASAFFECTEDBY VARIATIONSOFTHE REYNOLDSNUMBER 231
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Fmwmz 5.—N. A_ O. A. 031S.
AIRFOILSECTIONCHARACTERISTICSASAFFECTEDBY VARIATIONSOFTHE REYNOLDSNUMBER 233
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FIGUEE 6.—N. A. O. A. !24Lz
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234 REPORTNO..580-NATIONALADVISORYCOMMITTEEFORAERONAUTICS
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l?muEE9.—N. A. O.A.449.
.
AIRFOILSECTIONCHARACTERISTICSASAFFECTEDBY VARIATIONSOFTHE REYNOLDSITUMBER 235
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Fxoumi 10.—N.& C.A.44L5.
1 I I I I I I I I Ill
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‘Reynolds Number
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236 REPORTNO.586-NATIONAL ADVISORYCOMMITTEEFORAERONAUTICS
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