Zalovcik, J. A.

English

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ACE Jo. L431 
NATIONAL ADVISORY COMMITTEE FOR AERONAUTICS 
to V...,11ARTIM1I IRI PORT 
z 0M'd ORIGINALLY ISSUED 
*e3 _ct:_- May 194-4 as 
Advance " Resort 14331 
' PROPF -MlE COEFFICINTS OF COII7UTIONAL AND LOW-DRAG 
E4
=u) C AIRFO]lS AS OBTAINfED IN FLIGHT 
S14 * By John A. Zalovik 
H 10 Langley Memorial Aeroaiutical Laboratory
 
Langley Fieldj, Va.
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nic e been reproduced without change in order to expedite general distribution.
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NOTICE
 
THIS DOCUMENT HAS BEEN REPRODUCED 
FROM THE BEST COPY FURNISHED US BY 
THE SPONSORING AGENCY. ALTHOUGH-IT 
IS RECOGNIZED THAT CERTAIN PORTIONS 
ARE ILLEGIBLE, IT IS BEING REL'EASED 
IN THE INTEREST OF MAKING AVAILABLE 
AS MUCH INFORMATION AS POSSIBLE. 
NACA ACK No. 14P.31 
NATIONAL ADVISORY COMI,ITTEE FOR AERONAUTICS 
ADVANCE CONFIDEN7TIAL REPORT 
PROFILE-DRAG COEFFICIENTS OF CONVENTIONAL ADD LOW-DRAG 
AIRVOILS AS OBTAIMED IN FLIGHT 
By John A. Zalovcik
 
S UI'2 !ARY 
The results of flight investigations of the rrofile 
dreg of several csrefully finished conventional and low­
drag airfoils are presented. The results indicated that
 
in all cases lower profile-drag coefficients were
 
obtained with the low-drag than with the conventional 
airfoils over the range of lift coefficient tested and
 
that, for comparable conditions of lift coefficient a _,
 
Reynolds number, the low-drag airfoils may have profile­
drag coefficients which are ab least 27 percent lower
 
than the profile-drag coefficients of The conventional
 
airfoils.
 
INTRODUC TION
 
A number of flight investigations have been 
conducted by the National Advisory Connittee for 
Aeronautics during the past several years to determine 
the profile drag of .various conventional and low-drag 
airfoils. The purpose of this report is to present
 
.he principal results of these investigations in order
 
to provide information that may be of assistance in
 
judging the relative merits of conventional and low­
drag airfoils.
 
AIRFO ILS TESTED 
The various airfoils tested were the NACA 27-212,
 
NACA 35-215, NACA 66,2-2,(14.7), NACA 64,2-(1.4)(13.5),
 
4eproduced ro 
2 NACA ACR No. LE5
 
NACA 241l.5, N-22, and two Republic S-3 sections, one 
11 nercent thick and the other 13 percent thick. These 
two sections are designated Republic S-5,11 and Repub­
lic S-.13 in tbis parer. Flight tests of the 
NACA 6 _,2--(l.4)(13.5) and the NACA 2414.5 airfoils are 
reoorted in references 1 and 2, resnectivel. The pro­
files of the airfoils tested are shown in figure 1. 
The NACA 27-212 and NACA 35-215 airfoil sections were 
built into panels around the wings of the airplanes on 
which they were tested. The others were sections of 
the actual wings of the rest airplanes. The arrangement
 
of the test panels and the spanwise positions of the
 
wing sections tested are shown in plan form in figure 2.
 
The airfoil designation NACA 6452-(1.4)(13.5), which is
 
the test section of the NACA-NAA (North American Avia­
tion, Inc.) compromise low-drag wing, was based on the
 
rnaxtmLn thickness and on the pressure-distribution 
characteristics computed from the measured ordinates 
of the test section. The designation NACA 66,2-2(14.7) 
was similarly determined. 
The NACA 2414.5, Republic S-3,11, Republic S-3,13,
 
and N-22 sections may be classified as conventional
 
airfoils and the NACA 6h,2-(1.4)(13.5), NACA 27-212,
 
NACA 35-215, and NACA 66,2-2(14.7) sections, as low­
drag airfoils.
 
All the airfoils tested were carefully smoothed
 
and faired to eliminate perceptible protuberances due 
to rivets, skin joints, and access doors. Surface
 
waviness, however, was present to various degrees on
 
the different airfoils. Surface waviness was measured
 
by use of a curvature gage of the type shown in fig­
ure 5 on the uDoer surfaces of the NACA 35-215 and 
Republic S-5,15 airfoils and on the upper and lower 
surfaces of the NACA 64,2-(1.4)(13-5), NACA 66 2-2(14.7), 
and Republic S-3,11 airfoils. No waviness measurements 
were obtained for the other airfoils. 
The curvature-gage measurements on the NACA 55-215,
 
NACA 64,2-(1.4)(13.5), NACA 66,2-2(14.7), Republic S-3,11,
 
and Republic S-3,13 airfoils were made with the legs
 
of the gage spaced 1.2, 5.8, 4.0, 4.0 and 5.0 percent
 
of the section chord, respectively. In order to
 
present these measurements on a comparable basis, the
 
measurements on the NACA 35-215, RTACA 6h,2-(l.h)(13.5),
 
and Republic S-3,13 airfoils were reduced to values d
 
WACA ACR No. L4E31 3
 
that a gage would give if the legs were spaced 4.0 per­
cent of the section chord o. This reduction was made
 
to the first order of approximation on the assumption
 
that the readings of a curvature gage were proportional
 
to the square of the leg spacing. The reduced measure­
ments together with the measurements on the
 
NACA 66,2-2(14.7) and Republic S-5,11 sections are
 
presented in figure L as plots of a/c against s/c,
 
where a is the distance along the surface from-the
 
leading edge. The dashed lines in figure 4 indicate
 
the approximate curvature-gage readings that would be
 
obtained if the surfaces were free of waviness,
 
It should be pointed out that wing distortion
 
iftflight may introduce waviness considerably different
 
from'that measured. This effect is probably adverse
 
and may be expected to vary considerably with wing
 
construction.
 
The destabilizing effect on the laminar boundary
 
layer due to waviness of a given magnitude increases
 
as the chordwise velocity gradient becomes less favorable
 
(or more adverse). The chordwise velocity distribution
 
for the various airfoils at a section lift coeffi­
cient cy of 0.20 have therefore been included in
 
figure 4. The velocity distributions were calculated
 
for the undistorted airfoil profiles by the method of
 
reference 3. The velocity distributions are given as a
 
rlot of the ratio U/u. against s/c, where U is
 
the local velocity outside the boundary layer and
 
Uo is the free-stream velocity.
 
PROFILE DRAG
 
The profile-drag coefficients were evaluated from
 
wake surveys of the various airfoils by the method of
 
reference 4 and compressibility corrections were applied
 
as in reference 5. In figure 5 the section profile­
drag coefficients cdo and the corresponding Reynolds
 
numbers R are plotted against section lift coeffi­
cient 0o. The Mach numbers of the tests were less
 
than 0.55.
 
From figure 5 it may be seen that all the low-drag
 
airfoils gave lower profile-drag coefficients than the
 
L. NACA ACR No. I14E31
 
conventional airfoils over the range of lift coefficient 
tested. The lowest profile-drag coefficient, a value 
of 0.0040, was measured on the ITACA 27-212 section at 
a lift coefficient of 0.28 and.a Reynolds number 
of 7.4 x 10E. The NACA 27-212 airfoil, however, is not 
considered a particularly desirable airfoil because, as 
indicated by wind-tunnel tests, lcw drag is obtained 
only over a relatively small range of lift coefficient 
and the pressure gradient at the trailing edge is 
tmnecessarilyK severe. At Reynolds numbers in the range 
from 15 x10 to 20 x 106, now commonly encountered by
 
fighter-type aircraft, orofile-drag coefficients
 
of 0.00;45 and 0.0052 were measured on the NACA 66,2-2(14.7)

and NACA 64,2-(l. .(1 .5) airfoils, respegtively. At
 
Reynolds numbers from,22 x .00 to 31 x-l0b,a orofile­
drag coefficient of 0.0049 vas obtained on the
 
NAGA 35-215 airfoil.
 
The lowest profile-drag coefficient obtained on
 
the conventional wing secticns was 0.0062 and was
 
measrured on he Republic S-3 II.The lowest profile­
drag coefficients obuained on the other conventional
 
sctions were 0.0067 for the Republic S-3,13 and 0.0066
 
for t e NACA 2414.5. All these values were obtained at
 
low lift coefficients in the range of Reynolds number 
from 15 x lo6 to 20 x 106. On the N-22 section-only 
one value of profile-drag coefficient, 0.0070, was 
obtained, which was at the relatively high lift coeff4­10.
 
cient of 0.50 and the low Renolds number of 474 X 

The results for the NACA 66,2-2(14.7) and Repub­
lic S-5,11 sections were obtained for the most nearly
 
comparable test conditions - that is, lift coefficient,
 
Reynolds number, and wing-surface preparation - and are
 
therefore best suited for the comparison of the profile­
dreg characteristics of low-drag and conventional air­
foils. At a lift coefficient of 0.20 and a Reynolds
 
number of 16 x 106 the profile-drag coefficients for
 
the NACA 66,2-2(14.7) and Republic S-3,11 sections were
 
0.0045 and 0.0062, respectively. The profile-drag coef­
ficient of the NACA 66,2-2(lL.7) section is thus 0.0017,
 
or 27 percent, lower than the profile-drag coefficient
 
of the Republic S-3,11 section.
 
Unpublished tests in the NACA two-dimensional low­
turbulence pressure tunnel of a section aporoximating
 
_UAOA ACR No. l491
 
the NACA 66,2-2(14.7) indicated a profile-drag coeffi­
clent of 0.003J4 at a lift coefficient of 0.20 and a 
Reynolds number of 16 x 106. Similar tests (unpublished) 
of NACA 230-series airfoils indicated a profile-drag 
ccefficient of 0.0063 for an NACA 23011 section at a 
lift coefficient of 0,20 and a Reynolds number 
of 9 x 100 The Republic 8-3 sections have pressure­
distribution characteristics that are very nearly those 
of the WACA 230-series sections and may therefore be 
expected to have the same drag characteristics. Inas­
much as the surfaces of bhe NACA 66,2-2(14,7) airfoil 
tested in flight were carefully finished to give a very 
low degree of waviness (figs. 4(g) and (h)), probably 
comparable with that of the tunnel model, the con­
siderably greater drag measurad in flight as compared 
with the value obtained in the tunnel is believed to be 
due to an increase in surface waviness associated with
 
wing distortion under air loads. The.better agreement
 
between the fligbt and tunnel results for the conven­
tional sections may indicate that the position of
 
transition is 3o far forward on these sections that it
 
is not materially affected by an increase in surface 
waviness resulting from loads imposed on the wing in 
flight.
 
CONCLUDING REMARKS 
Ihe results of profile-drag tesbs of various
 
smoothed airfoils indicated that in all cases lower
 
profIle-drag coefficients were obtained on low-drag
 
airfoils than on conventional airfoils over the range
 
of lift coefficient tested. The results also indicated
 
that, for comparable conditions of lift coefficient and
 
Reynolds number, the low-drag airfoils may have profile­
drag coefficients which are at least 27 nercent lower
 
than the profile-drag coefficients for the conventional
 
airfoils
 
Langley Memorial Aeronautical Laboratory
 
National Advisory Committee for Aeronautics
 
Langley Field, Va.
 
6 i NACA 	ACR No. LtE31 
REFERENCES
 
1. 	Zalovoik9 John A.: A Profile-Drag Investigation 
in Flight on an Emperimenbal Fighter-Type Air­
plane - The North American XP-51 (Air Corps 
Serial iio. 1l±-38). N&CA ACR, Now 1942. 
2. 	Bicknell, Joseph: Determination of the Profile
 
Drag of an Airplane Wing in Flight at High
 
Reynolds Phmmbers. NACA Ren, No. 667, 1939.
 
3. Theodorsen, T., and Garrick. I. E.: General Poten­
tial Theory of Arbitrary Wing Sections. NACA
 
Rep. No. i52, 1933.
 
Ij. The Cambridge University Aeronautics Laboratory:
 
The Measurement of Profile Drag by the Pitot-

Traverse Method. P. & M. No. 1638, British A.R.C.,
 
1936.
 
5. Silverstein, A., and Katzoff, S.: A Simplified
 
Method 	 for Determining Wing Profile Drag in 
Flight. Jor. Aero. Sci., vol0 7, no. 7,
 
May 1940, pp. 295-301. 
-7-

NACA ACR No. L4EZ1 Fig. 1 
NACA 2414.5 
-
NATIONAL ADVISORY 
CONFIDENTIAL , COMMITTEE FORAERONAUTICS 
Figure I.- Profiles of various airfoi- sections tested
 
in flight.
 
NACA ACR No. L4EZ1 
 Fig. 2
 
Panel having
k WAZA 35-215 airfoi
 
,Tetsection N-? 
>1 f4 
SEl N-2 wing 
E 
Panel having - - Tes sec -jl7 -NAGA57-ZIZ airfol 
________-1 
S-3 wing 
Test section MP.CA (6,2-Z0(4.7) 
NASA 67(115)-213,a=0.7 RTest &ectio 
Rep ub I 
-ACA 66(2I5)-[(165), aLO -3 wng 
Test section approy. 
- Tst section 
NACA 64,Z- (.4 5) VW-- ACA Z414.5 
M~ACA-NAk compromi-se 
I low-drag Ong I NACA 2409 
NACA 2415 
CONFIDENTIAL 9 - 0"10 1
,3ca 1e, in. I NATIONAL ,ADvISORY
,Sca e.,t).COMMITTEE F'OR AERONAUTICS 
Figure 2.-
 Plan forms of various wings of which tests 
were made. 
z 
CA 
r 
..
Fgure 3.- Curvature gge. ++++++++ 

NACA ACR No. L4E31 	 Fig. 4a,b,c
 
U 0 
.o ,2_0 40 - 60 .70 
.001o
 
.0006 ,
 
.0006 
-­
-
.0004-­
0-	 -3 1,4­
6o0 ,0 ,2O ,30 40 '.50 74Y0 . 
s/c
 
.00 10------------------­
.0008 i -	 -- -, -,-­
0006 d/c I 
.0004- -­
-	
J4
 0­
-z --.00027 
I-CONFIDENTIAL,a
 
0 	 .10 2.0 .,30 40 .50 .60 70 NATIONAL ADVISORY 
S/C COIMFTrEE R ANAUTICS 
(al Republic S-3,11 airfoil, lower surface.
 
(b) Republic S-3,11 airfoil, upper surface.
 
(c) Republic S-3,13 airfoil, upper surface.
 
Figure 4.- Surface waviness and velocity distribution
 
(at Cj = 0.20) over various airfoils. (Dashed lines 
indicate approx. gage readings for surfaces free of 
waves.) 
-Il-

NACA ACR No. L4E31 Fig. 4d,e,f,g,h
 
\ - c(d) 
-4 0 
.0 002­
00 
0 10 O20 .30 440 0 .60 .70­
.0002- \N If-I L4 
Q y1

C ,8
 
0 ,O , 30 .,40 ,0 .60 70
 
. 0 0 ,0 3.0-. 40 0 . 
.0004
-0 ,10 ,20 .30 ,40 .50 .60 .70 
S NATIONAL ADVISORY 
MCOCMITTEE FORaAEROAUTIC .
 
o '10 .20 30 .4 50 .60 .70s/c.
000o - 'e-.. 
- -=-- .
 
0 .10 ,20 30 .40 .,50 .60 .70 
SIC
 
(d) NACA 64,2-(1.4)(13.5) airfoil, lower surface.
 
(e) NACA 64,2-(1.4)(13.5) airfoil, upper surface.
 
(f) NACA 35-215 airfoil, upper surface..
 
(g) NACA 66,2-2(14.7) airfoil, lower surface.
 
(ni MACA 66,2-2(14.7) airfoil, upper surface.
 
Figure 4.- Concluded.
 
NACA ACR No. L4E31 Fig. 5
 
32x10 6 
- __ ____ 
G\ 
28 
____ 
\"-NACA 35 -215B a irfoil 
24
 
11=e-Repubs .5-3j IS 
IS J 64 )2-1.013.6) 
I? x ACA 6654-e214.) 
-~ NACA 24145 
. >A4A 
NACAA 7-f511 
CC4 Yc S-3,1 
0 -08 .16 .24 .32 .40 48 .56 .64 .72
,5ection lift1 coefficient, c, 
.0080 Pub I,-
_IF /-VCZ41.___-//
 
o .0070 j_-ACA 
o .0060 -Repubhc -3 1I 
0050 NACA5-15 
.0NACA 27-El2 
0040 

'-NACAG6,a-2(14 7 ) 
ZVACA 64-,(4)(S.5) j CONFIDENTIAL 
0 .08 .16 24 3? .40 48 .56 .64 .72Section lift coefficient , NATIONAL ADVISORYCONMITTEFFOAERONAUTICS 
Figure 5.- Comparison of profile-drag coefficients ob­
tained in flight on various conventional and low-drag
 
airfoils. Reynolds number for corresponding lift
 
cefficients given above.
 
PIOPERTY UF JET PROPULSION LABORATORY IBRARY 
cAuFORNIA 111STI1UTE OF 1EChK, POY 


Profile-drag coefficients of conventional and low-drag airfoils as obtained in flight

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