This paper presents the results of wind-tunnel tests of several airfoils of low aspect ratio. The airfoils included three circular Clark Y airfoils with different amounts of dihedral, two Clark Y airfoils with slots in their portions, and three flat-plate airfoils. Lift, drag, and pitching-moment characteristics of the slotted airfoils with slots open and closed; pitching moment characteristics of one of the slotted airfoils with slots open and closed; and lift characteristics of the flat-plate airfoils are included. The results reveal a definite improvement of lift, drag, and pitching-moment characteristics with increase in dihedral of the circular Clark Y wing. Lift characteristics near the stall were found to depend markedly on the shape of the extreme tip but were not greatly affected by slots through the after portion of the airfoils. Changes in plan form of the flat-plate airfoils gave erroneous indications of the effect to be expected from changes in plan form of an airfoil of Clark Y section. The minimum drag characteristics of the circular Clark Y airfoils were found to be substantially the same as for a Clark Y airfoil of conventional aspect ratio

Zimmerman, C H

English

NASA Technical Reports Server

TECH_CAZ N0_S
NATIONAL ADVISORY COMMITTEE FOR AERONAUTICS
j.-
AERODYNAMIC CHARACTERISTICS OF SEVERAL AIRFOILS
OF LOW ASPECT RATIO
By C, H. Zimmerman
Langley Memorial Aeronautical Laboratory
Washington
A_g_ st 19_5
https://ntrs.nasa.gov/search.jsp?R=19930081303 2020-03-17T04:07:32+00:00Z
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L •
NATIONAL ADVISORY CON_ITTEE FOR AERONAUTICS
TECHNICAL NOTE NO. 539
AERODYNANIC CHARACTERISTICS OF SEVERAL AIRFOILS
OF LOW ASPECT PATIO
By C. H. Zimmerman
SUmmARY
This paper presents tile results of wind-tunnel tests
of several airfoils of low aspect ratio. The airfoils in-
cluded three circular Clark Y airfoils with different
amounts of dihedral, two Clark Y airfoils with slots in
their after portions, and three flat-_plate airfoils. Lift,
drag, and pitching-moment characteristics of the circular
Clark Y airfoils; lift characteristics of the slotted air-
foils with slots open and closed; pitching-moment charac-
teristics of one of the slotted airfoils with slots open
and closed; and lift characteristics of the flat-plate
airfoils are included,
The results reveal a definite improvement of lift,
drag, and pitching-moment characteristics with increase
in dii_edral of the circular Clari_ Y wing. Lift character-
istics near thG stall were found to depend markedly on the
shape of the extreme tip but were not greatly affected by
slots through the after portion of the airfoils. Changes
in plan form of the flat-plate airfoils gave erroneous in-
dicaticns of the effect to be expected from changes in
plan form of an airfoil of Clark Y section. The mini_um
drag characteristics of the circular Clark Y airfoils were
found to be substantially the same as for a Clark Y air-
foil of conventional aspect ratio.
INTRODUCT ION
Wind-tunnel tests of a series of Clark Y airfoils
(reference l) revealed interesting aerodynamic character-
istics for airfoils having aspect ratios of the order of
1.27. Experimental airplanes utilizing airfoils of very
low aspect ratio have added to the interest in the possi-
bilities of designs embodying such airfoils. The tests
I_.A C.A Tecnnical }_ote _,_o. 559
reported herein are ,,pa_rt of an exploration started to de-
termine the combination of physical f6aturcs of a low-
,..._. .aspect-ratio wing for ,.p_._mum aerodynamic characteristics.
The ex_loration has been discontinued because of the pres-
sure of more urgent work.
Included° in the present paper are the results of
tests of the following airfoils cf low asT:_ect ratio:
throe circular Clark Y airfoils with various amounts of
dihedral, two slotted Clark Y airfoils, and three flat-
plate airfoils. The tests ,of theslotted and flat-plate
airfoils were in the nature of preliminary .work to be _sed
in the preparation of a general program.
APPARATUS AI_D Y.0DELS
All tests were m._de on the regular balances installed
in the I_.A.C.A. 7- by 10-look atmospheric wind tunnel (ref-
erence B).
The circular Clark Y airfoils with various amounts of
dihedral are shown in figure I. These airfoils were built
of laminated mahogany to a precision of O.01 inch and fin-
ished _ith shellac. The Clark Y airfoil _ection was p're-
served to a point as _ear the tio as practicable, the chord
lines being kept parallel to the root chord. The airfoils
were ii.14 inches .in diameter. The dihedral was varied by
changing the curve, of the intersection of the chord lines
with a plane normal to the chord• The basic curve adopted
is given by the relation
Z = K(0maxr - 0max)
where _ is the _-_erpendicular distance from a plane in-
cluding the root chord line to the chord line of the indi-
vidual section; K is an arbitrary constant; 0max r is
the maximum - " _e o. re t se " is theo.rd._na _ _: the o ction, Oma x
maximum ordinate o.¢ the ,_rtic_.lar section Values of K
for th 9 airfoils in this investigation wore 0, I, and 2.
The slotted Clark Y airfoils tested are shown in fig-
ures 2 and 3. The eiliD_ical airfoil of aspect ratio I
.(fig. 2) is the airfoil without slots for which test re-
sults appear in reference I. The root chord was 14.14
inch.cs and th.e span !l,ll inches, the construction :being
• , • .. .. , ,,
[
N.A.C.A. Technical Note No. 539 3
similar to that of the circular airfoil with K = I. The
airfoil shown in figure 3 was derived from the circular
Clark Y airfoil with K = 1 by placing the quarter-chord
points of the individual sections in a plane perpendicular
to the root chord. The root chord and the span for the
airfoil tested were each 14.14 inches. The area was that
of a circular airfoil 14.14 inches in diameter. The con-
struction of this airfoil was similar to that of the cir-
cular airfoils.
The flat-plate airfoils were each 1/16 inch thick,
14.14 inches root chord, and 14.14 inches span. One was
circular in plan form; the other was the _ame in plan form
as the airfoil shown in figure 3, This latter airfoil was
tested with two different edges at the leading edge so
that it served as two airfoils in the test series. These
airfoils were made from flat stock steel with the edges
smooth and rounded,
TE $T $
The lift, drag, and pitching moment were determined
for each of the Clark Y airfoils at angles of attack from
-50 to 60 °. The elliptical airfoil was tested with slots
closed, slots open, and with the outer lip of each of the
slots on the under surface extended to form an air scoop.
The slotted airfoil of aspect ratio 1.27 was tested with
slots open and slots closed.
Values of lift and drag were determined for the flat-
plate airfoils at angles of attack from 0° to 60 o .
All tests were made at an air speed ef approximately
80 miles per hour, giving a Reynolds N_mber of approxi-
mately 860,000 based on the root chord.
RESULTS
Results of the tests are presented in standard coeffi-
cient form in figures 4 to ll. The effects of varying the
dihedral upon the shape of the lift-coefficient curve and
upon the value of the maximum lift coefficient appear in
figures 4 and 5. Variations of minimum drag coefficient,
of the shape of the polar curves, and of pitchlng-moment
coefficients with changes in the dihedral of the airfoil
r i/_" • • •
4 N.A.C.A. Technical Note No. 539
/
I)
are given in figures 5, 6, and 7, respectively. The pitch-
ing-moment coefficients are referred to the quarter-chord
point of the root chord. Variations of the curves of lift
coefficient against angle of attack with changes in tip
shape and plan form, with the addition of slots through
the airfoil, and with change of plan form in the case of
flat plates, appear in figures 8, 9, and 10, respectively.
The pitching-moment characteristics of the slotted airfoil
with aspect ratio of 1.27 with slots open and closed are
given in figure ll.
The results have not been corrected for tunnel-wall
or blocking effects. The probable errors in measurements
are as given in reference 2.
DISCUSSION
The circular Clark Y airfoils with various amounts of
dihedral were tested as part of a program designed to re-
veal physical features of low-aspect-ratio airfoils that
most markedly affect their aerodynamic characteristics. A
circular airfoil with the maximum ordinates of the upper
surface in a plane parallel to the root chord of the air-
foil had given the best characteristics of the series of
low-aspect-ratio airfoils reported in reference 1. The
first series of additional tests undertaken involved
changlng the dihedral angle as indicated in figure 1. The
method of variation of the dihedral adopted was chosen be-
cause it did not necessitate sharp changes in the shape of
the airfoils as seen in a front elevation and because it
permitted including the circular airfoil of reference I in
the regular series.
As viii be seen from figure 4, increasing the dihe-
dral moved the angle of attack for zero lift to a lower
value and increased the value of the maximum lift coeffi-
cient. The trend of the increase in maximum lift coeffi-
cient is more clearly shown in figure 5.
The test results indicated the variation of minimum
drag coefficient with dihedral shown in figure 5. It is
quite possible, however, that slight differences in the
accuracy of construction or in the surface finish of the
airfoils masked the true effect of the dihesral. It is of
especial interest that the airfoils of low aspect ratio
gave values of minimum drag coefficient substantially the
_i ¸¸ _
N.A.C.A. Technical Note No.539
same as those given by conventional Clark Y airfoils in
the 7- by 10-foot tunnel. (See flg. 5.) This result is
contradictory to the results presented in reference 1 but
confirms tests in the variable-density tunnel with the
circular airfoil after it had been refinished, The dSffer-
ence between the present results and those given in refer-
ence 1 is thought to have been caused by the difference
in finish of the airfoils.
Increasing the dihedral resulted in large reductions
in the drag at values of lift coefficient corresponding
to climbing and slow cruising speeds. With K = 2 the
total drag was less than the induced drag computed from
c2
the relation CDi - b_- for values of CL between 0.8
S
and 1.2. The total drag of this airfoil was approximately
80 percent that of the airfoil with K = 0 and 88 percent
that of the airfoil with K = 1 for values of lift coef-
ficient from 0.3 to 0.7.
Increasing the dihedral resulted in no important ef-
fect upon the curves of pitching-moment coefficient re-
ferred to the quarter-chord point of the root chord at low
values of the lift coefficient but did result in a decrease
in the diving moment at high values of the lift coefficient.
This effect is favorable in that less control moment would
be necessary to obtain a given value of CL for the air-
foil with the greatest dihedral. It is interesting to
note that almost the same pitching-moment coefficient was
obtained at maximum lift coefficient for each of the air-
foils.
Details of tip shape and changes in plan form had
large effects upon the maximum lift coefficient obtaina-
ble, as shown in figure 8. The only differences between
the circular Clark Y airfoil with K = 1 reported herein
and the original circular Clark Y airfoil reported in ref-
erence 1 were in the surface finish and in the shape of
the extreme tips. In the case of the original airfoil the
tips were slightly rounded; whereas in the new airfoil the
tips were carried to a sharp edge. As will be seen from
figure 8, the original airfoil apparently had a slightly
smaller effective aspect ratio and reached a higher value
of maximum lift coefficient. The slotted airfoil of as-
pect ratio 1.27, when tested with the slots closed, gave
lower values of maximum lift coefficient than either of
8 N.A.C.A, Technical Note No. _39
/,
,!
the circular airfoils. The extreme tip shape of this air-
foil was the same as that for the circular airfoil with
E = 1 in the present series of tests.
The preservation of unburbled flow to very high an-
gles of attack, in the case of the airfoils of low aspect
ratio, is apparently due to the action of the tip vortices
in removing the boundary layer that tends to buildup near
the trailing edge of the upper• surface of the airfoil. A
qualitative preliminary exploration of the direction of
flow over the upper surface indicated that possibly sl0ts
through the airfoils opening upward and outward would furl
ther delay the separation of the flow. Two Clark Y air-
foils of very low aspect ratio were fitted with slots and
tested with slots open and slots closed. As shown bY fig-
ure 9, there was a slight increase in maximum lift coeffi-
cient, but the increase was not sufficient to warrant ad-
ditional investigations along this line. Fitting air
scoops to the•lower surface of the airfoil to direct air
through the slots had but slight effect.
The diving moment of the slotted airfoil of aspect
ratio 1.2V with the slots closed was less than that for
the circular airfoil with K = i throughout the lift-coef-
ficient range. Opening the slots produced increases of
the order of 30 percent in diving moment, The slope of
the curves with slots open and with slots closed was sub-
stantially the same.
Flat plates were tested to determine whether the ef-
fect of plan form upon the maximum lift coefficient could
be predicted qualitatively for other airfoil sections from
such t_sts. That such a prediction would be reasonably
accurate had seemed possible because the tests of refer-
ence 1 showed a very great influence of plan form upon the
characteristics at very low aspect ratios. The results in-
dicate that an airfoil with the quarter-chord points of
the individual sections in a plane perpendicular to the
root chord and 25 percent of the root chord back of the
leading point of the root section should give higher val-
ues of maximum lift coefficient than one of circular plan
form. This prediction was not substantiated in the case
of the Clark Y airfoil, as may be seen from figure 8.
N.A.C.A. Technical Note No. 539 7
/,
CONCLUSIONS
I. The minimum drag coefficients of circular Clark Y
airfoils do not differ greatly from those for a Clark Y
airfoil of conventional aspect ratio•
2. Increasing the dihedral of airfoils of low aspect
ratio results in large decreases in drag at values of lift
coefficient corresponding to climbing and slow cruising
speeds,
3. The value of maximum lift coefficient and of the
angle of attack at which it occurs is greatly affected by
the shape of the extreme tip of the airfoil•
4• Slots of the t_pe used in this investigation af-
fect but slightly the lift characteristics of low-aspect-
ratio airfoils.
5. The nature of the variation of maximum lift coef-
ficient with plan form for airfoils of very low aspect
ratio cannot be predicted from tests with flat plates•
Langley Memorial Aeronautical Laboratory,
National Advisory Committee for Aeronautics,
Langley Field, Va., July 15, 1935.
lieFE lIENCE S
I •
•
Zimmerman, C.H.: Characteristics of Clark Y Airfoils
of Small Aspect liatios. T.li. No, 431, N.A.C.A.,
1952.
Harris, Thomas A.: The 7 by l0 Foot Wind Tunnel of
the National Advisory Committee for Aeronautics•
T.li. No. 412, N.A.C,A., 1931.
k
N.A.C.A. Technical Note No.539 Figs. 1,2,3
K--2
0_max _ ._
K'I
/
K=0
Fron[ elevation Side elevat ion
Figure 1.- Circular Clark Y airfoils. Z=K(0maxr-0max)
L,
Alr scoop
Sect ion A-A
Figure 2.- Slotted low-aspect-ratio
airfoil, b2/S=l
Airfoil section - Clark Y.
Figure 3.- Slotted low-aspect-ratio
airfoil, b_/S_ 1.27
Airfoil section-Clark Y.

r4--
8 16 24 32 40 48
Angle of attackl _, desrees
L)
0
-,04 -
c
._-.08 -
o-.i_-
-
-.16 -
o
_-20
-24
56
.,-._
-.28
.2
Lift coefficient, CL
.4
I
I
--.4
i
I
.6 .8
i _
L- L-
1.0
J
[_
1.2 1.4 1.6' 1.8
I __I _ _ _I
o-------I<--0
= 1
" =2
h _
---%_
z
-d
c_
Fisure 7.-Effect of dihedral on pitchins-
moment coefficient.
Circular Clark Y airfoils.
Tigure4-- Effect of dihedral on lift coefficient.Circular Clark Y ,airfoils.
_=_,
c_
0
6_
_t
0
ol
t_
to
oq
[h

N_1.8
o
(D
O
o
-_1.6
N
_3
::N
: _ ! , I ii , l , I , , , i_t I
t 1 i I I 4--! l'
t I .._.._---- i i _ t I ! I 1 I I 1
I ! i _ _ l t 1 ! i ! t I I
_i , , , , , T- , ! , ! I i!
I , _ ______ I .__[ _
t t i 1 i , t * _
__ ...._, 'i............._ _ **,_.....-_ i ....I ---
I 1 i ' _ i - I ' .L -±......_ -
.r4
o .0_
CD
o
o
h0
.r.I
r i ;
I 1 I 1 I
t 1 t 1 '*
4- '.o_ ! ...............
o_ i ;_ -i- ,
_ -'--, -=--4--- -_
.01 ! '
i
i I
..... "-t •
t
I , I '
I
i t
._ .........
i
!
t '
•--:------'---F---_ ..... t....i l
I
tVal,_ofrom_o_ts"
I airfoil of,refer_
[]' iValuelgrom }ests _
l nel of airfoil o_
_._____
n atmospheric tunnel of
mce 1_ rout% finish.
__._____._.-_---4......_ .....
i reference 4, ref_nishel
I....:....
F,
cyl
',.o
1
Dihedral factor, K
Figure 5.- Effect of dihedral on maximum lift coefficient and minimum drag coefficient_
.Circular Clark Y airfoils. o]

b 2
Drag coefficient, CD Cni : CL2/7T -_"
.04 .08 .12 .18 .20 .24 .28 0 }(= 0 _=_
1.o" . _'-1_1 I _- 'i: 1
, t. //_1"_"/f_--J--_ x_ Circular flat plate
I . " 1.6
.g u
.2
0
.16 .32 ,48 .64 .80 .96 1.12
Drag coefficient, CD
Figure 6.- Effect of dihedral on polar diagram. Circular ClarkY airfoils.
f%
t%
0
t_
0
c_
c_
CO
_ v
i L
- k • '
. 7 -
56
Figure 8.- Effect of tip shape and plan form on
oefficien% Clark Y airfoils,lift c
b2/S = l.g7
56
Figure 9 - Efiec% of slots on lif_ coefficient-
" Clark Y airfoils.
o_
¢o

00
0
Eo_-.04
._ -.os
t)
,r.._
m -o12
0
u
-.l_
G]
0
-.20
b_
-.24
P_
-.28
G4
Lift coefficienl, CL
.£ .4 .6 .8 1.0 1.2 1.4 1.6
- t I I o----- Slols closed
!!!
]
]
]
Figure ll.-Effecl of slols on pitching-
momenl coefficient.
ClarkY airfoil, b2/S = I.Z7.
Figure 10.- Effect of plan form on lilt coefficienio
Flal plate air foils.
Z
b
f0
0
o
co
o



Aerodynamic characteristics of several airfoils of low aspect ratio

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