Cahen, George L

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" 
RM L52C20 
NACA 
RESEARCH MEMORANDUM 
A PRELJMINARY GUST - TUNNEL INVESTIGATION OF LEADING -EDGE 
SEPARATION ON SWEPT WINGS 
By George L. Cahen 
Langley Aeronautical Laboratory 
Langley Field, Va. 
NATIONAL ADVISORY COMMITTEE 
FOR AERONAUTICS 
WASHINGTON 
June 6, 1952 
https://ntrs.nasa.gov/search.jsp?R=19930087139 2020-06-17T10:11:35+00:00Z
NACA RM L52C20 
NATIONAL ADVISORY COMMITTEE FOR AERONAUTICS 
RESEARCH MEMORANDUM 
A PRELIMINARY GUST -TUNNEL INVESTIGATION OF LEADING -EDGE 
SEPARATION ON SWEPT WINGS 
By George L. Cahen 
SUMMARY 
The results of a series of qualitative studies with tufts on three 
wings having sweepback angles of 300 , 450 , and 600 show that under cer-
tain conditions a leading-edge vortex can exist in the unsteady flow 
associated with a gust. It was indicated that, if a wing in steady flight 
prior to entering a gust is at an angle of attack several degrees less 
than that at which vortex flow first begins in steady flow, it may pene-
trate the gust without having the vortex develop, even if its angle of 
attack is increased by the gust into the vortex- flow regime. However, 
if the wing is within the vortex-flow regime prior to entering a gust, 
the vortex flow can progress rapidly. 
INTRODUCTION 
Wings having certain degrees of sweep and leading-edge radii have 
been found to develop a flow separation in the form of a leading-edge 
vortex in steady flow (references 1 and 2) . These vortices have been 
found to affect the total lift of the wings as well as the chordwise 
pressure distribution and other characteristics . During past investi-
gations in the Langley gust tunnel with swept-wing models, conditions 
have occurred which according to steady-flow data should be conducive 
to vortex formation. The gust condition, however, is one of unsteady 
flow and it is not immediately apparent that the flow behavior over a 
swept wing in a gust would be the same as that in steady flow. Some 
qualitative tuft studies were therefore made in the Langley gust tunnel 
to determine whether leading-edge vortices would occur in the unsteady 
flow conditions of a gust. 
A series of swept-wing models, previously used in load studies, were 
flown in the gust tunnel under the same conditions as in the load studies 
to determine whether leading-edge vortices were present during the load 
tests. Additional studies were then made with two of the models in order 
2 NACA RM L52C20 
to compare the re sults for steady and unsteady f l ow conditions . This 
paper present s the re sults of these studie s . 
APPARATUS 
Three model s having the semichor d lines sweptback 300 , 450 , and 600 
we re used for t he tests . The 300 and 600 sweptback- '-ling models used for 
these tests were those of references 3 and 4 . A new 450 sweptback-wing 
model identical to the one used in the test of r efer ence 5 was used for 
the pre sent tests . All of the model characteristics were maintained as 
in the tests of refe r ences 3, 4, and 5 except that tufts were placed on 
the upper surfaces of the left semispan of each wing. Photographs of 
the thr ee models are shown as figure 1, and some pert inent characteristics 
are given in the following table : 
0 30 swept - 450 swept - 600 swept -
back wing back wing back wing 
Weight , Ib 9 .75 9 . 25 9 . 63 
Wing area , sq ft 6 . 05 6 . 05 6 .17 
Wing l oading , Ib/sq ft 1. 61 1. 53 1.56 
Span, ft 5 . 2 4.25 3 .00 
Aspect ratio 4 . 44 2 .99 1.44 
Chord measured parallel to 
plane of symmetry : 
Mean geometric chord, ft 1.16 1.48 2 . 06 
Root chord, ft 1. 55 1.90 2 . 29 
Tip chor d , ft 0 .77 0 . 95 1.412 
Taper r atio 0 . 50 0 . 50 0 . 50 
Air foil section perpendicular 
to 50-percent - chor d line NACA 0012 NACA 0012 NACA 0012 
The Langley gust tunnel and its standard equipment are described in 
reference 6 . Special photogr aphic equipment , consisting of five speed 
flash lamps, an electronic timing and t rigge r ing deVice , a photoelectr ic 
cell, and three overhead cameras wer e used for these tests . Two of the 
cameras we r e placed in the upper section of the gust tunnel so as to 
view straight down on the flight path of the model and the third was 
placed above and in front of the test section so as to view it at an 
angle of about 450 • 
NACA RM L52C20 3 
TESTS 
The tests consisted of flights t hrough sharp- edge gusts in the gust 
tunnel under the conditions indicated in the following table : 
Sweep back of For ward velocity, Gust veloc i ty, Angle of attack 
wing , fl. , in steady flight , 
deg fps fps 0." deg 
30 88 10 3.7 
45 88 10 and 15 3 . 6 
60 {88 10 6 65 5 and 10 7 
Photogr aphs wer e taken f r om above so that five views we r e obtained 
of the model for each flight through the gust . 
Photogr aphs wer e also made of the tufts on the 450 and 600 sweptback 
wings through a range of angles of attack in steady flow. The 450 swept -
back wing was tested with a str eam velocity of 88 feet per second in the 
6- by 6- foot test section of the Langley stability tunnel , and the 
600 sweptback wing , with a str eam ve l ocity of 65 feet per second in the 
Langley f r ee -flight tunnel ( r efer ence 7) . The Reynolds number for the 
gust - tunnel flights and the steady- flow tests was of the or de r of 
800 , 000 . 
PRECISION 
The measur ed quantities ar e estimated to be accur ate within the 
following limits: 
For ward velocity, ft/sec •.•.•.•. 
Gust velocity, ft/sec .••...• . • 
Angle of attack fo r gust - tunnel flights , deg 
Angle of attack for wind- tunnel tests , deg 
• ±0 . 5 
±O . l 
±0 . 5 
±0 . 25 
The angles of attack in a gust fo r the gust - tunnel flights we r e 
determined by measur ing the angle of the wing with respect to the dir e c-
tion of flight , adding the gust angle , and subtr acting the angle resulting 
f r om the instantaneous ver tical velocity of the model . The steady-flight 
4 NACA RM L52C20 
angles of attack for the gust - tunnel flights were obtained by measuring 
the angle of the wing with respect to the flight path during steady free 
flight with no gust . Those for the wind - tunnel tests were measured with 
respect to the horizontal center line of the tunnel. 
RESULTS AND DISCUSSION 
The results from the tests are presented in the form of photographs 
grouped to show comparisons of the tuft patterns for the three wings 
under the various test conditions . The nature of leading-edge separation 
on swept wings in steady flow is described in references 1 and 2. The 
characteristic pattern which is formed by surface tufts in the presence 
of a leading-edge vortex is identified by the first row of tufts being 
lined up along the leading edge of the wing and those just aft showing 
a flow almost normal to the direction of flight. Where the vortex region 
does not extend over the full chord of the wing, the tufts near the 
trailing edge remain more in line with the stream direction and thus 
indicate reattachment of the flow behind the region of the vortex. 
Figure 2 shows the tufts on the three wings flying under conditions 
identical to those of the load tests of references 3, 4, and 5. Under 
these conditions the wings were flown at identical velocities (88 ft/sec) 
and lift coefficients through a sharp -edge gust with a velocity of 10 feet 
per second. Consequently, the three wings were subjected to the same 
initial angle -of-attack change due to the gust. The photographs in fig-
ure 2 show some spanwise flow near the trailing edges of the 300 and 
450 sweptback wings, but the pattern of the tufts on the 600 sweptback 
wing indicates that a leading-edge vortex was present during this flight. 
Thus, it was shown by these preliminary flights that a leading-edge 
vortex can exist under certain conditions in the unsteady flow associated 
with a gust. 
Studies of the initiation of the vortex flow were made by comparing 
the flow behavior at corresponding angles of attack for the wind-tunnel 
and gust-tunnel tests . The steady-flow test of the 450 sweptback wing 
model indicated that a leading -edge vortex developed over this wing at 
an angle of attack between 8.60 and 9.60 • Since figure 2 indicates that 
there was no leading-edge vortex over the 450 sweptback wing at 7.90 , it 
would appear, as might be expected, that the unsteady conditions of a gust 
will not cause a leading-edge vortex to develop at a lower angle of attack 
than it would or dinarily develop in steady flow. 
The results from the steady- flow and unsteady-flow tests of the 
600 sweptback wings are shown in figure 3. At 110 and 150 angle of attack 
NACA RM L52C20 5 
in steady flow, and at 110 and 15 .40 angle of attack in unsteady flow, 
leading-edge vortex patterns are shown by the tufts on the 600 sweptback 
wing. The tuft pattern before the model entered the gust was the same 
as shown for ,(0 angle of attack in the gust tunnel. It is interesting 
to note in figure 3 that the vortex appears to become stronger before 
the wing is completely within the gust. The tuft patterns for the steady 
and unsteady flow conditions appear to be almost identical at the same 
angle of attack . The reason for the difference between the two steady-
flow tuft patterns for the 70 angle of attack is unknown, however it may 
be due to differences in the flow conditions of the gust tunnel and the 
free -flight tunnel. Both of the patterns are indicative of a leading-
edge vortex, that for the wind tunnel indicating a somewhat stronger 
vortex. In figure 4, the tufts on the 450 sweptback wing in steady flow 
at 3 . 60 and 7.60 and in unsteady flow at 7.90 and 120 angle ·of attack 
show no indication of a leading-edge vortex. At 9.60 in steady flow, 
however , a vortex pattern is shown. In unsteady flow at 10.10 angle of 
attack there is a region of separated flow at the leading edge near the 
wing tip. The flow appears to r eattach behind this region as it does 
when a vortex occurs , but the pattern of the tufts in this case is not 
definitely characteristic of a leading-edge vortex. 
The data as presented appear inconsistent in that the vortex pat -
terns for the 600 sweptback wing in steady and unsteady flow are almost 
identical at given angles of attack, whereas there is no definite vortex 
pattern for the 450 sweptback wing in unsteady flow at 10 .10 and 120 angle 
of attack, and yet, in steady flow, a vortex pattern is shown at an angle 
of attack of 9.60 • This apparent anomaly suggests that the type of flow 
prevalent about a wing prior to its entering a gust may dictate the manner 
of flow development within the gust . With the 600 sweptback wing in 
steady flight at an angle of attack of 70 , a vortex pattern was present. 
As the model penetrated the gust the vortex apparently became stronger . 
The 450 sweptback wing in steady flight , however, at 3 . 60 angle of attack 
was about 60 from the angle of attack at which the vortex began to develop 
in the wind tunnel . The angle -of - attack change due to the 10-foot-per-
second gust was about 6 . 50 . Thus , the total angle of attack of the 
450 swept back wing should have been great enough for a leading-edge vortex 
to develop in steady flow. It therefore seems that there must be a lag 
in flow development over wings in gusts if the flow must develop from 
one r egime to another. Further study in the gust tunnel appears to be 
necessary to augment the present knowledge of air-flow development about 
sweptback wings in unsteady flow conditions . 
CONCLUSIONS 
The indicated results of tuft studies made in the Langley gust tunnel 
with three wings having sweepback angles of 30 0, 45 0, and 600 may be 
stated as follows: 
6 NACA RM L52C20 
1. Under certain conditions, a 1eading-edge-vortex flow can exist 
over a swept wing in the unsteady flow associated with a gust. 
2. If a wing in steady flight prior to entering a gust is at an 
angle of attack several degrees less than that at which vortex flow first 
begins in steady flow, it may penetrate the gust without having the vortex 
develop immediately even if the angle of attack is increased by the gust 
into the vortex-flow regime . 
3. If a wing is within the vortex -flow regime prior to entering a 
gust, the vortex flow can progress rapidly. 
Langley Aeronautical Laboratory 
National Advisory Committee for Aeronautics 
Langley Field , Va. 
REFERENCES 
1. Cahill, Jones F ., and Gottlieb, Stanley M.: Low-Speed Aerodynamic 
Characteristics of a Series of Swept Wings Having NACA 65A006 Air-
foil Sections . NACA RM L50F16, 1950. 
2 . Fitzpatrick, James E., and Foster, Gerald V.: Static Longitudinal 
Aerodynamic Characteristics of a 520 Sweptback Wing of Aspect 
Ratio 2 .88 at Reynolds Numbers from 2,000,000 to 11,000,000. NACA 
RM L8H25 , 1948. 
3 . Reisert, Thomas D.: Gust -Tunnel Investigation of a Wing ' Model with 
Semichord Line Swept Back 300 . NACA TN 1794, 1949. 
4. Pierce, Harold B.: Gust-Tunnel Investigation of a Wing Model with 
Semichord Line Swept Back 600 • NACA TN 2204, 1950. 
5 . Pierce, Harold B.: Tests of a 450 Sweptback-Wing Model in the Langley 
Gust Tunnel . NACA TN 1528, 1948. 
6. Donely , Philip: Summary of Information Relating to Gust Loads on 
Airplanes. NACA Rep. 997, 1950. (Supersedes NACA TN 1976.) 
7. Shortal, Joseph A., and Osterhout, Clayton J.: Preliminary Stability 
and Control Tests in the NACA Free -Flight Wind Tunnel and Correlation 
with Full - Scale Flight Tests. NACA TN 810, 1941. 
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Figure 1.- Photographs of the models used. 
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F i gure 1 .- Concluded. 
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Figure 2 .- Compari son of flow over the family of wings in the 
Langley gust tunne l . 
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A Preliminary Gust-tunnel Investigation of Leading-edge Separation on Swept Wings

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