Low-Speed Longitudinal Aerodynamic Characteristics - CiteSeer

116 Pages·1999·1.36 MB·English

NASA Technical Paper 3503
Low-Speed Longitudinal Aerodynamic
Characteristics Through Poststall for
Twenty-One Novel Planform Shapes
Gregory M. Gatlin
Langley Research Center • Hampton, Virginia
Brian E. McGrath
Lockheed Engineering & Sciences Company • Hampton, Virginia
National Aeronautics and Space Administration
•
Langley Research Center Hampton, Virginia 23681-0001
August 1995
Available electronically at the following URL address: http://techreports.larc.nasa.gov/ltrs/ltrs.html
Printed copies available from the following:
NASA Center for AeroSpace Information National Technical Information Service (NTIS)
800 Elkridge Landing Road 5285 Port Royal Road
Linthicum Heights, MD 21090-2934 Springfield, VA 22161-2171
[phone] [phone]
Summary with substantially greater maneuverability than current
fighter aircraft configurations. However, to develop such
To identify planform characteristics which could a vehicle, appropriate initial studies would include an
have promise for a highly maneuverable vehicle, an investigation of the characteristics of various total con-
investigation to determine the low-speed longitudinal figuration planform shapes. A study of complete config-
aerodynamics of 21 planform geometries was conducted uration planforms could identify promising combinations
in the Langley Subsonic Basic Research Tunnel (SBRT). of wing and body shapes which would be appropriate for
Concepts that were studied included twin bodies, double a highly maneuverable vehicle. Thus, to assess alternate
wings, cutout wings, and serrated forebodies. The plan- planforms for efficient high-angle-of-attack perfor-
form models that were tested were all 1/4-in.-thick flat mance, a low-speed study was undertaken.
plates with beveled edges on the lower surfaces. A
1.0-in.-diameter cylindrical body with a hemispherical This report presents the low-speed aerodynamic
nose was used to house the six-component strain gauge characteristics of 21flat plate planform models. Because
balance on each configuration. Aerodynamic force and wings and bodies affect the overall configuration aero-
moment data were obtained over an angle-of-attack dynamics, both wings and bodies were varied as part of
range of 0(cid:176) to 70(cid:176) with zero sideslip at a free-stream the investigation. Therefore, the term planform is used
dynamic pressure of 30psf, which corresponds to a free- throughout this report to refer to the total configuration
stream Mach number of0.14. including wings and bodies. The goal of this investiga-
tion was not necessarily to define an optimum planform
Surface flow visualization studies were also con- shape but to identify promising planform concepts. High
ducted on selected configurations to aid in the interpreta- lift performance and benign stability characteristics at all
tion of the force and moment data. The surface flow was angles of attack were the primary features of interest.
made visible through the use of fluorescent minitufts and
ultraviolet lighting. Minitufts were applied to model Current fighter aircraft are typically designed for
upper surfaces in a 1/2-in.-square grid pattern, and photo- both speed and maneuverability, but because air-to-air
graphs of the complete flow patterns on the configura- combat generally occurs at subsonic speeds, maneuver-
tions were taken. ability is of utmost importance. (See ref.14.) These
requirements drive fighter aircraft to designs with wing
Results of the investigation indicate that, when leading-edge sweeps ranging from 20(cid:176) to 70(cid:176) , depending
compared with single-component configurations, twin-
on the mission, and low aspect ratios ranging from 2 to4.
body planforms have minimal effects on lift and longitu-
(See refs.14 and15.) The resulting planforms are typi-
dinal stability characteristics, while double wings can
cally delta, arrow, and low-aspect-ratio swept wings. The
improve lift and longitudinal stability characteristics.
objective of providing pilots with maximum visibility
The investigation of cutout wing planforms indicates
has also resulted in designs of slender forebodies with
that, when compared with noncutout configurations, cut-
cockpits located as far forward on the vehicles as possi-
out planforms can improve lift characteristics; however,
ble. These fundamental design characteristics of low-
cutout size, shape, and position and wing leading-edge
aspect-ratio swept wings and slender forebodies are still
sweep all influence the effectiveness of the cutout con-
present in the planforms that are discussed in this report.
figuration. Tests of serrated forebodies identify this con-
cept as an extremely effective means of improving The 21planforms that were investigated included
configuration lift characteristics. Increases of up to flat plate representations of double-wing and cutout wing
25percent in the values of maximum lift coefficient can configurations as well as twin-body and serrated fore-
be obtained when compared with a nonserrated forebody body concepts. Double-wing and cutout wing configura-
configuration. tions were expected to benefit from a forward and an aft
lifting surface, which would act independently at moder-
Introduction ate to high angles of attack. Previous research on close-
coupled wing-canard configurations (refs.16–18) has
With continuing demand for increased maneuver- shown favorable improvements in maximum lift co-
ability of fighter aircraft, especially at high angles of efficients when a close-coupled canard was added to the
attack (ref.1), novel control concepts are receiving more basic wing planform. Thus, in this investigation, it was
and more attention. A number of concepts such as anticipated that two separate wings would generate lift
deployable strakes (refs.2–5), blowing and suction sys- more effectively than one. In addition, the effects of cut-
tems (refs.5–10), and porous surfaces (refs.11–13) have out size, shape, and location were investigated. Twin-
been developed for a variety of applications. These novel body and serrated forebody configurations were included
means of control could be incorporated into a newly to investigate the effects of multiple forebody vortices on
developed vehicle design and result in a configuration the high-angle-of-attack characteristics. The serrated
forebody was specifically designed so that a vortex Model Description
would form on each forebody serration; thus, multiple
vortices would extend over the length of the configura- In this wind tunnel investigation 21 different plan-
tion. These multiple vortices were expected to enhance forms were tested. Six planforms were chosen as refer-
lift-generating capability at moderate to high angles of ence configurations and are listed in tableI. The
attack. remaining 15 planforms were identified as alternate con-
figurations and are listed in tableII. The alternate config-
In typical planform studies such as those presented urations were designed as variations of the reference
in references19–21, the effects of wing aspect ratio, configurations. The reference area, length, and span used
leading-edge sweep, and trailing-edge sweep on the lon- to calculate the aerodynamic coefficients for all configu-
gitudinal aerodynamic characteristics must be investi- rations are presented in tablesI andII. In all cases, the
gated. Therefore, baseline planform shapes that included configuration planform reference area includes the body
variations in these parameters were designed, built, and and wings; the cutout area of a cutout wing configuration
tested; the acquired data were used as a reference point was not included. The photograph in figure1 shows the
for comparison with the alternate planform concepts. installation of a typical planform model in the test section
of the Langley Subsonic Basic Research Tunnel (SBRT).
Longitudinal aerodynamic data were obtained in the
Sketches of all planform models that were tested are pre-
Langley Subsonic Basic Research Tunnel (SBRT) on
sented in figures2 and3.
each of the 21planforms for angles of attack ranging
from 0(cid:176) to 70(cid:176) at zero sideslip. These data were taken at a Eleven of the planforms were models that were spe-
free-stream dynamic pressure of 30psf, which corre- cifically designed and built in accordance with predeter-
sponds to a free-stream Mach number of0.14. Surface mined specifications for improvement in maneuver-
flow visualization was also conducted on selected plan- ability at high lift. These planforms are referred to as the
forms to aid in the interpretation of the longitudinal force original planform models and are presented in the
and moment data. sketches in figure2. Each sketch includes side and bot-
tom views of the model as tested with the balance hous-
Symbols ing. The top surface of each planform model is flat and
lies on the centerline of the balance housing. The balance
All measurements are presented in U.S. Customary
housing is a 1.0-in.-diameter cylinder with a hemispheri-
Units. All data have been reduced to standard coefficient
cal nose. To ensure uniform flow separation and, thus,
form, and longitudinal data are presented in the stability-
uniform vortex formation at positive angles of attack, the
axis system.
leading edges were made sharp by beveling the lower
surface. Trailing edges were beveled on the lower sur-
b wing span, in.
faces of all configurations except on the two delta wing
Drag
C drag coefficient, ------------ planforms. Because the trailing edges of the delta wings
D q S
¥ were not swept, beveling would have had no effect and
Lift was deemed unnecessary. The 11 original planform mod-
C lift coefficient, ----------
L q¥ S els are all 1/4-in.-thick plates and include 55(cid:176) and 65(cid:176)
delta wings, 30(cid:176) and 40(cid:176) diamond wings, a 47.5(cid:176) dia-
C maximum lift coefficient
L,max mond with a wide forebody, 30(cid:176) and 40(cid:176) diamond wings
C pitching-moment coefficient, with twin forebodies, 30(cid:176) and 40(cid:176) diamond cutout wings,
m
Pitchingmoment and two double-wing configurations.
-----------------------------------------
q Sc
¥
The remaining 10 planforms were configured during
c planform reference length, in.
the test by modifying several of the original planform
q free-stream dynamic pressure, 1---r V2, psf models. These planforms, referred to as the modified
¥ 2 ¥
planforms, were also designed to improve maneuver-
S planform reference area, ft2
ability at high lift and are presented in the sketches in
V free-stream velocity, ft/sec figure3. They are identified by asterisks in tableII. Each
¥
modified planform was generated by attaching 1/16-in.-
X distance from model nose to moment refer-
ref thick flat plates to the top of an existing model to pro-
ence center, in.
duce the desired planform. In all cases, the edges of the
a angle of attack, deg flat plates were taped over to minimize the effects of any
forward- and rearward-facing steps. Because the original
L leading-edge sweep, deg
complete configurations are defined in figure2, only the
r density, slugs/ft3 modified overall planform shapes are presented in
2
figure3. The first modified planform is referred to as the Test Conditions
“55(cid:176) delta with trailing-edge serrations.” It was produced
by adding a forebody and a sawtooth trailing edge to the All 21 planforms were tested at a free-stream
55(cid:176) delta wing. The forebody is the same size and shape dynamic pressure of 30psf (159ft/sec), which corre-
(L =80(cid:176) ) as the standard forebody on the majority of the sponds to a nominal Reynolds number of 1.0· 106 per
original configurations. The 55(cid:176) delta with trailing-edge foot with the model positioned on the tunnel centerline.
serrations configuration was used to investigate the Longitudinal data were obtained at increments of 10(cid:176) for
effects of a forebody and trailing-edge serrations on a angles of attack ranging from 0(cid:176) to 70(cid:176) . Longitudinal
55(cid:176) delta wing. data were also obtained at angles of attack of 25(cid:176) and 35(cid:176)
to provide a better definition of the aerodynamic charac-
The next five modified planforms resulted from teristics around CL,max. All tests were conducted at zero
changing the cutout area on the original 30(cid:176) diamond sideslip. Because of the sharp edges on the configura-
cutout wing planform. These modifications are shown in tions and the vortex-dominated flow fields, the applica-
figures3(b)–3(f). By covering parts of the original cut- tion of transition grit was deemed unnecessary in this
out, configurations with forward, aft, and reduced-area investigation.
cutouts were assembled. In addition, a planform with a
diamond-shaped cutout was produced. The cutout area
Test Techniques
on the 30(cid:176) diamond with diamond cutout configuration is
equal to the cutout area on the 30(cid:176) diamond with the
The aerodynamic forces and moments on each con-
smallest cutout. This was done so that the effects of cut-
figuration were measured with an internal six-component
out shape alone could be investigated. The 30(cid:176) diamond
strain gauge balance. Even though all six force and
with reduced cutout configuration has a ratio of cutout
moment components were measured, only the longitudi-
area to total planform area of0.1 to1.0, which is the
nal components were of interest in this study and are the
same ratio as that of the 40(cid:176) diamond cutout wing config-
only data presented. Lift and pitching-moment data are
uration. Thus, these two configurations provided a means
of primary interest in identifying the aerodynamic char-
of identifying leading-edge sweep effects on a cutout
acteristics of each planform and, therefore, are analyzed
configuration. One last configuration was created by
in this report as each configuration is discussed. All lon-
reshaping the 30(cid:176) diamond cutout wing planform into a
gitudinal data are presented in tableIII. No base drag or
55(cid:176) delta cutout wing, as illustrated in figure3(g). The
base pressure corrections were made to any of the data
remaining modified planforms were produced to study
because these corrections were not expected to have a
the effects of serrated forebodies. These included a delta
significant effect on the overall longitudinal aerodynamic
sawtooth forebody, which was added to the 65(cid:176) delta
characteristics of the flat plate configurations; therefore,
wing, and two diamond sawtooth forebody configura-
the zero-lift drag has been included in all drag data
tions, which were added to the 40(cid:176) diamond wing. The
presented.
serrated forebody configurations are presented in
figures3(h)–3(j).
To present the pitching-moment data in a manner in
which the planforms could be appropriately compared
Test Conditions and Techniques with one another, these data were adjusted so that all
configurations would be neutrally stable at 0(cid:176) angle of
attack. This was accomplished by determining the
Wind Tunnel Description moment arm increment associated with the change in
pitching moment as the configuration was moved from
The investigation was conducted in the Langley Sub- an angle of attack of 0(cid:176) to 10(cid:176) . This moment arm incre-
sonic Basic Research Tunnel. This facility is an open- ment was then subtracted from the moment arm at each
circuit atmospheric wind tunnel capable of producing a data point to produce the desired pitching-moment data.
maximum continuous test-section speed of 194ft/sec For each configuration, the moment reference center
(q = 45 psf). The test-section dimensions are 22.5in. resulting from this procedure is identified by the distance
¥
wide by 32.25in. high by 73in. long. The model support X , as measured back from the nose, and is presented in
ref
system provides an angle-of-attack capability of 0(cid:176) tableIII. A desirable pitching-moment curve is indicated
to70(cid:176) in increments of 5(cid:176) while maintaining the model by a linear distribution of the data as presented. A linear
on the tunnel centerline. Sketches of the model support distribution is desirable because near-neutral stability can
system and the balance housing used in this investigation be obtained across the angle-of-attack range by proper
are presented in figures4 and5. location of the aircraft center of gravity.
3
The longitudinal stability characteristics of each con- The force and moment balance used in this investi-
figuration will be independently addressed as the overall gation was designated as SWT-01 and is a standard
results for each planform are presented throughout the Langley six-component strain gauge balance. The maxi-
report. However, because a desirable pitching-moment mum static load errors for this balance have been deter-
curve is indicated by a linear distribution, a linear regres- mined to be no greater than– 0.3percent of the maximum
sion was performed on the pitching-moment data for load of each component. Maximum errors are presented
each planform to provide a quantitative assessment of the in tableV for the normal, axial, and pitch components in
planform stability characteristics. The correlation co- both load and coefficient form. Even though all six bal-
efficients, as derived from the linear regressions, are val- ance components were measured, only the longitudinal
ues between zero and one and indicate the degree of lin- components were of interest in this study; therefore, only
earity of the pitching-moment data. The greater the value the errors for these components are presented.
of the correlation coefficient, the more linear the data; a
value of one indicates perfect linearity. Correlation co- Discussion
efficients for the pitching-moment data of each of the
The aerodynamic data figures for configuration com-
21planforms are presented in tableIV in decreasing
parisons and the flow visualization figures are listed in
order from most linear to least linear. A general assess-
tablesI andII. To discuss the aerodynamic character-
ment of these data indicates that the pitching-moment
istics of the planforms in a meaningful fashion, the data
data is fairly linear for correlation coefficients of 0.90 or
are presented in groups according to specific design
greater. Thus, configurations with correlation co-
characteristics.
efficients greater than 0.90 were assumed to have desir-
able longitudinal stability characteristics, while marginal
Diamond Wings and Twin Bodies
longitudinal stability characteristics were identified with
configurations having correlation coefficients between The first set of data, presented in figure6 provides
0.70 and 0.90. for a comparison between the 30(cid:176) and 40(cid:176) diamond
wings as well as identifies the effects of twin-body plan-
In addition to the force and moment data, flow visu- forms. The single-body planforms are shown in
alization studies were conducted on selected config- figures2(c) and2(d) and the twin-body planforms in
urations, and photographs illustrating surface flow pat- figures2(f) and2(g). The lift coefficient data indicate
terns were taken. Monofilament nylon fluorescent that at angles of attack up to 20(cid:176) , the 30(cid:176) diamond wing
minitufts, 0.0019in. in diameter, were applied to the generates a slightly greater lift coefficient than the 40(cid:176)
models in a 1/2-in.-square grid pattern and used with diamond wing, but at angles of attack greater than 25(cid:176) ,
ultraviolet lighting to identify flow patterns on the upper the 40(cid:176) diamond wing is clearly more effective in gener-
surface of each configuration. Minitufts respond to the ating lift than the 30(cid:176) diamond wing. When the 30(cid:176) dia-
surface flow and indicate regions of attached or separated mond wing with twin bodies is compared with the single-
flow as well as areas influenced by vortical flow. body configuration, a slight decrease in lift is noted at
Attached flow in this investigation is generally indicated angles of attack below 20(cid:176) , but a more substantial
when the minitufts are aligned with the free-stream flow increase in lift occurs at angles of attack of 25(cid:176) and
direction, while separated flow is indicated by minitufts greater. The effect of twin bodies on the 40(cid:176) diamond
that are lifted off the model surface and/or randomly ori- wing is a decrease in lift at angles of attack up to 40(cid:176) and
ented when compared with one another. When a vortex an increase in lift at angles of attack from 50(cid:176) to 70(cid:176) . At
lies above the model surface, the swirling vortical flow, CL,max, the twin bodies improve lift on the 30(cid:176) diamond
which initially comes up over the swept leading edge and wing but reduce lift on the 40(cid:176) diamond wing. Therefore,
then moves around the vortex core, will orient the mini- the overall effect of twin bodies on both diamond wings
tufts so that they are tangent to this flow. Thus, the pres- that were tested is that no increase in lift is generated at
ence of a vortex above a wing or forebody surface with a angles of attack below 20(cid:176) , while the most substantial
swept leading edge is generally indicated by minitufts increase in lift is generated by twin bodies on the 30(cid:176) dia-
swept in an outboard direction. Additional documenta- mond wing at angles of attack greater than 20(cid:176) . This
tion of the fluorescent minituft flow visualization tech- occurs because at angles of attack below 20(cid:176) strong fore-
nique is presented in references 22 and 23. Flow body vortices have not yet formed, and thus, the addition
visualization photographs, where available, are presented of a second forebody provides no lift benefit. However,
and discussed along with the longitudinal force and at angles of attack of 50(cid:176) and greater, an additional fore-
moment data. Photographs were generally taken at the body produces an additional set of forebody vortices, and
angle of attack corresponding with CL,max as well as at thus, the lift-generating capability is enhanced. At angles
angles of attack of 20(cid:176) and 50(cid:176) . of attack between 20(cid:176) and 50(cid:176) , the effectiveness of the
4
forebody vortices in producing additional lift is most surface or to be oriented on the surface in a random
likely dependent on the interaction of the forebody and disheveled fashion. Examples of each of these tuft pat-
wing vortices. The drag coefficient data (fig.6(b)) indi- tern interpretations are presented in reference22.
cate a slight improvement in the drag characteristics in
the region of C of the 30(cid:176) diamond wing with the The sketches presented along with the photographs
L,max in figures7 and8 of both diamond wings show the rela-
addition of the second body; however, no appreciable
benefit is obtained from the 40(cid:176) diamond wing with twin tive location of the primary regions affected by vortical
flow, attached flow, and/or separated flow. In each
bodies.
sketch presented in this report, the primary area affected
As mentioned previously when analyzing the by a forebody and/or wing vortex is identified by a
pitching-moment data, note that a desirable pitching- shaded region. In many cases, the influence of the fore-
moment versus angle-of-attack curve is indicated by a body vortices cannot be separated from the influence of
linear distribution. With this in mind, the pitching- the wing vortices, and a shaded region indicating the
moment data presented in figure6(c) indicate only a effects of both vortices is presented.
small effect due to twin bodies for both diamond wing
configurations. These data also indicate that the 40(cid:176) dia- When the flow visualization photographs and
sketches of both diamond wings (figs.7 and8) are exam-
mond wing would have slightly better longitudinal stabil-
ity characteristics than the 30(cid:176) diamond wing because of ined at angles of attack up to 35(cid:176) , the influence of a fore-
body vortex and a wing vortex can be seen on each side
the undesirable change in sign of the slope of the
pitching-moment data for the 30(cid:176) diamond wing as it of the configuration. These vortices are typical for a con-
passes through 20(cid:176) angle of attack. Also, note that the figuration consisting of a forebody and a swept wing, and
additional documentation of such vortices is presented in
twin bodies on both diamond wing configurations gener-
ate a nose-up increment in pitching moment at 0(cid:176) angle references24 and25. A vortex is present on each side of
the forebody with its core swept aft at an angle slightly
of attack, which may be due to the additional forward-
greater than the forebody sweep angle. A wing vortex is
facing surface of the beveled leading edge of the second
also present on each side of the configuration with its
forebody. Overall, however, the longitudinal stability
core originating at the wing-body junction. The wing
characteristics of each of these configurations are reason-
vortex is swept aft at an angle slightly greater than the
able, and the twin-body planforms do not produce any
wing leading-edge sweep angle. These well-behaved
significant adverse effects. This conclusion is further
vortical flow patterns are indicative of a prestall condi-
supported by the correlation coefficients presented for
tion. Note that, as the forebody vortices flow aft of the
these configurations in tableIV.
wing leading edge, the forebody and wing vortices are
In addition to the force and moment data, surface likely to interact and the influence of both vortices will
flow visualization photographs were taken of both 30(cid:176) be present on the configuration. An attached flow region
and 40(cid:176) diamond wing configurations. These photo- is indicated on the aft inboard portion of both diamond
graphs, which show surface flow conditions as indicated wing configurations at 20(cid:176) angle of attack and illustrates
by fluorescent minitufts, are presented in figures7 and8. that vortical flows are not influencing this area; however,
Each fluorescent minituft photograph presented in this some effect from the balance housing may be present. At
report is accompanied by a sketch which indicates the 50(cid:176) angle of attack (figs.7(d) and8(c)), minitufts on the
primary regions influenced by vortical flows as well as forebodies are still generally swept outboard, which indi-
regions of attached and separated flows. These sketches cates that vortices still exist in this area. However, fully
are interpretations of the minituft patterns and are pre- separated flows on the wings of both diamond planforms
sented as an aid to the reader. In general in this report, are indicated by the minitufts being lifted up off the
tufts which are swept outboard on forebodies and swept model surface, and evidence of wing vortices no longer
wings indicate regions influenced by the presence of a exists. Furthermore, the dynamics of separated flow con-
vortex lying just above the model surface. A feed sheet ditions are indicated in the photograph presented in
originating at the leading edge of a swept wing will lift figure7(d); completely separated flow is indicated by the
up off the surface and then revolve around the vortex minitufts on the majority of the right wing, while the
core. If the vortex core lies close enough to the surface of minitufts on the left wing are generally well behaved and
the wing, the vortex will scrub the model surface and oriented in the free-stream direction. This asymmetric
deflect the tufts outboard and toward the leading edge. surface flow pattern was typical of many of the configu-
When vortex influence is not present and the surface rations at 50(cid:176) angle of attack and was observed to oscil-
flow is attached and well behaved, the tufts are generally late back and forth between indications of separated flow
aligned with the free-stream flow direction. Separated and indications of relatively well-behaved flow on either
flows often consist of some reversed flow and some side of the configuration. The significant amount of sepa-
dynamic erratic flow, which causes tufts to lift off the rated flow present at this angle of attack indicates a
5
poststall condition in which lift is reduced and drag is angles of attack above 20(cid:176) for the double-wing configu-
increased. Thus, the flow visualization photographs rations when compared with the single-wing configura-
effectively illustrate the surface flow conditions and sup- tions. The pitching-moment data indicate only slightly
port the trends identified in the force data. Because the better longitudinal stability characteristics for the double-
twin-body planforms did not produce any substantial wing configurations than for the 30(cid:176) diamond wing;
effects in the force and moment data, flow visualization however, the absence of significant nonlinearities in the
was not conducted on these configurations. data for all configurations results in relatively compara-
ble pitching-moment characteristics. This conclusion is
The overall results of this phase of the investigation also supported by the correlation coefficients presented
indicate that the 30(cid:176) and 40(cid:176) diamond wings and twin- in tableIV.
body configurations all perform similarly. However, the
40(cid:176) diamond wing generates a slightly greater lift co- Fluorescent minituft photographs were taken of both
efficient than the 30(cid:176) diamond wing for angles of attack double-wing configurations and are presented in
greater than 25(cid:176) . The twin bodies produce a slight figures10 and11. As noted in previous flow visualiza-
improvement in lift on both diamond wing configura- tion photographs, the minitufts being swept outboard on
tions at angles of attack beyond CL,max but are more the forebody indicate that forebody vortices are present
effective on the 30(cid:176) diamond wing configuration. on both configurations at all angles of attack presented
up to 50(cid:176) . On the 30(cid:176) diamond twin wing, vortical flow
Double Wings regions are indicated on both wings at angles of attack of
20(cid:176) and 35(cid:176) . Other promising surface flow conditions are
The next phase of the investigation dealt with the indicated at 50(cid:176) angle of attack (fig.10(c)) where the sur-
effects of double wings. As mentioned previously, dou- face flow appears to have less separation than on the 30(cid:176)
ble wings were expected to benefit from forward and aft diamond wing. (See fig.7(d).) Specifically, at 50(cid:176) angle
lifting surfaces which would act independently to effec- of attack, the minitufts on the leading edges of the wings
tively generate more lift than that of a single wing at of the 30(cid:176) diamond twin wing are lying in a more stream-
moderate to high angles of attack. A 30(cid:176) diamond twin wise direction than the minitufts on the 30(cid:176) diamond
wing (fig.2(j)) and a 60(cid:176) double-arrow configuration wing. The aft wing is benefitting from operation at a
(fig.2(k)) were tested and these data are presented for lower effective angle of attack because of the downwash
comparison with the 30(cid:176) diamond wing data in figure9. (ref.27) from the forward wing while the forward wing
The lift coefficient data indicate a 24-percent increase in may simultaneously be benefitting from a similar influ-
CL,max when comparing the 30(cid:176) diamond twin wing ence because of the presence of the aft wing. The possi-
with the 30(cid:176) diamond wing. The 30(cid:176) diamond twin wing bility of both wings operating at effectively reduced
also produces a greater lift coefficient than the 30(cid:176) dia- angles of attack would generate more lift in the poststall
mond wing at the angles of attack beyond CL,max. This angle-of-attack range and could explain the greater lift
indicates that, in the moderate- to high-angle-of-attack generated by the 30(cid:176) diamond twin wing than by the 30(cid:176)
region, an additional wing, whether it is strongly influ- diamond wing. The surface flow visualization of the 60(cid:176)
enced by the other wing or effectively acting indepen- double-arrow configuration (fig.11(c)) indicates the
dently, adds significantly to the lift-generating capability same general trends on the forward wing as on the wings
of the configuration. One explanation is that two sets of of the 30(cid:176) diamond twin wing configuration. However,
vortices may effectively generate a larger low-pressure the 60(cid:176) leading-edge sweep produces a wing vortex
region on the upper surface and produce greater overall which lies farther inboard than the wing vortex on the
lift on the surface of two small wings than would just one 30(cid:176) diamond wing. The more inboard location of the
set of vortices on a single larger wing. The 60(cid:176) double- wing vortex on the 60(cid:176) double arrow has a greater influ-
arrow configuration generates a CL,max comparable with ence on the overall configuration and, thus, could
that of the 30(cid:176) diamond twin wing while generating more account for the improved lift characteristics beyond
lift than the 30(cid:176) diamond wing or the 30(cid:176) diamond twin CL,max. In addition, the more inboard location of the
wing configurations at angles of attack beyond CL,max. wing vortex could possibly result in a beneficial inter-
The greater leading-edge sweep on the 60(cid:176) double arrow action between the forebody and wing vortices. How-
may well account for some of the lift increases beyond ever, the flow over the outboard portion of the aft wing
CL,max because the nonlinear portion of lift increases of the 60(cid:176) double arrow is very different from the flow
with increasing leading-edge sweep. (See ref.26.) In a over the forward wing. At each of the angles of attack
similar flat plate planform study presented in ref- presented, the random orientation of the minitufts and the
erence24, increases in lift at and beyond CL,max were fact that they are generally lifted up off the model surface
noted as wing sweep was increased. The drag coefficient indicate that the flow over the aft wing appears to be sep-
data (fig.9(b)) indicate increases in both C and C at arated. This means that the outboard region of the aft
L D
6
wing is not generating as much lift as would a wing with patterns are very different. The 55(cid:176) delta wing
attached flow. (fig.13(c)) has separated flow over the majority of the
configuration, while the 55(cid:176) delta with trailing-edge ser-
The overall results of the double-wing tests indicate
rations (fig.14(b)) has very little, if any, separated flow.
that double-wing planforms produce increased lift and This explains why the 55(cid:176) delta with trailing-edge serra-
improved stability characteristics when compared with tions generates much more lift than does the 55(cid:176) delta
single-wing planforms. However, the aft-wing planform
wing at this test condition. Furthermore, note in ref-
may require specific attention to issues such as vertical
erence24 that forebody vortices resulting from the addi-
and horizontal placement as well as size to ensure that
tion of a forebody to a delta wing will have a stabilizing
the flow over the aft wing is not separated.
effect on the wing vortices and actually delay wing vor-
tex breakdown. This appears to be occurring in the flow
Forebody Effects visualization photographs just discussed at an angle of
attack of 35(cid:176) . At an angle of attack of 50(cid:176) , the photo-
The effects of a wider forebody as well as the effects
graphs of both configurations indicate substantial regions
of adding a forebody to the 55(cid:176) delta wing were investi-
of separated flow. Examination of the surface flow pat-
gated, and the results are presented in figure12. When
tern on the sawtooth trailing-edge region of the 55(cid:176) delta
compared with the 40(cid:176) diamond wing, the 47.5(cid:176) diamond
with trailing-edge serrations did not reveal any unex-
with wide forebody (fig.2(e)) produces a CL,max that is pected or significant flow phenomenon in this area at any
11.5percent greater. This increased CL,max may be due of the test angles of attack.
to the wider forebody and/or to the increased sweep of
the main wing; aerodynamic data in reference24 indicate In summarizing these comparisons, note that fore-
that a wider forebody will increase CL,max. When the body effects can be dependent on three-dimensional
55(cid:176) delta with trailing-edge serrations (fig.3(a)) is com- characteristics; more specifically, forebody vortex loca-
pared with the original 55(cid:176) delta wing, a less abrupt drop- tion will depend on the three-dimensional shape of the
off in CL occurs after CL,max for the 55(cid:176) delta with forebody. The results in this report show only two-
trailing-edge serrations. Although the 55(cid:176) delta wing pro- dimensional effects because the models are flat plates;
duces a greater CL,max, the 55(cid:176) delta with trailing-edge therefore, further tests would be required to represent
serrations produces greater lift coefficients at angles of three-dimensional configurations properly and to deter-
attack of 30(cid:176) to 70(cid:176) . This indicates that the presence of mine their associated aerodynamic characteristics. This
the forebody and the trailing-edge serrations on the 55(cid:176) study shows that a wider forebody on a diamond wing
delta wing significantly improve the lift characteristics at planform or the addition of a forebody and trailing-edge
angles of attack beyond stall. Previous studies have iden- serrations to the 55(cid:176) delta wing generally improves lift at
tified lift increases due to the addition of a forebody to a angles of attack of 30(cid:176) and above. The addition of a fore-
delta planform (ref.24); the addition of the forebody to body to the 55(cid:176) delta wing produces beneficial vortical
the 55(cid:176) delta wing, rather than the trailing-edge serra- flows over the configuration for angles of attack up to
tions, is expected to account for most of the lift increase. approximately 35(cid:176) . However, the longitudinal stability
When pitching-moment data are compared, the 55(cid:176) delta characteristics, generally, were not improved with the
with trailing-edge serrations shows the least desirable widening or addition of a forebody.
longitudinal stability characteristics, while the other
three configurations are substantially better. In fact, the Cutout Wings
correlation coefficients presented in tableIV indicate
that the 47.5(cid:176) diamond with wide forebody, the 55(cid:176) delta The evaluation of the effects of wing cutout regions
wing, and the 40(cid:176) diamond wing have the most desirable was a significant portion of this wind tunnel investiga-
tion. The first data to be presented are for the 30(cid:176) and 40(cid:176)
longitudinal stability characteristics of all the configura-
diamond wings with the original cutouts. (See fig.15.)
tions tested.
The intent of wing cutouts was to postpone flow separa-
Surface flow visualization photographs were taken tion on the wing by energizing the flow aft of the cutout
of the 55(cid:176) delta wing and the 55(cid:176) delta with trailing-edge and to produce a forward and an aft wing with reduced
serrations and are presented in figures13 and14. When chords when compared with the noncutout wing. Thus,
these configurations are compared at 20(cid:176) angle of attack, attached flow on the forward portion of the wing would
both indicate the influence of strong vortical flows over be maintained at higher angles of attack. The lift coeffi-
the majority of the configuration. As expected from the cient data for the 30(cid:176) diamond wing and the 30(cid:176) diamond
previous discussion of flow visualization photographs, cutout wing indicate that the cutout wing produces
strong vortical flows are noted on the forebody of the 55(cid:176) decreased lift coefficients at angles of attack between 10(cid:176)
delta with trailing-edge serrations. When the two config- and 35(cid:176) and a decreased CL,max. However, this same
urations are compared at 35(cid:176) angle of attack, the flow comparison for the 40(cid:176) diamond wings indicates that the
7
cutout improves the lift characteristics in the angle-of- ence area and high-lift regions results in a greater overall
attack region around CL,max and postpones CL,max by lift coefficient for the configuration. This conclusion is
approximately another 5(cid:176) angle of attack. At angles of derived from the fact that the lift coefficients for the cut-
attack between 40(cid:176) and 70(cid:176) , the cutout configurations out configurations are based on smaller reference areas.
generate greater lift coefficients on both diamond wing
The results from this initial data of cutout effects
planforms. The drag data for the 30(cid:176) diamond wing con-
indicate that improved lift can be generated by a cutout
figurations indicate that the cutout configuration pro-
duces a greater drag coefficient than the noncutout wing design at angles of attack beyond CL,max. How-
ever, the exact physics of how this improvement occurs
configuration across the angle-of-attack range. However,
is not yet well understood. To more systematically iden-
at prestall conditions, the 40(cid:176) diamond wing configura-
tify cutout wing effects, several tests were conducted
tions have roughly the same drag characteristics. When
with various cutout sizes, shapes, and locations.
evaluating the pitching-moment data, the trends indicate
that the wing cutouts tend to degrade the longitudinal sta- Determination of the effects of varying the wing
bility characteristics. The cause of increased drag and leading-edge sweep on a cutout wing configuration while
degraded longitudinal stability characteristics produced keeping the ratio of cutout area to total planform area
by the cutout configurations is not clear; however, the aft constant was of interest. To do this, the cutout region on
portion of the wing may be immersed in or unfavorably the 30(cid:176) diamond cutout wing was reduced so that the
affected by the wake from the forward portion of the ratio of cutout area to total planform area was equal to
wing. that for the 40(cid:176) diamond cutout wing. This area ratio of
0.1 to 1.0 was the design criteria for the configuration
Surface flow visualization was conducted on the 40(cid:176) identified as the 30(cid:176) diamond wing with reduced cutout.
diamond cutout wing, and photographs of this configura- (See fig.3(d).) Thus, this configuration is compared with
tion are presented in figure16. The conditions presented the 40(cid:176) diamond cutout wing and the data are presented
in figure16 can be compared directly with the flow visu- in figure17. For angles of attack of 30(cid:176) and above, the
alization photographs of the 40(cid:176) diamond wing presented 30(cid:176) diamond with reduced cutout generates essentially
in figure8. If the cutout configuration is thought of as the same increase in lift coefficient over the noncutout
having both a forward and an aft wing surface, then the configuration as does the 40(cid:176) diamond cutout wing when
flow over the aft wing is shown to be separated at all compared with the 40(cid:176) diamond wing. Thus, in this
angles of attack presented (i.e., 20(cid:176) , 35(cid:176) , and 50(cid:176) ). The angle-of-attack range, wing leading-edge sweep has neg-
surface flow on the 40(cid:176) diamond wing is not separated at ligible impact on the effect of a cutout wing. At angles of
either 20(cid:176) or 35(cid:176) angle of attack. This explains why the attack of 0(cid:176) and 10(cid:176) , the 30(cid:176) diamond with reduced cut-
40(cid:176) diamond wing produces more lift at 20(cid:176) angle of out generates greater lift coefficients than the noncutout
attack than the 40(cid:176) diamond cutout wing. This same configuration, whereas the 40(cid:176) diamond cutout wing
trend would be expected at 35(cid:176) angle of attack; however, does not. This may be due to an unintentional cambering
why both configurations produce the same lift coefficient effect created when the 30(cid:176) diamond cutout wing plan-
at this condition is not clear. When the photographs of form was modified by attaching flat plates to create the
the 40(cid:176) diamond cutout wing (figs.16(a) and16(b)) are reduced cutout configuration.
compared with those of the 40(cid:176) diamond wing (figs.8(a)
When the pitching-moment data for the cutout con-
and8(b)) at angles of attack of 20(cid:176) and 35(cid:176) , note that the
figurations are compared, the cutouts are shown to have a
presence of the cutout affects the wing vortex and may
similar effect on both 30(cid:176) and 40(cid:176) diamond wings. In
cause it to burst prematurely or to be moved farther off
both cases the cutouts tend to degrade the longitudinal
the body. This conclusion is derived from the fact that
stability characteristics; however, the cutout effect is
the presence of a vortex is indicated by the outboard-
greater on the 30(cid:176) diamond wing configuration. The data
swept minitufts on the aft portion of the 40(cid:176) diamond
presented thus far on the effects of cutouts indicate that a
wing, whereas the minitufts on the aft portion of the 40(cid:176)
cutout configuration can generate increased lift coeffi-
diamond cutout wing do not indicate the presence of a
cients at angles of attack beyond stall and that wing
vortex at all. The reason for the improved lift perfor-
leading-edge sweep in the range of 30(cid:176) to 40(cid:176) has little
mance of the cutout wing (fig.16(c)) when compared
impact on cutout effects.
with the 40(cid:176) diamond wing configuration (fig.8(c)) at
50(cid:176) angle of attack is not evident from the flow visualiza- The effect of cutout size was investigated next. The
tion photographs as both indicate separated flow over the 30(cid:176) diamond cutout wing, which was modified once to
wings. However, lift coefficients on the cutout configura- reduce the cutout, was modified again to further reduce
tion may be greater because the cutout regions produce the cutout size resulting in a configuration which will be
less lift than the regions that are retained (e.g., forebody referred to as the “30(cid:176) diamond with smallest cutout.”
region). Therefore, the combination of a smaller refer- (See fig.3(e).) Longitudinal data for the 30(cid:176) diamond
8