Patterson, J. C., Jr.

English

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18. WIND-TUNNEL STUDIES OF NACELLE INTERFERENCE DRAG
AT HIGH SUBSONIC SPEEDS INCLUDING I
THE EFFECT OF POWERED JETS
By James C. __atterson, Jr.
NASA Langley Research Center
SUMMARY
Experimental wind-tunnel investigations have been conducted recently by
the National Aeronautics and Space Administration to determine the aerody-
namic interference associated with engine-pylon installations. The studies
included the effects of powered jets. Tests were conducted on an aft-
fuselage-mounted nacelle configuration over a Mach number range from 0.665
to 0.82 at Reynolds numbers of 2.74 × 106 to 3.94 × 106 based on the mean
geometric chord of 3.2 inches. Tests were also conducted on a semispan
model of a cargo-type logistic transport configuration with an underwing pylon--
mounted powered-englne nacelle and on a O.0576-scale semlspan model of the
Lockheed C-SA transport configuration. The effects of powered fan-Jet model
englaes were included. These tests were conducted over a Mach number range
from 0.70 to 0.825 at Reynolds numbers of 6.10 × 106 to 7.13 × lO6 based on
the mean geometric chord of 21.158 inches.
The results of these investigations indicate that favorable inter-
ference drag may be obtained by detailed tailoring of the engine nacelles
and pylons to the airplane itself. The effect of the powered jet on aero-
dynamic interference is such that the favorable interference noted at the
cruise Mach number was doubled throughout the higher lift-coefficient range
of this investigation.
_2RODUCTION
One important fact to be considered in the design of airplanes of today
is aerodynamic interference. Investigations to determine aerodynamic inter-
ference resulti_ from engine-pylon installations have been conducted
recently by the NASA on a small transport configuration having aft-fuselage-
mounted nacelles and on the more familiar underwing pylon-mounted nacelle
configurations. The studies of the underwing configurations included the
effects of a powered model of a fan-jet engine.
The ability to simulate the full-scale engine and nacelle aerodynamic
and geometric effects in the _-ind tunnel is unique and offers the potential
of obtaining engine-Jet-stream interference effects which could very pos-
sibly result in new engine-installation methods.
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259
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SYMBOLS
CD
ADD
CL
_Cp
M
Pt,e
Pt ,_
q
S
x/z
drag coefficient, Drag/qS
interference drag coefficient
lift coefficient
incremental pressure coefficient
Mach number
engine fan exit total pressure
free-stream total pressure
free-stream dynamic pressure
Reynolds number based on mean geometric chord
wing area
ratio of longitudinal distance from powered-engine leading edge to
total length of powered engine
DISCUSSION
Aft-Fuselage-Mounted Nacelle Configuration
A sting-supported model of a small transport configuration with engine
nacelles pylon-mounted On the rear of the fuselage is shown in figure 1. Tests
were conducted on this configuration with the engine nacelles mounted in various
longitudinal locations, with the engine-nacelle incidence angle and cant angle
varied to aline the engines with the local flow, and with the pylons and engine
nacelles extended chordwise (as shown by the dashed nacelle-pylon outline in
fig. l) in an attempt to improve the area distribution in the vicinity of the
engine nacelles. The test Mach number ranged from 0.665 to 0.82.
The results of this investigation are presented as interference drag coef-
ficient 2C D for the various configurations tested. These interference-drag
values are obtained from the difference between the drag level of the basic air-
plane configuration and that of the basic configuration less pylons and engine
nacelles plus the calculated skin-friction drag of the pylons and engine nacelle_
throughout the Mach number range for a lift coefficient of 0.25. (See fig. 2.)
The interference-drag results obtained for the four previously mentioned
configurations tested - engine longitudinal location, nacelle incidence angle,
260
• A
cant angle_ and engine and pylon extension - are shown in figure 3_ The
interference-drag results of the basic configuration are shown in each of the
four plots for reference. The negative values of 2_CD represent favorable
interference whereas the positive values represent interference drag. The
results obtained for the basic configuration indicate interference drag in the
lower Mach number range and favorable interference above a Mach number of 0.79.
The optimum longitudinal nacelle location, determined from tests conducted
on a series of chordwise nacelle locations, was obtained by moving the nacelle-
pylon combination rearward 27 percent of the wing mean aerodynamic chord from
its original position on the basic configuration. The interference-drag results
obtained for the improved longitudinal location indicate a reduction in inter-
ference drag coefficients over that obtained for the original location through-
out the Mach number range with an increase in the favorable interference at the
higher test Mach numbers.
To aline the engine nacelles with the local flow, tests were conducted
through a nacelle-incidence-angle range from approximately 0 ° to 4° . The
lowest interference drag results were obtained at an incidence angle of 2.5 ° .
The effect of this incidence angle is shown in the lower left-hand plot of
figure 3_ throughout the Mach number range_ the interference drag coefficient
is reduced.
A cant angle of 3.5 ° was investlgated (nacelle inlet directed outward),
which resulted in a slight decrease in drag in the lower Mach number range,
followed by a reduction in the favorable interference at higher Mach numbers.
Since only one cant angle was investigated, it is possible that further improve-
ment may very well be obtained at other cant angles.
The rearward chordwise engine nacelle and pylon extension, of approximately
25 percent of the nacelle length 3 increases the favorable interference from a
Mach number of 0.77_ to a Mach number of 0.82 as a result of an improvement in
the local area distribution.
Unfortunately, tests were not conducted with the nacelle-pylon configura-
tion in its most favorable location, but the configuration changes investigated
are such that their contributions to interference drag may very possibly be
additive. Nevertheless, the data presented indicate that the aerodynamic inter-
ference can be reduced and a favorable interference produced in many cases as a
result of tailoring the nacelle-pylon combination to the airplane itself by
taking into consideration the area distributions and local flow conditions.
Underwing Pylon-Mounted Engine Nacelles Including Power Effect
Investigations including the effects of the fan-jet flow on the aerody-
namic interference have also been made of the more familiar underwing pylon-
mounted engine configuration. With the development of the current high-bypass
fan-jet engines, the exact effect that the fan-jet flow has on aerodynamic inter-
fcrence has become vely important. An investigation to determine this power
261
effect has recently been conducted with the use of a semispan model of a car@
type airplane configuration. Figure 4 shows this model installed in the Langley
8-foot transonic pressure tunnel. The fuselage is mounted directly on the tun-
nel wall while the wing is mounted directly on the force balance. A powered
model of a fan-Jet engine is pylon-mounted under the wing. The force results
were obtained for the wing-pylon-engine combination and included the influence
of the presence of the fuselage on these components.
The interference drag coefficient _C D for the powered-model test was
obtained by reducing the total drag measured with the force balance by the com-
puted thrust and by the drag of the three individual components of the model:
wing, engine, and pylon. This procedure is shown by the following equation:
AC D = (CD)tota I + CF - (CD)wing - (CD)engin e - (CD, f)pylo n
where
(CD)total total measured wing-pylon-engine drag coefficient, obtained
from wall-mounted strain-gage force balance
CF engine net thrust coefficient based on wing area, computed from
total-pressure and static-pressure measurements taken in fan
inlet, fan exit, turbine inlet, and turbine exit
(CD)wing wing-drag coefficient, obtained from wall-mounted strain-gage
force balance during tests made with the engine and pylon
removed
(CD)engine measured drag coefficient less pylon drag coefficient and less
engine net thrust coefficient for the engine alone
(CD, f)pylon pylon skin-friction drag coefficient, calculated by using skin-
friction values from the Sommer and ShortT' method (see
ref. l)
The engine-alone measurements were obtained during tests of the engine mounted
on an elongated pylon that was mounted directly on the balance. The pylon drag
was measured during tests performed with the engine removed from this configura-
tion. The engine net thrust was obtained from the same type of pressure meas-
urements and computing method used during the complete-configuration test.
Fan-Jet engine.- The general outline of the full-scale fan-Jet engine which
is to be used on the C-5A logistic transport is shown in figure 5. The engine
is approximately 25 feet long and has a maximum fan-cowl diameter of 9 feet.
Eighty-five percent of the maximum thrust of 41 000 pounds is produced by this
fan. With a bypass ratio of 8.1, approximately 1335 lb/sec of the total engine
weight flow of 1500 lb/sec passes through the fan, whereas 165 lb/sec of air
enters the primary discharge nozzle.
262
JAn effort was made during the design of this engine to maintain an area
distribution which would be compatible to that of the overall area_buildup of
the C-SAtype airplane configuration. This effort has resulted in a smooth
total area distribution for the engine-airplane combination.
Model engine.- A cross-sectional view of the powered model of the C-_A
engine used during this investigation is shown in figure 6. The two-stage
fan is connected directly to the nitrogen-driven three-stage turbine. This
model engine was designed to produce the same mass-flow ratio and exit-pressure
ratio as the actual full-scale engine. At the maximum design speed of
h_ 000 rpm, the model engine, which has a maximum diameter of _.9 inches and an
overall length of 18 inches, develops approximately 130 horsepower.
Total-pressure rakes and static-pressure taps were located in the fan
inlet, in the fan exit, and in the turbine exit to obtain the measurements to
be used in computing thrust. A total of 276 pressures were recorded during
this investigation and included surface pressures on the fan cowl, turbine,
turbine exit plug, pylon surfaces, and wing upper and lower surfaces in the
vicinity of the wing-pylon Juncture.
Jet effects.- The effect of the powered fan-Jet engine on the aerodynamic
interference is shown in figure 7 as the interference drag coefficient _CD
plotted against lift coefficient for fan-exit pressure ratios of 1.O and 1.47.
A pressure ratio of 1.O is obtained with the engine operating Just fast enough
to overcome the internal losses of the engine, and zero thrust is produced. A
pressure ratio of 1.47 is obtained for the maximum design speed, where maximum
thrust is produced. The data indicate that with power on, the favorable inter-
ference is increased throughout the lift-coefficient range, with a maximum
increase of approximately lO drag counts at the highest lift coefficient.
A possible explanation for the increase in favorable interference may be
that with the engine operating at the zero thrust condition, there is a tend-
ency for the pressure to become equalized on the inboard and outboard sides of
the pylon. This balance of pylon pressure is believed to be caused by a pres-
sure leakage that takes place between the fan cowl and the turbine cowl, as
shown by the arrow in the cutawayoblique view of the engine in figure 7. The
change in pressure coefficient (ACp) between the inboard side and the outboard
side of the pylon measured along the nacelle-pylon Juncture is presented in the
lower plot of figure 7 as a function of nacelle length.
The pressure difference is small for the zero thrust condition; however,
with the engine producing maximum thrust, the fan exit is choked and the pres-
sure leakage from the inboard side to the outboard side of the pylon cannot
occur. This is shown in figure 7 as an increase in the difference in pressure
coefficient. This increase in pylon_normal force is believed to be a result of
an increase in the end-plate effectiveness of the engine nacelle with the fan
exit choked plus the effect of the increase in the fan-Jet flow over the rear
portion of the pylon. This pylon normal force, when reduced to a lift vector
perpendicular to the local flow, results in a thrust vector in the stream
direction, because of the direction of this local flow associated with the
263
swept wing. This induced thrust may possibly account, in part, for the favor-
able interference obtained with power on.
C-SA lo6istics transport.- Further investigations have been made by using
a semispan model of the C-SA logistics transport configuration mounted in the
Langley 8-foot transonic pressure tunnel. Again, the fuselage was mounted
directly on the tunnel wall and the wing was mounted directly on the force
balance. Two powered fan-jet engines were pylon-mounted under the wing of the
C-SA model as shown in figure 8. A total of 552 surface pressures and total
rake pressures were measured during this two-engine investigation.
En6ine position.- The results obtained froman investigation of engine
position are shown as interference drag plotted against Mach number in figure 9
for the basic engine position, for the engine moved rearward, and for the engine
moved rearward and vertically upward. The data indicate that the greatest
favorable interference effect is obtained with the engine in the most forward
position. These results substantiate results obtained previously (not shown
herein) for the one-engine test where it was also found that with the engine in
the foremost longitudinal and lowest vertical position the most favorable inter-
ference results were obtained.
The data in figure 9 indicate that interference drag is very sensitive to
engine position. With the engine moved rearward approximately l0 percent of the
engine length, favorable interference is obtained in the lower Mach range;
before the cruise Mach number is reached, however_the interference drag coef-
ficient becomes unfavorable. The reduction in pylon leading-edge sweep associ-
ated with this rearward movement of the engine from 77.5 ° to 74o results in an
increase in pylon side force which may possibly account for the favorable inter-
ference obtained for this configuration at the lower Mach numbers. The unfavor-
able interference drag obtained in the higher Mach number range would result
from the change in the area distribution caused by this change in engine posi-
tion. As the engine is moved closer to the wing, the interference drag is
increased even further and, in fact, no favorable effects occur throughout the
Mach number range. These results, in the lower Mach number range, may be
attributed to the increase in pylon leading-edge sweep and the associated loss
in side force; in the higher Mach number range, they may be attributed to a
magnification of the already unfavorable effects associated with the change in
area distribution as the engine is moved even closer to the wing.
P_lon leadin6-ed6e extension.- The effect of pylon leading-edge extension
was also investigated. Figure l0 shows the two pylon configurations tested.
The first pylon has a leading edge sweep of 74o and is attached to the lower
surface of the wing, just behind the wing leading edge, as proposed for the
C-SA airplane. The second pylon, having the same sweep, has its leading edge
extended forward approximately l0 percent of the pylon chord. This extension
results in the pylon leading edge extending over the wing# as shown by the
dashed outline on the sketch in the upper part of the figure.
The data indicate that the configuration having the originally proposed
pylon configuration provided favorable interference at the cruise Mach number.
The extended-pylon configuration produced approximately zero interference drag
264
Iin the lower Mach number range, becoming unfavorable at the higher test Mach
numbers. Further wind-tunnel investigations will be necessary before an
explanation can be given for this difference in interference drag resulting
from this change in pylon extension.
CONCLUSIONS
Wind-tunnel studies of nacelle interference drag at high subsonic speeds
including the effect of powered jets have been presented. The data indicate
that the aerodynamic interference resulting from nacelle-pylon installation may
be minimized or even made favorable by properly locating the engine nacelles
and by detailed tailoring of the engine-pylon configuration. It has also been
shown that the effect of the powered jet is such that the favorable interference
noted in this investigation was doubled throughout the higher lift-coefficient
range. Inasmuch as only a few possible engine installations have been studied
herein, it should be emphasized that more extensive wind-tunnel investigations
will be required before the exact effect that the powered jets have on inter-
ference drag can be fully understood.
REFERENCE
1. Peterson, John B., Jr.: A Comparison of Experimental and Theoretical Results
for the Compressible Turbulent-Boundary-Layer Skin Friction With Zero
Pressure Gradient. NASA TN D-1795, 1963.
265
AFT-FUSELAGE- MOUNTED NACELLES
WING AREA, 51.5SQ IN.; MEAN AERODYNAMICCHORD,3.2 IN.;
Rf.=2.74 TO 3.94 xlO 6
._ AXIS OF ..1__.T.AT!ON
',d
Figure1
ENGINE NACELLE-PYLON DRAG
CL = 0.25
.04
CD
.03
.02-
.01 -
BASIC CONFIGURATION
BASIC CONFIGURATION
LESS ENGINES AND PYLONS
PLUS SKIN FRICTION OF
ENGINES AND PYLONS
I I I
m
0 .65 .'tO .75
M
I I
.80 .85
Figure2
266
/INTERFERENCE DRAG
-- BASIC
.... IMPROVED
ACD "OO2IO/LONGITUDINAL__._LOCATION I _'CANT ANGLE
-O02L I I I I I I I I I I
I NCIDENCE ANGLE E.NGINE AND PYLON EXTENSION
I I I l I I I t I I
65 .70 .75 .80 .85 0 85 .70 .75 .80 .85
M
Figure3
CARGO-TYPE AIRPLANE MODEL WITH POWERED ENGINE
Figure4 L-2683-6
/."-,
CONFIGURATION OF C-SA FAN-JET ENGINE
_ DISCHARGE
FAN DISCHARGE
Figure 5
MODEL-ENGINE CROSS SECTION
___w_<__._ I
_T_ll: _" -- -- PRIMARY DISCHARGE
"-" FAN DISCHARGE FLOW
Figure6
268
ENGINE-JETEFFECT
M--0.78
Pt,e
Pt,ao
,c;°:[ _o
I I r "T-- t //_-
-.002_" .3 .4 Ci .5 .6 .7
.i"_/ CL: 0.4 ,,. ._.
0 _
.5 I.O
x/l
Figure 7
C-5A MODEL WITH POWERED ENGINES
Figure8 L-2683-5
269
EFFECT OF ENGINE POSITION
CRUISE MACH NUMBER = 0.775
.006 CL= 0.5
oo4 ,i,,/,,
.00Z if," /
•..O02L I I I
0.70 .75 .80 .85
M
Figure 9
EFFECT OF PYLON LEADING-EDGE EXTENSION
CRUISE MACH NUMBER = 0.775
CL = 0.5
.002
AC D 0
_.00 o i I , J
.7O .75 .80 .85
M
Figure 10
_ 27o


Wind tunnel studies of nacelle interference drag at high subsonic speeds including the effect of powered jets

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