Nacelle-pylon configuration for an aircraft and method of using the same

A below-wing engine nacelle mounted by a pylon forwardly of a wing having a rearward sweep. The pylon upper outboard edge that extends forwardly of the wing is formed with a relatively sharp radius of curvature. Thus, airflow which flows over the top of the pylon in an outboard direction separates over the sharp pylon edge to form a vortex which passes beneath the wing adjacent the outboard surface of the pylon. This vortex maintains attached flow at the outboard pylon surface immediately beneath the wing to reduce drag.

DESCRIPTION 
1. Technical Field 
The present invention relates to method and apparatus adapted for reducing 
drag in the airflow over the top forward surface of a pylon and thence 
beneath the wing of an aircraft. 
2. Background Art 
Quite often, to reduce aerodynamic drag, the engine and its nacelle are 
positioned below the level of the chord of the wing and extend quite far 
forward of the wing leading edge. The pylon by which the nacelle is 
mounted to the wing also extends quite far forward of the wing leading 
edge. Typically, drag minimization requires that the top of the pylon 
intersect the wing below the leading edge of the wing, rather than extend 
over the wing upper surface. 
With a swept wing configuration, the resulting flow over the top surface of 
the pylon forward of the wing leading edge (and also some of the flow from 
the inboard side of the pylon) is rearwardly and in a lateral direction 
over the top side of the pylon toward the outboard side of the pylon and 
thence beneath the wing. The severity of this "cross flow" increases as 
the flow over the top surface of the pylon gets closer to the wing leading 
edge. 
To reduce aerodynamic drag resulting from this "cross flow", a typical 
solution is to make the upper edge portions of the pylon rounded as much 
as possible to allow the airflow to follow a fairly gentle curved path 
from the top of the outboard side of the pylon and thence beneath the 
wing. There are two problems with this approach. First of all, the 
constraints of the pylon structure usually do not allow sufficient 
rounding of the outboard "shoulder" to attain the desired aerodynamic 
curvature. Secondly, the requirement for rounding is most critical near 
the wing leading edge, very close to the wing lower surface. At this 
point, the intersection between the vertical side of the pylon and a 
relatively horizontal wing lower surface is approaching a sharp corner 
rather than a rounded shape desired just upstream. Therefore, the 
intersection itself hinders the attainment of the aerodynamic lines 
desired. As a result, there is a tendency of the flow along the outboard 
wing pylon intersection to proceed aft from the wing leading edge in an 
aerodynamically rather confused pattern. More specifically, the flow from 
the top of the pylon moves far down the outboard side of the pylon leaving 
an area of slow, low energy, turbulent air in the intersection region. 
This results in high drag for this area and local vibration due to 
turbulence and separation. 
A search of the patent literature has not disclosed any patents having a 
teaching which the applicants believe to be particularly relevant to this 
particular problem. However, the following patents are noted as being of 
general interest. These are as follows: 
U.S. Pat. No. 2,090,775, Wright, shows an aircraft where struts are 
connected from the fuselage downwardly and outwardly to the wing. The 
aerodynamic contour of the struts is varied along its length to match the 
airflow at different locations. 
U.S. Pat. No. 3,101,920, Fradenburgh, shows a helicopter having a pylon on 
top of the fuselage, with the rotor being mounted to the pylon. Boundary 
layer air is blown outwardly through slots in the member by which the 
rotor head is mounted to the pylon. 
U.S. Pat. No. 3,370,810, Shevell et al., shows a device to alleviate the 
problems of premature stall at the wing tips. This comprises a member 
positioned beneath the wing and extending moderately forwardly of the 
wing. The nose of the device extends forward of the wing leading edge 
sufficiently to intersect the air stagnation streamline near aircraft 
stall. Thus, there is created a vortex which begins at the leading edge of 
the device and travels upwardly over the wing. 
U.S. Pat. No. 3,471,107, Ornberg, discloses various vortex generating 
devices to create vortices over the top of a delta wing. 
In U.S. Pat. No. 3,744,745, Kerker et al, a pair of lifting vanes are 
attached to the upper forward side surfaces of an engine nacelle. These 
two vanes produce a downwash to alleviate the condition of a strong upwash 
which otherwise would flow upwardly around the nacelle and over the wing 
to cause separation of the flow over the wing. The downwash field is 
bordered on each side by two vortices. 
In U.S. Pat. No. 3,960,345, Lippert, Jr., a pair of strakes are mounted on 
the top side of an engine nacelle to reduce or prevent the formation of 
vortices usually occurring with nacelle-wing combinations. 
In U.S. Pat. No. 3,968,946, Cole, there is shown an extendable fairing for 
use between an engine nacelle and the aircraft wing. When the leading edge 
slat of the wing is extended, the extendable fairing is moved outwardly to 
fill the gap between a fixed fairing section on the nacelle and the 
leading edge flap. 
U.S. Pat. No. 4,176,813, Headley et al, discloses a nose for an aircraft 
having a particular configuration to optimize vortex patterns over the 
aircraft nose. 
In British patent specification No. 523,357, there is a shield-like member 
positioned between the wing and a propulsion unit having a propeller. This 
is to provide a separation of the normal airflow under the wing from the 
slipstream produced by the propeller. 
Therefore, it is an object of the present invention to provide for an 
apparatus and method to alleviate the problem of drag in the nacelle/pylon 
configuration described above, and also to accomplish this in a relatively 
simple and effective manner. 
DISCLOSURE OF THE INVENTION 
The present invention is adapted to be utilized in an aircraft having: 
a. a swept wing which has a first end and a second end, a leading edge, a 
trailing edge, an upper aerodynamic surface and a lower aerodynamic 
surface, said wing slanting rearwardly from said first end to said second 
end, 
b. a nacelle positioned at a level below said wing, at least a portion of 
said nacelle being located forwardly of the wing leading edge, 
c. a pylon by which said nacelle is mounted to the wing, said pylon having 
a forward pylon portion extending forwardly of the wing leading edge and a 
rear pylon portion positioned beneath the lower surface of the wing, said 
forward pylon portion having a first side forward surface portion 
positioned closer to the wing first end, and a second side forward surface 
portion positioned closer to the wing second end, and a top forward 
surface portion extending between the first and second forward side 
surface portions, said rear pylon portion having a first rear side surface 
portion located closer to the first wing end and a second rear side 
surface portion located closer to the wing second, and 
d. said wing, nacelle and pylon being arranged relative to one another so 
that airflow over the top forward surface portion of the pylon follows a 
direction of rearward sweep of the wing laterally to pass beneath the wing 
lower surface adjacent the second rear side surface portion. 
The improvement of the present invention comprises means to create a vortex 
in the airflow that passes over the top forward surface portion in a 
manner that the vortex travels along an area adjacent the wing lower 
surface and adjacent the second rear side surface portion. The vortex is 
sufficiently strong to maintain attached flow in that area. 
In the particular embodiment shown herein, this is accomplished by having 
the top forward surface portion and the forward second side surface 
portion shaped to meet one another along a forward upper edge surface 
portion having a lengthwise axis. The forward upper edge surface portion 
has a sufficiently small radius of curvature in sectional planes taken 
perpendicular to the lengthwise axis to cause airflow over said forward 
upper edge surface portion to separate and create the vortex in the 
airflow. 
In the preferred embodiment shown herein, the radius of curvature decreases 
along the lengthwise axis of the pylon in a rearward direction. Also, the 
second forward side surface portion extends downwardly at a slant 
moderately towards said first side surface portion. 
Also, as a further specific improvement, the first side forward surface 
portion and the top forward surface portion are shaped to meet one another 
along a second forward upper edge surface portion having a lengthwise 
axis. The second forward upper edge portion extends a sufficient distance 
upwardly adjacent the wing leading edge to form a seal with an extended 
leading edge slat of the wing to improve aerodynamic flow over the slat 
and the pylon. 
In the method of the present invention, a vortex is created as described 
above, and desirably the vortex is created by passing the airflow over the 
top upper surface portion over the relatively sharp forward upper edge 
surface portion which is nearer the second end of the wing. 
Other features will become apparent from the following detailed description 
.

BEST MODE FOR CARRYING OUT THE INVENTION 
It is believed that a clear understanding of the present invention will be 
obtained by first describing a conventional prior art nacelle/pylon 
configuration for an engine nacelle mounted beneath and forwardly of the 
wing. Such a prior art nacelle/pylon configuration is shown in FIGS. 1 and 
2, where there is a wing 10, pylon 12 and engine nacelle 14. For 
convenience of illustration, only a portion of the engine nacelle 14 and 
wing 10 are shown. The wing 10 sweeps rearwardly from the inboard end to 
the outboard end and has an upper surface 16, a lower surface 18, a 
leading edge 20 and a trailing edge 21. 
The engine nacelle 14 is or may be of conventional configuration, and as 
shown herein is moderately elongate, with a generally circular 
configuration in transverse section. The lengthwise contours of the 
nacelle 14 are moderately rounded so that the cross sectional area from 
the nacelle inlet 22 expands moderately to the middle section 24 of the 
nacelle and again converges radially inwardly at the rear exhaust end 26. 
The pylon 12 can be considered as having two portions, namely a forward 
portion 28 extending forwardly of the wing leading edge 20, and a rear 
portion 30 positioned below the wing lower surface 18 and rearwardly of 
the wing leading edge 20. Also, as shown herein, the rear end 26 of the 
engine nacelle 14 terminates just a short distance behind the wing leading 
edge 20, and the major part of the rear portion 30 of the pylon 12 is 
located rearwardly of the exhaust end 26 of the nacelle 14. 
The lengthwise axis of the pylon 12 extends generally parallel to the 
center axis of the engine contained in the nacelle 14 and also generally 
parallel to the chordwise axis of the wing 10. The cross sectional 
configuration of the pylon 12 a short distance forward of the wing leading 
edge 20 is shown in FIG. 2A. The pylon 12 can be considered as having a 
first inboard side surface portion 32, a second outboard side surface 
portion 34, and a top surface portion 36. 
The two side surface portions 32 and 34 are generally vertically aligned, 
and the top surface 36 is generally horizontally aligned. The forward ends 
of these surface portions 32, 34 and 36 converge at the forward end of the 
pylon 12 to blend aerodynamically into the top forward surface of the 
nacelle 14. The inboard side surface portion 32 and the top surface 
portion 36 meet one another at an upper inboard edge surface portion 38, 
and in like manner the outboard side surface portion 34 and the top 
surface portion 36 meet one another along an upper outboard edge surface 
portion 40. 
Since the wing 10 sweeps rearwardly in an outboard direction, the typical 
flow streamlines over the pylon forward top surface portion are, as shown 
in FIG. 1, slanted in an outboard direction. It is apparent that the 
outboard or lateral component of flow increases substantially as the flow 
over the pylon upper surface gets closer to the wing leading edge 20. As 
indicated previously, the typical prior art approach has been to round the 
upper edge surface portions 38 and 40 as much as possible to allow the 
airflow a fairly gentle curved path across the top surface portion 36 of 
the pylon 12. The effect of this rounded configuration will now be 
discussed further with reference to FIG. 2, which shows the streamlines of 
flow along the side of the outboard side surface portion 34 of the forward 
pylon portion and along the outboard side surface portion 42 of the rear 
portion 30 of the pylon 12. It can be seen that the streamlines which pass 
over the top surface portion 36 just forward of the wing leading edge 20 
tend to continue to flow at a moderate downward slant rearwardly away from 
the lower surface 18 of the wing 10. The result of this is that at the 
area 44 where the vertical side outboard surface portion 42 of the pylon 
meets the lower surface 18 of the wing 10 there is created an area of 
slow, low energy, turbulent air, which, as indicated previously, results 
in high drag for this area and local vibration due to turbulence and 
separation. 
It is to be understood that the immediately preceding discussion is 
directed to a typical prior art configuration to illustrate the problems 
associated therewith. With the foregoing in mind, the present invention 
will now be described. 
Since the major components and the general configuration of the present 
invention have substantial similarities to the prior art configuration 
described above, components of the present invention will be given 
numerical designations the same as corresponding components of the prior 
art configuration, with an "a" suffixed distinguishing those of the 
present invention. Accordingly, in the present invention, there is a wing 
10a, pylon 12a and engine nacelle 14a. The wing 10a and engine nacelle 14a 
are, or may be, substantially the same as the corresponding wing 10 and 
engine nacelle 14 of the prior art configuration described above. Thus, no 
further description of the wing 10a and nacelle 14a will be given, and the 
same numerical designations as in the prior art configuration will simply 
be applied in place of any further description. Thus, the wing 10a has an 
upper surface 16a, lower surface 18a, etc. 
The surface configuration of the pylon 12 is critical in the present 
invention. For purposes of description, the pylon 12a is considered as 
having a forward portion 28a, rear portion 30a, first inboard side surface 
portion 32a, second outboard side surface portion 34a, top surface portion 
36a, upper inboard edge surface portion 38a, upper outboard edge surface 
portion 40a and rear outboard side surface portion 42a. 
Attention is now directed to FIGS. 5A through 5G to describe the 
configuration of the pylon 12a. As indicated previously, the cross 
sectional configuration of the pylon 12a of the present invention is shown 
in solid lines in FIGS. 5A through 5F, while those of the conventional 
prior art pylon 12 are shown in broken lines. It can be seen that in the 
present invention the upper edge surface portions 38a and 40a are both 
made relatively sharp in comparison with the prior art configuration. The 
configuration of the upper outboard edge surface portion 40a is believed 
to more critical in the present invention, so the configuration of the 
inboard edge portion 38a will be discussed first rather briefly. The 
inboard edge portion 38a increases in sharpness in a rearward direction 
along the lengthwise axis of the pylon 12 and is raised moderately at the 
location where it approaches closely to the wing leading edge 20a. The 
reason for this is that when the wing 10a is in its landing configuration 
and a forward slat is placed outwardly from the wing leading edge 20a, 
because of the cross sectional configuration shown in FIG. 5F, the inboard 
side surface portion 32 makes a seal with the slat to improve aerodynamic 
flow in that area. This is not closely relevant to the present invention 
since the present invention is designed primarily to reduce drag in the 
cruise configuration of the wing 10a. However, a discussion of this 
feature is included to insure that there is a complete disclosure of the 
preferred embodiment. As will be explained below, the relatively sharp 
curve of the outboard edge surface portion 40a serves a distinctly 
different purpose. 
With regard to the upper outboard edge surface portion 40a, the radius of 
curvature, indicated at 46 is made sufficiently small so that the airflow 
over the pylon top surface 36a is not able to remain attached at the edge 
surface portion 40a. Thus, as the airflow passes over the edge surface 
40a, it separates from it, rolling into a discrete vortex which trails 
back along the edge surface portion 40a and beneath the wing 10a adjacent 
the rear outboard pylon surface portion 42a in the area 44a. This vortex 
is indicated at 48 in FIG. 3. The effect of this vortex 48 is to induce 
free stream air into this area 44a and therefore maintain clean, attached 
flow back to the pylon trailing edge 50a. 
The strength of the vortex is a function of the length of the sharp 
outboard edge surface portion 40a and the sharpness of this edge. In 
applying the present invention to any particular aircraft design, this 
sharpness must be tailored carefully through wind tunnel testing to 
produce a vortex strong enough to maintain the attached flow, and yet not 
strong enough to cause periodic vibration on local skin panels. Since this 
is well within the skill already present in the prior art, no further 
discussion of that subject will be included herein. In the present 
configuration, as can be seen in FIGS. 5A-5F, the sharpness of edge 40a 
increases in a rearward direction toward the leading edge 20a of the wing 
10a. 
To give approximate values mathematically of the degree of sharpness of the 
upper outboard edge surface portion 40a, the following formula is given: 
##EQU1## 
where .sup.L effective is the length over which 
##EQU2## 
.sup.R corner equals the radius of curvature of the upper outboard edge 
surface portion 
W.sub.max equals the maximum width of the pylon. 
It is to be understood that while the above formula is believed to be a 
reasonable approximation for many situations, for any particular 
application wind tunnel experimentation should be carried out to optimize 
the design and possibly arrive at suitable configurations beyond the 
limits of the above formula. 
Also, it should be understood that the present invention has been described 
relative to a wing configuration having a rearward sweep in an outboard 
direction. Obviously, if the wing had a forward sweep in an outboard 
direction, the same teachings would apply, but the arrangement of the 
parts would simply be reversed. 
Wind tunnel testing has confirmed that the present invention, in comparison 
with the prior art configuration disclosed herein has provided a reduction 
in cruise drag of approximately one-half percent to two-thirds percent for 
a particular airplane configuration. 
It is to be understood that while the present invention has been described 
in reference to a particular application of an engine nacelle, pylon and 
wing, within the broader aspects of the present invention, it could have 
broader application. Thus, in the present invention, the terms "nacelle", 
"wing" and "pylon" are intended to apply as well to components having 
generally similar aerodynamic functions, and are not intended to apply 
only to those components having that particular location or function on an 
aircraft. 
Also, it is to be understood that while the preferred embodiment is to 
provide the relatively sharp edge at the top pylon surface to form the 
vortex at the desired location, within the broader aspects of the present 
invention other devices could be used, such as a vortex creating vane or 
possibly blown air. However, the present embodiment provides particular 
advantages in terms of cost, simplicity, minimum drag and no power 
requirements for operation. Obviously, various modifications could be made 
without departing from the basic teachings of the present invention.