VORTEX GENERATORS
Original idea from Donald E. Stein
Vortex generators have been utilized
on most of commercial aircraft. If
observed carefully, one can see
vortex generators installed at specific
locations on Boeing aircraft: for
instance, on wing upper surface & on
the engine nacelles for the B737 &
the B767. (see pictures, page 5)
Some specific examples of vortex
generator applications are shown in
the accompanying photographs.
These devices have been installed in each case to enhance flying qualities, but each application has its
own story. A brief discussion of what vortex generators do is necessary before the design decision
process for specific applications can be discussed. As can be observed from the previous
photographs, these devices are used in assorted sizes and combinations, and can be mounted in
various locations on an airplane. What they all have in common, however, is that they all act like
miniature wings, each creating lift perpendicular to its own surface. By creating lift, they each shed a
downstream vortex which can influence airflow in two distinct ways:
•
•
The vortex interacts with the boundary
layer air on the aircraft surface behind the
device by inducing high energy air from
outside the boundary layer down to the
surface displacing low energy air in the
process as shown in Figure 1.
The air adjacent to the surface is reenergized, and by suitable tailoring of the
configuration, the vortex generators can be
used to delay, control, or sometimes
prevent separation of the boundary layer
from the surface.
The wing vortex
generators installed on the 737, 757 and
767 are applications which take advantage
of this mechanism.
Figure 1. A vortex generator reduces the boundary layer height &
reduces boundary layer separation
The vortex is oriented by appropriate placement of the vortex generator in order to redirect airflow
in the flow field so that adverse interactions are prevented or delayed. With this mechanism, the
generators act as a flow deflector. The large vortex generators installed on the 767 and 737-300
nacelles are examples of applications which take advantage of this mechanism.
Vortex generators have been used to increase aircraft speeds, improve initial buffet boundaries,
improve control authority, and reduce vibrations induced by boundary layer separation on some aircraft.
However, these reasons do not explain why vortex generators have been used on the Boeing 737, 757,
and 767 aircraft. Vortex generators are used on these aircraft to improve high Mach pitch
characteristics beyond initial buffet and to lower stall speeds in the landing configuration.
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To further indicate why vortex generators were used on these latest aircraft, the case history for the
767 is presented to explain why the airplane was configured with wing vortex generators and with the
large vortex generator installed on each engine fan cowl.
WING VORTEX GENERATORS
During the early design phase of the 767, more
stringent high angle of attack stability
requirements were established. Specifically, the
new requirements established criteria for
acceptable stick force vs. g (load factor)
characteristics for pitch maneuvers above the
angle of attack for initial buffet.
Reduced stick force gradients at angles of attack
beyond initial buffet are typical for low tail, swept
wing aircraft due to the tendency for the
boundary layer air on the outboard wing panel to
separate prior to the inboard wing. Although the
probability of encountering these characteristics
in normal service is very small, history has
shown that high speed upsets followed by high
Figure 2. A wing is designed to achieve an approximately
load factor recoveries do occur. It was the
elliptical spanwise lift distribution at cruise angle of attack
Boeing Company's desire that this new
requirement be met by aerodynamic means,
although a solution by means of a pitch augmentation control system was carefully considered . This
alternative was not desired since it would add cost and complexity to the airplane.
Early 767 wind tunnel test results showed, as expected, that the configuration with the best
aerodynamic cruise efficiency displayed predicted stick force per g characteristics similar to previous
low tail, swept wing transports and did not meet the more stringent design criteria beyond initial buffet.
For optimum efficiency, a wing is designed to achieve an approximate elliptic spanwise lift distribution
at cruise angle of
attack as shown in
Figure 2.
This loading minimizes
lift induced drag and
thereby maximizes lift
to drag ratio.
The
elliptical lift distribution
is accomplished by
proper selection of
airfoil camber and
Figure 3. Sectional lift coefficient variation
Figure 4. On swept wing airplanes, the greatest
twist along the wing
increase in section lift coefficient with
relative to span
increasing angle of attack occurs on the
span.
outboard region of the span
The resulting sectional lift coefficient variation with span is presented in Figure 3. On a swept wing
planform, the largest increase in section lift coefficient with increasing angle of attack occurs on the
outboard region of the span as seen in Figure 4. This is because wing sweepback causes the outboard
wing to operate in a local upwash field created by the inboard wing; therefore, the outboard wing
effectively operates at a higher angle of attack than the inboard wing.
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Another important factor is the spanwise flow of the low energy boundary layer air which makes the
outboard wing more susceptible to initial flow separation. The loss of outboard wing lift at high angles
of attack is the direct cause of the reduction in stick force per g for swept wing airplanes.
Several candidate design modifications were studied as configuration options for improving the stick
force per g characteristics to meet the new design requirements:
•
•
•
Several T-tail configurations were studied in order to separate the wing flow field from the
horizontal tail
The wing span loading was modified by retwisting the wing to unload the outboard wing
The inboard wing airfoils were modified in order to promote initial separation on the inboard wing
Figure 5. Model 767 wing vortex generators
Figure 6. Relationship of landing gear length to nacelle lip
height on high-bypass ratio engines.
Each of the above options resulted in a significantly less efficient airplane. A better solution was
desired, and vortex generators provided that solution. A 1/10 scale model of the 767 airplane was built
and tested at a high Reynolds number wind tunnel in order to obtain data simulating full scale 767
conditions. Vortex generators were evaluated in detail. The test results were very encouraging
because it was determined that only a few small vortex generators located on the wing just outboard of
the nacelle were very effective in improving the wing stall pattern and hence the stick force
characteristics. It remained to be proven on the flight vehicle.
The early flight tests on the 767 airplane without the vortex generators confirmed the initial wind tunnel
test results. When vortex generators were added, the stick force characteristics beyond initial buffet
met the new Boeing design requirements. The vortex generators also provided increased buffet
intensity with increasing load factor thereby contributing additional deterrence to a pilot as he pulled
into these conditions. The production vortex generator configuration required only seven 3/4 inch high
vortex generators per wing shown in Figure 5. The effect on weight and drag were negligible.
NACELLE CHINE
The large vortex generator installed on the inboard side of the nacelle is commonly called a nacelle
chine as shown in the 767 photograph at the end of the article. These devices are used on both the
767 and 737 airplanes. Modem efficient aircraft utilize high bypass ratio engines mounted from pylons
off the wing. In order to minimize landing gear length (minimize weight) and to maintain adequate
runway clearance (minimize foreign object ingestion), the engines are installed in relative close
proximity to the wing as shown in Figure 6.
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This close coupling of the large engines with the
wing results in increased flowfield interaction
between the engines and the wing at high angles of
attack, and can result in reduced airplane
performance unless special consideration is given
to counteracting these effects. One such adverse
interaction is a loss in maximum wing lift capability
in the landing configuration. At the high angles of
attack required at low airspeeds, vortices are shed
from the fan cowl. For engine installations where
the nacelle is located further below the wing, such
as JT9D installations on the 747, these vortices
pass underneath the wing. For more close coupled
nacelle configurations, these vortices flow over the
top of the wing and interact with the wing flowfield. The effect of these vortices is generally favorable
as long as they remain intact. Unfortunately the wing, at high angle of attack, will impose large adverse
pressure fields on these vortices as they flow rearward along the wing surface as shown in Figure 7.
Figure 7. Wing vorticies resulting from close coupling of large
engines to the wing
These vortices will break up and burst, causing the boundary layer air
over the wing behind the engine to separate. This results in lower
maximum lift levels than would be the case with less closely coupled
nacelles as shown in Figure 8.
Figure 8. Effect of nacelle chines on lift
The solution was the development of a large vortex generator
installed on the inboard side of the engine nacelle which was
sufficient to delay the nacelle vortex bursting phenomenon. The
Boeing invention disclosure identifies this as a vortex control device
(VCD), but it is more commonly known as a nacelle chine. The
nacelle chine was
sized
and
positioned on the inboard side of the nacelle to
control where the nacelle vortex is shed so that it will
not attach to the wing. The strong vortex shed by
the nacelle chine will cause the nacelle vortex to
flow over the wing as shown in Figure 9 delaying the
wing influence to burst the vortices until a higher
angle of attack.
The result is that
Figure 9. Effect of the nacelle chine on the nacelle vortex
the
lift
loss
shown in Figure 9 is essentially regained as shown in Figure 10.
Figure 10. Regained lift loss due to the
use of chines
Due to air condensation under certain atmospheric conditions, the
vortex shed by the nacelle chine can be clearly viewed from the
cabin as shown in Figure 11. In terms of airplane performance, the
nacelle chine reduced approach speeds by 5 knots and landing
field lengths by approximately 250 feet for the 767-200 as shown in
Figure 12. The nacelle chine is a significant contributor to the
superior short field performance of the 767.
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OPERATIONAL CONSIDERATIONS
Each airplane in the fleet should be periodically inspected to determine if all the vortex generators are
installed. On most models, dispatch is allowed with a limited number of missing vortex generators.
The Configuration Deviation List (CDL) in the Airplane Flight Manual should be consulted to determine
the minimum number required for dispatch and whether operational limitations are to be imposed.
Repair and replacement of vortex generators is explained in the appropriate Maintenance Manual for
each model.
CONCLUSIONS
Vortex generators are a
valuable aerodynamic tool
which can be used by
aircraft
designers
to
enhance airplane flying
qualities. Judicious use of
vortex generators results
in optimum aerodynamic
characteristics over a wide
range of flight conditions
Figure 12. Approach speed & landing field length effect of nacelle chines
(e.g. from cruise flight to
high g and/or high angle of
attack maneuvers into heavy buffet). The use of these devices on the new Boeing aircraft have
contributed to:
•
•
more efficient aerodynamic designs with low fuel bum performance
aircraft with lower initial cost and maintenance expense
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