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tip from stalling first, thus reducing the
tendency to roll-off, which was also exhibited in
stalls with moderate amounts of sideslip of
some subsonic aircraft.
Some designs with no smooth
transition between the winglet and the wing
employ some faring with a triangular-shaped
trailing-edge extension to minimize interference
and wave drag (Fig. 9). The fairing acts
increasing the local Reynolds number and
reducing the airfoil section maximum thickness.
At the junction the flow is highly accelerated
and at transonic speeds strong shock waves
could appear.
Fig 8 – The Embraer 170 airliner has the
navigation lights casing placed at the wing lower
side in order to minimize interference drag
(Photo by Collin K. Work).
Fig 9 - Some conventional designs employ an
extended triangular-shaped trailing edge at the
wing/winglet junction (Photo Embraer).
The Boeing Co. employed a raked
wingtip for the 767-400 airliner instead of
whitcomb-like winglets and Fairchild Dornier
had selected the so-called Super Shark winglet
for the Envoy 7 business jet (Fig. 10), which
would be a derivative of its 728JET airliner.
Compared to the smaller B-737-800 the raked
wingtip provided more percentual drag
reduction4 (Fig. 11). However, this figure must
be taken carefully, since both aircraft have
different mission profiles and therefore lift
coefficients at the cruise condition. In addition,
raked wingtips may require wider hangars and
the some benefits of higher aspect-ratio wings
in place of wingtip devices will certainly be
overcome by the demand for additional engine
bleed air in order to deice the extended wingtip.
Fig. 10 – Super Shark Winglet of the Envoy 7
business jet.
Fig. 11 – Drag reduction of wingtip device.
Source: Boeing Aero Magazine
Besides the AEW&C aircraft, Embraer
installed winglets on two other variants of the
ERJ 145 regional jet: the 3,100 nm-capable
Legacy business jet and the ERJ 145 XR, which
has more powerful engines and an increased
maximum takeoff weight for greater range. The
twinjets of the new aircraft family 170/190 were
designed with winglets from the beginning.
Winglets are being also considered for a new
ERJ 145 maritime-patrol variant of the ERJ 145
airliner.
Computational fluid dynamics
Embraer employed the XFOIL10 (Fig.
12) and MSES7,8 codes from Massachusetts
Institute of Technology (MIT) to design the
airfoil of the winglets. The commercial fully
unstructured finite-volume FLUENT3 code from
Fluent Incorporated, Lebanon, New Hampshire,
was employed to analyze the three-dimensional
flow around the winglet. Fig. 13 shows typical
domain boundaries of the computational model
constructed for the FLUENT code. Parametric
studies were conducted to determine the best
shape and incidences for the winglet geometry.
The FLUENT code was very useful in the
designing of the transition surface between the
winglet and wing.
Fig. 12 – Viscous flow calculation around a
winglet airfoil with XFOIL
(M∞ = 0.65, Cl = 0.50, Reyn = 8 x 106).
Fig. 13 – Computational domain used for an
external aerodynamic calculation.
Regarding the ERJ 145 and its
commercial derivatives, a new, thinner, winglet
airfoil had to be designed as well as the toe and
twist angles of the winglet planform were
modified due to the flight regime at higher
speeds. A preliminary winglet and the transition
surface were created with the CATIA CAD
program. Then they were exported to the
Gambit grid generator, which was used to
produce an unstructured triangular surface
mesh. After that, this grid was imported into the
spatial mesh generator T-GRID and a mesh
consisting of one million cells was generated.
The analytical capabilities of T-GRID were
helpful in identifying and correcting skewed
faces that could decrease the accuracy of the
model and increase its execution time. Once the
mesh was complete, the CFD calculations were
performed on a Silicon Graphics Origin 2000
server. Fig. 14 shows Mach number contours on
a computational model of the ERJ 145 airliner.
Fig. 14 - Contours of Mach number on a
computational model of the ERJ 145.
The addition of the winglet contributes
to accelerate the flow at the portion of wing
close to the tip. Fig. 15 illustrates this situation
well for one of the first design attempts of the
Legacy’s winglet (M∞=0.76). Here, for the
station located at 94% of the semispan a shock
wave located in the aft region caused by the
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