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Velocity magnitude contours on an ERJ 190 computational model
Embraer, based in São José dos Campos, Brazil, ranks among
the four largest commercial aircraft manufacturers in the world. In 1989,
subsonic wind tunnel tests conducted by Embraer at Centro Tecnico Aeroespacial
(CTA), a Brazilian research agency of aerospace technology, indicated
the potential benefits of fitting winglets onto the tips of existing aircraft
wings. Since that time, Embraer has used CFD to help design winglets for
several of its aircraft models, bringing about considerable improvements
in the performance of all of them.
A winglet is a wing-like surface attached to the tip of the wing that
is used to improve fuel efficiency and climbing capability. It accomplishes
this by acting like a small sail with a lift component that generates
a traction force, draining energy from the tip vortices. Consequently,
it reduces the vortex strength, lowering induced drag. "Induced drag
represents 30-40 percent of the total drag of a transport air-plane at
the cruise condition, so it has a big impact on fuel consumption,"
explains Bento Silva de Mattos, senior engineer at Embraer. "Because
the induced drag coefficient is proportional to the square of the lift
coefficient, a reduction in drag improves climbing capability as well,
because the lift coefficient is high at this condition."
Example of aerodynamic optimization using CFD
Despite the benefits of winglets, there are some drawbacks that need
to be addressed. For example, the bending moment at the wing root is higher,
and may require additional structural reinforcement of the wing. The viscous
drag of the winglet can be too great, nullifying the reduction of the
induced drag. Winglets have to be carefully designed so that these and
other problems can be overcome. It is here that CFD can help by offering
rapid turnaround times for parametric configuration studies. For example,
it was through FLUENT simulations that the transition between the wing
and winglet was optimized to achieve the best performance.
Winglets were first used on the EMB 145 SA, an aircraft used for early-warning
duties and airspace control. They were then applied to the Legacy business
jet, a derivative of the ERJ 135 regional jet. For this model, a prototype
was designed, tested, and released within four months of the original
concept. The results from the flight tests clearly showed that the designed
winglet configuration fulfilled all expectations of the performance improvements.
The ERJ 170 and ERJ 190 are newer regional jets with winglets that were
incorporated during the original design phase.
Despite the fact that winglets have been in use for many years, optimization
of winglet design had been difficult because the complex airflow patterns
around these wing-mounted airfoils could not be understood through wind
tunnel testing. The Reynolds number has a big effect on the flow structure;
f light tests always showed larger drag reductions than the ones from
wind tunnel data. Using simulation results as a guide, the engineers fine-tuned
the winglet geometry to further reduce induced drag, improving fuel consumption.
"We evaluated seven designs in only one month, ending up with a shape
that was even more successful than we had hoped for," says Silva
de Mattos.
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