Flying on the Boeing 777
Tuesday, 11 December 2007 20:37
Hello Capt Lim,
Why doesn't the Boeing 777 have winglets?
This topic is also covered in my previous FAQ and here are some more information extracted from an article by George Larson in the Air & Space Magazine.
Basically, winglets reduce wingtip vortices, the swirling airflows formed by the difference between the pressure on the upper surface of an airplane's wing and that on the lower surface. High pressure on the lower surface creates a natural airflow that makes its way to the wingtip and curls upward around it. When flow around the wingtips streams out behind the airplane, a vortex is formed. These twisters represent an energy loss and are strong enough to flip airplanes that fly into them.
Winglets produce a good performance boost for jets by reducing drag, and that reduction could translate into slightly higher cruising speed. The Boeing 747-400s have winglets. The Boeing Business Jet, a derivative of the Boeing 737, has a set of the firm's eight-foot winglets as well.
After the energy crisis in 1976, Richard Whitcomb, a NASA aerodynamicist, in a research, compared a wing with a winglet and the same wing with a simple extension to increase its span. As a basis for comparing both devices, the extension and the winglet were sized so that both put an equal structural load on the wing. Whitcomb showed that winglets reduced drag by about 20 percent.
A wing with high aspect ratio will provide longer range at a given cruise speed than a short, stubby wing because the longer wing is less affected by the energy lost to the wingtip vortex. But long wings are prone to flex and have to be strengthened, which adds weight. Winglets provide the effect of increased aspect ratio without extending the wingspan.
If winglets are so great, why don't all airplanes have them? In the case of the Boeing 777, an airplane with exceptionally long range, the wings grew so long that folding wingtips were offered to get into tight airport gates. Dave Akiyama, manager of aerodynamics engineering in Boeing product development, points out that designing winglets can be tricky because they have a tendency to flutter. And so the computer came up with a Boeing 777 wing design that did away the winglets and fly just as efficiently.