Winglet

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A winglet is a device used to improve the efficiency of aircraft by lowering the lift-induced drag caused by wingtip vortices. The winglet is a vertical or angled extension at the tips of each wing.

Winglets work by increasing the effective aspect ratio of a wing without adding greatly to the structural stress and hence the necessary weight of its structure — an extension of wing span would also permit lowering of induced drag, though it would cause parasitic drag and would require boosting the strength of the wing and hence its weight — there would come a point at which no overall useful saving would be made. A winglet helps to solve this by effectively increasing the aspect ratio without adding to the span.

The exact upward angle (called cant) of the winglet, and its inward angle (or toe) is critical for correct performance, and is determined for each aircraft application. The vortex which rotates around from below the wing strikes the angled surface of the winglet, generating a small lift force that angles forwards relative to the direction of flight — thus the energy in the vortex contributes to thrust rather than drag as it normally would. This is analogous to a sailing boat sailing very close to the wind. This small contribution can be very worthwhile on long-distance flights. Another use for winglets is that airplanes without winglets create violent turbulent air flow, and if another airplane hits this air flow the plane might crash unexpectatly.

Some types of aircraft, especially airliners, have winglets, for example the Airbus A340, and the Boeing 747 -400. Other designs such as the Boeing 777 and the 747-8 omit them, because the gain available is very small and would make the aircraft just too large for a standard airport gate. Winglets are mainly good for short distances, while raked wingtips are better for long distances. Recently, blended winglets have been offered as an aftermarket retrofit for Boeing 737, 757 and Raytheon Hawker 800 series aircraft by Aviation Partners Inc., allowing these aircraft improved performance and efficiency characteristics. As of 2006, most major next-generation 737 and 757 operators have opted to retrofit their fleets due to high fuel prices. Aviation Partners is currently developing winglets for the 767-300ER.

Some airlines have taken advantage of the fact that winglets are visible to many passengers onboard through the airplane's windows. Some examples include Southwest Airlines and WestJet, which advertise their website addresses on the interior side of their 737's winglets.

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