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 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.
Many modern types of aircraft, especially airliners, have winglets, for example the Airbus A340, and the Boeing 747-400. Other designs such as the Boeing 777 omit them, because the gain available is very small and would make the aircraft just too large for a standard airport gate. The designers of the 777 considered folding wingtips to accommodate winglets, but in the end, customers decided that the extra complexity of the wing did not justify it.
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