The main purpose of winglets is to reduce induced drag. Induced drag is caused by wing tip vortices and are created when the high-pressure air from the lower surface of the wings comes around the wing tips. They are the cause of loss of efficiency and performance of the wing.
Winglets are vertical extensions of the wing on the wingtips. They increase the spread of the vortices along the trailing edge of the wing. This increases lift and reduces induced drag. On most jets, the maximum benefit of these winglets will be at the aircraft's cruise speed. An approximate 5%-10% fuel savings per airplane is estimated with their installation.
Most new airline aircraft are being delivered with winglets already installed, older aircraft are being retrofitted with them.
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