Aerodynamics: How do winglets (upswept wingtips on modern airliners) decrease drag?

They reduce one type of drag which is caused by the production of lift. They increase another type of drag which is caused by friction. In total, the reduction of the lift-related drag must be larger than the increase in friction drag, or winglets would not help. Therefore, you find them most often on growth versions of older designs where more lift needs to be produced than initially planned when the wing was designed.

Every wing creates lift by deflecting air downwards. This happens gradually over the wing's chord, and creates a reaction force perpendicular to the local speed of air. This means the reaction force is pointing up- and slightly backwards. This backwards component is called induced drag, and it grows with the lift and the deflection angle. At low speed, relatively less air per unit of time is available for deflection, so the angle must be larger. That is the reason why airliners deploy flaps and slats in slow flight.

If you fly fast, there is a lot of air mass streaming past the wing per unit of time, so you need to deflect the air only slightly. Your induced drag is small. Sames goes for a large span: It "captures" more air which can be deflected, so the induced drag is small.

Adding winglets makes slightly more air available for lift creation, so a slightly smaller deflection angle is needed. Extending the wing's span by the height of a winglet would help even more, but would produce a larger bending moment, since the far end of the wing has the biggest lever arm and increases the bending moment at the wing root disproportionally. Also, extending the span would lift some designs into a new size class, which brings more restrictions at airports (fewer airports to operate from, fewer available gates at the remaining airports, less clearance on taxiways).

Thanks for A2A.

This question has been asked, what feels like a million times, here on Quora and been answered just as many times.

So, without sounding like a broken record...

• Wnglets increase lift by reducing vertices created around the tip of the wings.
• They do not increase drag if designed correctly.

Watch the first segment of this Video to learn why Winglets are often used.

Video Credit: BBC and National Geographic.