Why do aircraft wings curve upward at the tips?

A wing generates lift because pressure below it is higher than above it. Everywhere along the wing except at the very tip, that difference stays orderly; but at the tip the air finds a shortcut and curls around from the lower surface to the upper surface. The result is a conical vortex that sheds from each tip and trails for miles behind the aircraft. These vortices are what force the mandatory separation between aircraft on approach, and on humid days they're the two white streaks seen trailing from the tips at takeoff.

For the aircraft, that vortex is burned money. By inducing a downward component in the flow reaching the wing, it tilts the lift vector backward and generates induced drag, which at cruise accounts for roughly a third of total drag and grows sharply at low speed and high weight. All the energy feeding those swirls comes out of the fuel.

A wingtip device acts as an aerodynamic barrier that makes it harder for air to curl around, pushing the vortex core outward and upward and diffusing it. The vertical surface also works in flow that's already turned, generating its own force with a component pointing forward: literally, a small amount of thrust.

The modern idea is credited to Richard Whitcomb at NASA's Langley center, who in the second half of the 1970s published the studies that turned wingtip fins into a technology applicable to transport aircraft. Since then variants have proliferated: the up-and-down split fins of the McDonnell Douglas MD-11, the classic angled winglets of the Boeing 747-400, the continuously curved blended winglets of many Boeing 737s, Airbus's sharklets, the split scimitar double fins, or the folding curved tips of the Boeing 777X, which fold on the ground so the aircraft fits existing gate dimensions.

A winglet isn't free: it adds weight, increases wetted area (and with it parasitic drag), and loads the wing structure more, sometimes requiring a reinforced spar. That's why its payoff depends on the mission. On long legs, where induced drag weighs heavily for hours, manufacturers typically claim fuel savings of between 3% and 5%. On very short-leg operations, with more time spent climbing and less in cruise, the balance is tighter.

There's an equally valid alternative: simply lengthen the wing. Aerodynamically it's even more efficient, since the governing parameter is aspect ratio. The problem is practical: wingspan is limited by airport gate and taxiway categories. The winglet is, at bottom, a way to gain effective aspect ratio by growing upward when you can't grow sideways; the 777X's folding wing is the opposite answer to the same problem.

Not every aircraft has them. The Boeing 787 and the Airbus A350 use high-aspect-ratio wings with sharp tips continuously curved upward, integrated into the design from the start, which makes an added fin unnecessary.