Winglets, sharklets and raked tips

A wing generates lift by holding higher pressure underneath than above, and at the wingtip, that pressure difference escapes around the end, spiralling off as a vortex. That vortex is pure waste: it is drag, it is noise, and it is fuel burned for nothing.

Wingtip devices interrupt it. A vertical winglet, an Airbus sharklet, or a Boeing raked tip each spreads the escaping air over a longer path, cutting induced drag by a few per cent. A few per cent of an airline's fuel bill is an enormous amount of money, which is why they appeared on almost everything after the 1990s.

There is a second, sneakier reason. Airports allocate gates by wingspan category, so a wing cannot simply be made longer without pushing the aircraft into a bigger, costlier gate class. A device that curves upward buys aerodynamic span without buying physical width, which is exactly why the 777X's wingtips fold.