Winglets all work in the same basic way, but they don't all look the same. In essence, winglets reduce drag by recovering some of the energy in the wingtip vortex. This provides an effective increase in wing aspect-ratio (span² divided by area - a measure of slenderness), and therefore a reduction in lift-induced drag, for a smaller increase in span, weight and profile drag compared with simply making the wing longer
Once rare, winglets have become commonplace, and over the years their design has evolved to make them more efficient and maximize the fuel savings they provide. Here are some examples of how winglets have changed - and how they could change in the future.
Challenger (photo via Wikipedia)
Bombardier's Challenger 601 (first flown 1987) was one of the first aircraft with production-standard winglets. They were simply angled upwards from the wingtip.
Boeing 737-800 and Airbus A319 (photo via Wikipedia)
Aviation Partners Boeing's blended winglet for the 737NG (above, foreground) is joined to the wing with a smooth curve to reduce interference drag, and provides a 4% fuel-burn reduction over long ranges. Airbus's signature wingtip fence (background, on an A319) is more endplate than winglet, and shaped to avoid the effects of winglet stall.
A320 Sharklet (photo: Airbus)
Airbus's Sharklet (above), introduced on the A320 at the end of 2012 and offering a 3.5% fuel-burn improvement, is so similar to Aviation Partners' blended design as to have sparked a lawsuit.
737 MAX (concept: Boeing)
Boeing, meanwhile, has designed a new "dual-feather" winglet (above) for the 737 MAX, claiming it offers another 1.5% fuel-burn improvement over the blended winglet.
737NG Scimitar (concept: Boeing)
Aviation Partners Boeing has launched the Split Scimitar (above) program to retrofit its 737 blended winglets with a new tip cap and ventral strake for an extra 2% fuel saving.
Falcon 50 Spiroid (photo: Aviation Partners)
Aviation Partners, for its part, continues to test the most extreme interpretation of a winglet yet, the closed Spiroid (above), chasing the promise of a 10% cruise fuel-burn reduction.
And it doesn't stop there, as these recent US patents show.
Winglets, as the name applies, are miniature wings, and as such they generate their own tip vortices that degrade their performance. The first Airbus design (above, US patent 8,366,056) seeks to mitigate this by curving the tip, sort of a winglet on a winglet - a bit like the idea behind Aviation Partners' Spiroid, but not as extreme.
Winglets reduce drag, but they also generate lift at the tip, increasing the bending moment at the wing root and requiring the structure to be beefed up, adding weight. The second Airbus design (above, US patent 8,342,456), the winglet still extracts energy from the wingtip vortex, but now is canted downwards so that part of it generates a downforce to counteract the extra lift and reduce the weight penalty.
A third recent patent, also assigned to Airbus (above, US patent 8,387,922), uses the winglet for more than just reducing wing drag. Actively controlled surfaces are added to the trailing edges of the winglets to introduce aerodynamic instabilities into the eddies shed by the wing. These instabilities would accelerate the dissipation of high-velocity wake vortices and allow aircraft to follow each other more closely. (in the diagram above the cylindrical object (20) at the bottom is a structural housing for the active-surface actuators).