What do winglets do to increase aircraft performance?

It seems like a lot of the newer airliners have winglets or wing fences. How do they improve aircraft performance?

they reduce the wingtip vortex and the associated drag by deflecting the air that wants to escape over the tip back down

• For lots more detail than you ever wanted to know about wingtip vorticies, their induced drag, and wingtip devices to counteract them check out Sighard Hoerner's book Fluid Dynamic Drag. The ZenithAir website also has some more accessible explanations (about 1/3 of the way down under "Wing Tips"). Commented Mar 7, 2014 at 20:36
• @voretaq7 How exactly does the induced drag of wingtip vortices work? The book doesn't really mention this.
– user7241
Commented Dec 24, 2017 at 6:17
• Wingtip vortices do not create drag; they are a consequence of the process that creates both lift and drag - see here and here. See page 4.6 in the latter for a discussion of this myth, and page 4.8 for an explanation of induced drag reduction as a consequence of how a non-planar wing has a different (flatter) least-induced-drag lift distribution than a planar wing of the same span. Commented Nov 17, 2020 at 21:25

Wing generates lift by creating area of higher pressure below and area of lower pressure above.

At the wing tip, some air flows around the tip, reducing the pressure differential and thus the efficiency of the wing. This is called transverse flow. The wing tip reduces this flow, thus improving the efficiency.

The effect of wing tip is actually similar to making the wing a bit longer, but since the wing tip does not produce lift itself, it is less stressed and therefore can be lighter, even though the extension would be about 1/3 shorter for comparable efficiency.

Winglets, raked wingtips, fences, sharklets all do the same thing; reduce drag. When the wing is working hard (high AoA), it will produce a lot of this kind of drag since vortices of all kinds will be stronger. Winglets makes the passage of air from the bottom of the wing to the lower pressure top of the wing more difficult, thus reducing induced drag.

The big manufacturers explored offering winglet retrofit kits to their customers but at the time, it wasn't economical since fuel was so cheap. These days, however, the retrofits pay for themselves fairly quickly even on airplanes wings that have a fairly high aspect ratio. Every 737/757/767 guy that I talk to says they get in the neighborhood of a 4-6% boost in fuel savings.

On an tangentially related topic, the hump from the satellite internet antennas has a negligible impact on performance, less than 1% according to the few guys that I talked to.

• Lol, what? 1% of fuel for just an antenna? No thanks! Commented Jul 3, 2015 at 16:52
• OK, 5 years late here. @Alexus, agreed, 1% is a lot but he said less than 1% and the money they can make in exchange for the in-flight web connection is apparently enough to offset. Probably more likely that it is partially baked into ticket prices which is why it is not available on budget airlines. Commented Oct 16, 2020 at 19:28

Basically what happens with an airplane without a wingtip is the high pressure area comes over the lower pressure area and creates a giant vortex called a wing tip vortex and the winglet reduces the strength of the vortex reduces drag, increasing lift, and increasing the aircrafts range.

• an airplane without a wingtip has infinite wings Commented Aug 7, 2015 at 11:55
• or circular wings Commented May 27, 2017 at 23:59