image credit: Carla Thomas, NASA; image source; larger image
How Things Work: Winglets
The photo shows two NASA F/A18s. The smoke streaming from the wingtip of the one on the right reveals the wingtip vortex, which increases the wing's drag. This vortex occurs because the pressure underneath the wing is greater than the pressure above the wing; this excess pressure generates a flow of air around the wingtip, creating the vortex. These vortices can trail behind the aircraft for miles, creating a hazard for following aircraft, particularly small ones.
To reduce the drag caused by these vortices, "winglets" have been added to the wingtips of some airliners, as you can see in this Wikimedia photo. To learn more, visit How Things Work: Winglets.
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Make your own vortex at home--see Bottled tornado for how to do it. And be sure you have an adult with you.
Alternately, you can purchase a connector for the two bottles, with a hole in the middle--just google "vortex bottle" to find a vendor.