What Are Wingtip Vortices?

Answer:

A C-17 Globemaster III from the 14th Airlift Squadron, Charleston Air Force Base, South Carolina, flies off after releasing flares over the Atlantic Ocean near Charleston, South Carolina, during a training mission on Tuesday, May 16, 2006. The "smoke angel" is caused by wing vortices at the plane's wingtips.

Wingtip vortices are the spiral airflows of disturbed air flowing outwards and downwards from an aircraft’s wingtips when it is generating lift.

As the decreased pressure over the top of the wing is less than the ambient pressure, the air flowing over the top surface flows inwards. The air flowing over the lower surface, due to the relatively high pressure around it, flows outwards and curls upward over and around the wing tip. When the two airflows meet at the trailing edge they are flowing in opposite directions. Vortices are formed which merge into one large vortex at each wing tip, resulting in a resistant force on the aircraft called induced drag.

Winglets or tip-tanks affixed to the wingtips are effective in reducing the vortices and thus in reducing induced drag.