At the tip of every lifting surface, high-pressure air beneath spills around the edge and rolls into a vigorous whirl. These wing-tip vortices steal lift and leave a long trail of drag behind the aircraft. By turning a short, upright fence at the extremity, engineers interrupt the roll-up; the vortex is broken into weaker eddies and its core energy is dissipated in the direction of the flow. With the swirling motion suppressed, lift-induced drag falls and the wing no longer pays the same horsepower penalty. The savings translate directly into fuel: a modest 2-5% improvement that, on a fleet's yearly ledger, outweighs the price of the add-on.
Expert behind this article
Jim Goodrich
Jim Goodrich is a pilot, aviation expert and founder of Tsunami Air.
Why do some aircraft have upturned wingtips?
Some aircraft have upturned wingtips because flipping up the outer few feet of the wing reduces drag. Non-planar wingtips are normally angled upwards in a polyhedral wing configuration so that altered airflow cuts the strength of the tip vortex. Because they weaken that vortex, non-planar wingtips have less parasitic-drag penalty if designed carefully. Upturned surfaces increase local dihedral, adding roll stability without enlarging the whole wing. The Boeing 737 has upturned winglets and Boeing has increased the use of upturned winglets from 10% in 2001 to 50% recently.
What do upturned wingtips do?
Upturned wingtips, called winglets, diminish wingtip vortices and reduce drag by increasing the height of the lifting system without greatly increasing wingspan. They operate in the whirlpool of air at the airplane's wingtips, turning part of the sideways energy into forward thrust, which improves fuel efficiency.
In nature, soaring eagles have splayed upturned feathers at their wingtips to reduce drag. Lanchester copied this into designs for model gliders. A similar principle guides the polyhedral wing configuration, where non-planar wingtips are angled upwards, providing wake control benefit of winglets while avoiding dangerous contact with the ground.
