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Vortex Drag

What are Those Whirls off the Wing Tips?

The spirals of air that trail off the tips of an airplane’s wings also contribute to drag. These wing tip vortices steal energy from the motion of the airplane, creating vortex drag.

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How Do Wing Tip Vortices Affect an Airplane?

The pressure imbalance that produces lift creates a problem at the wing tips. The higher-pressure air below a wing spills up over the wing tip into the area of lower-pressure air above. The wing’s forward motion spins this upward spill of air into a long spiral, like a small tornado, that trails off the wing tip. These wing tip vortices create a form of pressure drag called vortex drag.

Vortices reduce the air pressure along the entire rear edge of the wing, which increases the pressure drag on the airplane. The energy required to produce a vortex comes at the expense of the forward motion of the airplane.

Tilting the airplane’s wings upward makes the vortices stronger and increases vortex drag. Vortices are especially strong during takeoff and landing, when an airplane is flying slowly with its wings tilted upward.

How Do Wings with a High Aspect Ratio Reduce Vortex Drag?

The farther a vortex is from the main body of the wing, the less influence it has on the wing. So long, narrow wings, like those of an airliner, or this Lockheed U-2 spy plane, will produce less vortex drag than a short, stubby wing with the same surface area. But to make long wings strong enough adds weight and reduces maneuverability.

But Low Aspect Ratio Wings Have Advantages Too

An attack aircraft, like the Douglas A4-C Skyhawk, has shorter wings to enhance its maneuverability. Short wings also reduce the drag from shock waves that begin to develop at speeds approaching that of sound. Short wings also take up less space—no small matter on an aircraft carrier.

Ask an Explainer

Q:

Why do birds fly in a V formation?

A:

Birds have a low pressure zone over their wings and high pressure underneath. They create wingtip vortices just like airplanes. Geese fly in a V-formation to reduce the drag caused by these vortices. The vortex created by the lead bird is counteracted by the vortex created by the birds flying at either wingtip. The lead goose works hardest, and periodically drops back to let other geese lead. ... more

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Did You Know?

A rectangular wing creates more wingtip vortices than a tapered or elliptical wing. Few aircraft have elliptical wing shapes, because of the manufacturing difficulties. The most famous examples being the World War II Supermarine Spitfire and P-47 Thunderbolt.

Pop Quiz

Which airplane has the highest aspect ratio?

A) North American X-15
B) 1903 Wright Flyer
C) Rutan Voyager
D) Ryan NYP Spirit of St. Louis

The aspect ratio of a wing is essentially the ratio of its length to its breadth (from front to back), which is known as its chord length.  The Rutan Voyager has the highest ratio of wing length vs. chord length.