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.
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.
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.
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.