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.
What is Vortex Drag?
Trailing vortices deflect the flow behind the wing downward. This is called downwash, and it reduces the amount of lift produced by the wing. In order to make up for that lost lift, the wing must go to a higher angle of attack, which increases the drag generated by the wing.
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.