What is Drag?

If you have ever put your hand outside the window of a moving car, you have experienced drag. Your hand moving against the air causes two types of drag: pressure drag and skin friction drag.

Pressure drag is created when a stream of air runs into an object and separates to get around that object. You experience pressure drag when you hold your hand out the window of a moving car with your palm facing into the wind at a 90 degree angle to the road (like the hand signal meaning stop).

Skin Friction Drag

Skin friction drag is due to friction between air and the surface moving through it. You experience skin friction drag when you hold your hand out the window of a moving car with your palm parallel to the road. Friction on your hand occurs as the air rushes past your hand. The air near your skin tries to pull your hand backward, while at the same time your hand causes the air passing by to slow down.

Drag and an Airplane's Design

To improve an airplane's performance, scientists, engineers, and aircraft designers consider the purpose and requirements of an airplane before thinking of form or structure. To reduce drag, they also consider how fast the airplane will be flying, the stickiness of the air (viscosity), and the "tug of war" relationship between drag and thrust.

This is how we describe the battle between drag and thrust:

• If drag is greater than thrust, the airplane's speed decreases.
• If drag is less than thrust, the airplane's speed increases.
• If thrust and drag are equal, the airplane will fly at a constant speed.