# Friction Drag

## What is Friction Drag?

Friction is the resistance that happens when two things rub together—like air against an airplane. Friction is partly what causes drag.

## How Does Friction Work?

When an object moves through air, the air closest to the object’s surface is dragged along with it, pulling or rubbing at the air that it passes. This rubbing exerts a force on the object opposite to the direction of motion—friction drag.

The thin layer of air closest to the surface of a moving object is called the boundary layer. This is where friction drag occurs.

The boundary layer is a very thin layer of air flowing over the surface of an object (like a wing). As air moves past the wing, the molecules right next to the wing stick to the surface. Each layer of molecules in the boundary layer moves faster than the layer closer to the surface. The greater the distance (n) from the surface, the greater the velocity (V) of the molecules. At the outer edge of the boundary layer, the molecules move at the same velocity (free stream velocity) as the molecules outside the boundary layer. Ludwig Prandtl revolutionized fluid dynamics when he introduced the boundary layer concept in the early 1900s.

## Air "Sticks" to a Wing

Though air is much less "thick" than, say, honey, like all fluids it has viscosity—internal friction. The air directly touching the wing does not slip past it but stays "attached" to it. The air "stuck" to the wing rubs against the air just above it, which in turn rubs against the air just above it, and so on, up to the outer edge of the boundary layer.