What Is Induced Drag?

Induced drag is created as a result of lift. As your wing passes through the air, an area of lower air pressure is formed on the top of the wing.

Higher-pressure air below the wing seeks equilibrium with the lower pressure area above, resulting in a vortex flow from the bottom of the wing to the top.

These vortices change the direction and speed of the airflow behind the trailing edge of the wing. The airflow deflects downward, which is called downwash.

Downwash changes the relative wind downward, which is an important point, because lift is always perpendicular to the relative wind.

As downwash increases, the lift vector tilts backward, creating induced drag (see diagram below).

How Induced Drag Changes With Angle-Of-Attack

The higher your angle-of-attack, the larger your wingtip vortices are, assuming ground effect isn't playing a role (which we'll get to shortly).

In general, the three factors that produce large wingtip vortices are: heavy, clean (no flaps), and slow, because you need to fly at a higher angle-of-attack in all three scenarios.

How Ground Effect Changes Induced Drag

When you fly close to the ground, generally within one wingspan, you'll start to notice ground effect.

When you're flying close to the ground, your wingtip vortices are smaller, because they hit the ground and can't expand. This limiting factor flattens out and reduces downwash.

Because your downwash is flattened out, the relative wind is flatter as well. Lift is generated perpendicular to the relative wind, which means the lift vector tilts forward, resulting in a reduction of induced drag.

With less induced drag you'll have a more vertical lift vector, which can cause you to float during landing.

Wake Turbulence

Aside from induced drag, wingtip vortices can create a safety hazard as well.

As you fly through the air, the vortex you've generated creates a spiraling mass of air. If another aircraft were to fly through this spiraling air, they could encounter severe turbulence or worse.

Again, the strongest wake turbulence is created when an aircraft is heavy, clean (flaps up), and slow.

Putting It All Together

Wingtip vortices induce downwash, which changes the relative wind and tilts your lift vector backward. And as your lift vector tilts backward, you generate more induced drag.