Thursday, August 2nd, 2012 11:17 am GMT -6 Thursday, August 2nd, 2012 11:17 am GMT -6Thursday, August 2nd, 2012 11:17 am GMT -6
 
Airliner Wing in Flight

You probably do not know this, but there are actually two different types of airplane ground effects.


 

 

 

Introduction

Ground effect is when an airplane is flying close to the ground and it experiences a lift increase and a drag reduction. So what’s not to like, right?

There are actually two entirely different effects at play. One reduces drag, the other increases lift. The drag reduction starts about one wingspan above the ground. Both effects get much stronger the closer the wing is to the ground.

Drag Reduction

Wings create lift by having a lower air pressure above them than below them. The air pressure equalizes at the wing tips. This is where the air from the top and the bottom of the wing meets, and a vortex is created. This vortex represents lost energy, and is what we call induced drag.

When an airplane wing flies very close to the ground, two changes happen to this vortex. First, the ground pushes the vortex outward, effectively increasing the wing span of the airplane. Second, the ground itself interferes with the vortex formation process, decreasing its strength. The net effect is that the airplane has lower induced drag.

This reduction in drag is determined primarily by the wing span to height above ground ratio. For a reasonably close distance, the induced drag could be cut in half.

Lift Increase

The increase in lift is easier to understand. When the wing is close to the ground, there is a ram effect at play under it. It results in an increase in air pressure under the wing. Higher air pressure means more lift.

The increase in lift is proportional to the size of the wing chord versus the height above the ground. Once again, we could see a doubling in lift for wings flying close to the ground.

Summary

An airplane flying very close to the ground could easily experience a doubling in its lift to drag ratio. This is excellent! The lift to drag ratio (L/D) is a key measure of airplane efficiency.

You may have heard of ground effect vehicles that exploit this phenomenon. They always fly just a few feet off the ground, realizing dramatically improved efficiency over a conventional airplane.

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