# What is ground effect?

We've all heard the "acts like a cushion of air" explanation tossed casually around by CFIs.

There's also plenty of books and reference materials that give a detailed, accurate, and complete explanation of ground effect, and which are incomprehensible to most student pilots.

What's the simplest way to accurately explain ground effect?

• en.wikipedia.org/wiki/Ground_effect_%28aircraft%29 ? If fear the judging of the superlative "simplest" will be opinion-based. – yankeekilo Dec 23 '13 at 9:46
• @yankeekilo - That link is exactly the kind of explanation I'm not looking for. Not only is it written with as much technical language as possible, it skips steps in the explanation. – Steve V. Dec 23 '13 at 13:55
• O.k. - I am looking forward to answers, Getting rid of all the technical terms comes with the risk of oversimplification. – yankeekilo Dec 23 '13 at 14:58
• I'm disappointed in most of these answers because the correct ones are too complex, and the simple ones have been down voted probably because they're too simplistic. – rbp Dec 29 '14 at 1:03
• The "cushion of air" comparison is actually pretty accurate. Phillipe's explanation is more specific but it's really nothing more than how or why you get that cushion of air between the wings and ground. – KeithS Oct 28 '15 at 19:47

It's impossible to give a correct AND simple explanation I think. I would explain it like this if I was asked:

The downwash from the wings and the downward component of the wingtip vortices create a higher than normal pressure area below the wings because the air hits the ground and can't "escape", increasing lift.

Induced drag is reduced because wingtip vortices (causing drag) can't fully develop close to the ground.

• “downwash from the wings” and “downward component of the wingtip vortices” are only one thing, not two different things! – Jan Hudec Apr 29 '17 at 22:20
• Your second sentence suffices. – KorvinStarmast May 1 '17 at 2:10

For a simple and correct answer you need first to understand induced drag.

Short version: The wing creates lift by deflecting air downwards. Induced drag is the consequence of this deflecting, because the resulting force of this deflection process is tilted backwards by half the deflection angle. Due to the backward tilt, the force has a horizontal component, pointing backwards. This is induced drag.

The ground effect will prevent this flow field from developing fully, because air cannot flow into the ground. Consequently, the deflection angle is lower, and also its horizontal component, i.e. drag.

The same happens ahead of the wing, where the upward motion of air ahead of the stagnation point is also restricted. Another consequence of this is a reduced lift curve slope in ground effect.

Lift is higher relative to induced drag compared to the free-flow case because the wing somewhat blocks the outflow of air at the trailing edge, and this results in a higher pressure on the bottom of the wing compared to the free-flow case.

This "cushion-effect" explanation is actually quite correct. The pressure which lifts the airplane is not only created by accelerating air downwards, but also by ram pressure. This effect disappears once the wing is far enough from the ground.

The ground acts as an aerodynamic mirror. When the wing approaches the surface it is as if an inverted wing comes from below. Their high pressure areas amplify each other, increasing the efficiency and thereby reducing the drag.

• Technical terms: ground effect produces an "image force." This image-force provides 100% of the lift ...as long as the wing altitude is far smaller than one wingspan. Also, ground effect is a three-dimensional effect involving wingspan and vortex-shedding, therefore a typical airfoil diagram (airfoil section with flow-lines) cannot explain it. – wbeaty May 30 '20 at 6:49

Here's my simplification, it's not the full story, but it covers the essentials.

Lift: When you are in free space, the high pressure below your wing dissipates into the surrounding air. When you're in ground effect, the high pressure below the wing encounters an incompressible solid, and therefore cannot dissipate as quick, causing higher pressure below the wing, and therefore more lift.

Drag: When you're in free space, the wake (turbulent, low pressure) behind your aircraft dissipates unimpeded until it becomes uniform with the surrounding atmosphere. When you're in ground effect, the air near the ground absorbs this dissipation quicker because it has to shear against the ground (more friction that air in free space). Additionally, wing-tip vortices encounter the ground and dissipate much quicker than in free space.

I suspect the reason GE is tied to wing length is because the longer your wing is, the stronger your wing tip vortices, and the further air has to travel laterally to exit the high pressure zone under the wing.

Simple explanation:

When out of ground effect, air on the bottom of the wing has plenty of room to move around. When in ground effect, the air on the bottom of the wing can't move around nearly as well and therefore pushes the plane upward to try and make more room.

• How does that reduce induced drag? – Steve V. Dec 23 '13 at 18:27
• You asked for simplified. – Andy Levesque Dec 23 '13 at 18:30

in normal flight high above the ground, wings create lift by deflecting an air stream downwards. This is not possible close to the ground: air does not travel through the ground surface.

Picture the downward air stream being bounced back and increasing pressure underneath the wing. This happens partially at the wing, not only behind it, because at sub-sonic speeds the air is being split in front of the wing.

So in ground effect, lift is created by pushing air downwards and by increased static pressure underneath the wing, like a hovercraft. And the increased pressure does not come with an induced drag penalty. And that is the main difference: total drag is lower in ground effect than in normal flight.

The wing pushes air down.

In flight, that air pushes on other air that moves.

In ground effect, that air pushes on the ground that doesnt move.

Pushing things that move is more difficult than pushing things that dont move. Like walking on sand vs walking on pavement.

What happens behind the trailing edge is surely irrelevant.

The wing has an angle of incidence to the ground so there is a wedge effect under it, presumably increasing pressure under the forward part of the chord and less so under the after part due to venturi effects.

Looks like a cushion effect to me and the airflow over the wing is deflected, and pressure reduced, as it would be at altitude.

Take a look at Pelicans in flight and you can see "ground effect" hard at work! Not complicated and most readily observed over water and not Terra Firma. For great YouTube videos simply Google "ground effect vehicle" and in particular the fantastical creation by the USSR during the Cold War (1948-1989). "Ground effect" is lift...and indeed forward velocity...gained when "sailing" just above a preferably watery surface. In nautical terms it's known as "displacement"(as opposed to Buoyancy) but you are in fact displacing the air just above the water instead of being on or in that physical surface. Their are many finite and interesting realities that exist in this "space" as well.

• You should add the link t the video you mention. Moreover, you may emphasize what your answer adds to existing one. – Manu H Jul 22 '19 at 8:27

Near the ground, the ground can push up on the airplane, giving it more lift.

• I thought that reduced drag was the main factor in ground effect, which causes increased lift? – egid Dec 25 '13 at 21:25
• There's no difference. Increased lift for same drag is the same as reduced drag for the same lift. – David Schwartz Dec 25 '13 at 22:01
• @DavidSchwartz No it's not the same. Suppose you go to the market with \$5. If you accept the first stall's offer of a dozen apples for \$5, you go home with 12 apples and no money. If you accept the second stall's offer of a dozen apples for \$4, you go home with 12 apples and \$1 in change. If you accept the third stall's offer of 15 apples for \\$5, you go home with 15 apples and no money. The second and third stalls are giving you the same lift/drag ratio but are behaving noticeably differently. – David Richerby Jun 22 '14 at 14:56
• @DavidRicherby: David is on to something here, even if he does not explain it. In the end, the ground supports all aircraft aloft, but in case of the wing in ground effect this support is more focused. – Peter Kämpf Dec 29 '14 at 9:09
• @DavidRicherby Your analogy shows why your argument is incorrect. In your analogy, there's no ability to choose the total taken. But a pilot can choose the total amount of lift+drag to take by moving the stick forwards and backwards. – David Schwartz Dec 29 '14 at 17:46