Love it or hate it, ground effect plays a big part in your landings (and takeoffs too). If your approach to landing is too fast, ground effect can really get the best of you, as you float, and float, and float down the runway.
How It Happens
We’ve all been there. You start your flare for landing, you’re too fast, and you wait for your wheels to touch down. You wait a little longer, and a little longer, and pretty soon, you’re halfway down the runway.
That floating happens because of ground effect. But what it really comes down to are wingtip vortices, and what happens to them as your wings get close to the ground.
How The Ground Limits Wingtip Vortices
Ground effect basically comes down to how big your wingtip vortices are, and how much downwash they’re creating. Take a look at the Cirrus SR-22 examples below. Notice the difference in wingtip vortex size at altitude, vs. near the ground?
When your wing is close to the ground, wingtip vortices can’t get as big, because as they spin around your wingtip, they impact the ground and dissipate. The result? A reduction in something called downwash.
Less Downwash, Less Drag
As the air (and vortices) roll off the back of your wing, they angle down, which is known as downwash. Check out the difference in downwash at altitude, vs. near the ground:
Downwash points the relative wind downward, so the more downwash you have, the more your relative wind points downward. That’s important for one very good reason: lift is always perpendicular to the relative wind.
Scroll up and look at the diagrams again. You can see that when you have less downwash, your lift vector is more vertical, opposing gravity. And when you have more downwash, your lift vector points back more, causing induced drag. On top of that, it takes energy for your wings to create downwash and vortices, and that energy creates drag as well.
As you get close to the ground, your downwash is reduced and your vortices are reduced, which means your induced drag is reduced as well.
You can really break ground effect into three major categories to identify what’s happening as you approach the runway for landing:
- You have more vertical lift, which opposes weight
- You have less rearward lift, which reduces drag
- You have smaller vortices and less downwash, which reduces drag
How Close To The Ground Do You Need To Be For Ground Effect?
If you’ve flown a low-wing and high-wing airplane, you know that low-wing planes experience a lot more ground effect during landing.
Check out the chart below. You can see that ground effect doesn’t come into play until you’re within 1 wingspan of the ground. But as you get closer, your induced drag reduces significantly, amplifying ground effect.
Let’s look at a couple of real-world examples. First, let’s look at a Cessna 172, which has a 36-foot wingspan.
When a 172 is on the ground, its wing is about 7 feet off the ground, or 20% of the span length. Scroll up to the diagram above, and you can see that just before you touch down in a 172, your induced drag is reduced to 60% of your normal induced drag.
Now let’s look at a low-wing aircraft, the Piper Warrior. The Warrior has a 35-foot wingspan, and when it’s on the ground, its wing is about 3.5 feet off the ground, which is about 10% of the span length. Just before you touch down in a Warrior, your induced drag is reduced to only 40% of your normal induced drag. That’s significant, and it’s also why you float so much as you land in a low-wing airplane.
Ground Effect: Some Aircraft Are Designed For It
While ground effect might cause you headaches when you land, it’s the exact opposite for other planes. Some aircraft, like the Ekranoplan, were designed to only fly in ground effect. The Ekranoplan never got higher than a few feet from the Earth’s surface, allowing it to carry more payload than what would normally be possible. Check it out:
Putting It All Together
When you’re in ground effect, you have smaller wingtip vortices, less downwash, and more vertical lift, all of which dramatically reduce induced drag.
It all happens within one wingspan or less of the ground. So the next time you find yourself floating down the runway, go-around, and try another landing at a little slower speed.
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