# Ground effect on planes (and cars?)

1. Sep 6, 2004

### Gonzolo

Hi, I got a few questions on ground effect :

1. When a plane lands, there is a period of ground effect, when the plane is less than 10 feet above the ground and the flight is very smooth. Can someone remind me why it is so smooth?

2. Does ground effect have any effect on regular or race cars? And do skirt kits for street cars have any use at all besides looks?

2. Sep 7, 2004

### Clausius2

I don't remind well what was the ground effect. I think it provides with an additional lift force to the aero-plane, isn't it?. If so, it has an explanation with Fluid Mech. theory. To be honest I'm not able to explain it well using mathematical tools. Maybe, the airstream which flows onto the aircraft has the same behaviour like oil inside a journal bearing. This is a typical viscous flow at low speeds, such the aircraft landing process. The oil inside the journal bearing provides it with a lifting force when it passes through the narrowest gap section.

If you are not agree employing viscous approaching with this case, then I have a contradiction to discuss. Let's yield high #Reynolds and a very low #Mach. Then incompressible Bernoulli equation governs this process. See the figure below attached I've tried to draw, please.

Thus, the air passing through the section A-B must experiment a significant increasing of velocity, due to at low #Mach $$\rho$$ is almost constant, and section is shortened.

According to Bernoulli equation, static pressure must be smaller in A-B section as the flow is accelerated, so the lift force is decreased.

What is going wrong?. I don't know.

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3. Sep 7, 2004

### Gokul43201

Staff Emeritus
I think the "ground effect" is due to a boundary layer of relatively motionless air - but that's just an educated guess, at best.

4. Sep 7, 2004

### Marijn

I do know that any lift on a true racing car is eliminated through the construction of the bottom of the car.

It is formed in such a way that air is traveling under the car as fast as possible.
Because the air travels faster under the car than it does over the car, the car is sucked to the ground.
Some supersport cars even have a fan onderneath it which sucks the air away from underneath it.
This was however banned from most racing classes.

So i don't think ground effect will take place with a racingcar.

5. Sep 7, 2004

### Staff: Mentor

Ground effect for cars and planes are opposites - for planes it pushes up, cars it pulls down. In either case, I know of no reason a plane would be "smoother" just before it lands, other than that its engines are throttled back.

Ground effect has two components, iirc: First the wingtip vortices, which reduce lift, are disrupted. Second, air traveling below the plane gets more pressurized because it can't 'get out of the way' when pushed down by the wing. For this reason, planes with larger wing area are better at riding their ground effect.

For cars, ground effects are designed to create a suction. You can do this easily enough by having the front of the car lower than the back. The side skirts block air from getting under the car from the sides. Some work, some (like on my new Mazda 6) are just for decoration. On race cars, the downforce from the combination of the wings and ground effects is huge - Indy cars have strict regulations on ground clearance, as the lower the car is, the better the ground effects.

6. Sep 7, 2004

### Clausius2

Can you explain that second reason?. I mean, give a mathematical reason for what are you saying. I've tried to explain it above, but I've reach a paradox using bernoulli equation. I'm wrong, but I don't know where.
I don't agree. Perhaps this enhances the top air stream being deflected upwards and creating a downwards force. But it is not what is called ground effect. F1 cars have half of a convergent-diverging nozzle at their bottom. The proper F1 bottom structure is curved like the narrower part of the half part of a convergent-divergent nozzle. I mean, like this:

-------\____________/------- F1 bottom
--->
-----------------------------ground

Here, a narrower section accelerates the flow, creating a force downwards over the F1 car. Hey! can you see this? the question is: what is the difference between this analysis and the other for airfoils?. They have opposite effects but I see the same thing.

Change the label "F1 bottom" by "airfoil bottom", what happens? :surprised

7. Sep 7, 2004

### Staff: Mentor

Actually, I was under a common misconception here. See http://www.aa.washington.edu/faculty/eberhardt/lift.htm [Broken] site.
The description is a little tough, but it sounds like the ground inhibits airflow under the wing, causing more to flow over the top.
You're right. HERE is a link:

Last edited by a moderator: May 1, 2017
8. Sep 7, 2004

### Gonzolo

I think it is because there is less turbulence when there is a smooth airflow below the plane, perhaps due to the higher pressure underneath you mentioned. I have definitely experienced this on a flight I made last week in a 737. All turbulence practically stopped just before touch down. If there is no turbulence in the first place, it may well not be any smoother.

Last edited by a moderator: Sep 7, 2004
9. Sep 8, 2004

### Clausius2

Gonzolo: I've taken a look at your link. Although I tried to look into it carefully, I've not understood nothing . Maybe the things are not as simple as we usually imagine. Anyway, thanks for trying to clear it up.

10. Sep 8, 2004

### Gonzolo

I think you mean russ_watters.

11. Sep 9, 2004

### Clausius2

Ah! :yuck: Excuse me. I meant russ_watters. Thank you for your link.

12. Sep 16, 2004

### Ivan Seeking

Staff Emeritus
Speaking of ground effect

FYI

The Russian Ekranoplan

http://www.gizmohighway.com/transport/ekranoplan.htm

13. Sep 17, 2004

### LURCH

Perhaps http://www.se-technology.com/wig/html/main.php?open=aero&code=0 [Broken] explanation will be a bit clearer. It has a better illustration, at least.

And http://reprieve.iwarp.com/other.html [Broken] with many more "Wing In Ground effect" craft (all recreational).

Last edited by a moderator: May 1, 2017
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