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Ground effect on planes (and cars?) |
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| Sep6-04, 03:42 PM | #1 |
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Ground effect on planes (and cars?)
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? |
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| Sep7-04, 07:03 AM | #2 |
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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 [tex]\rho[/tex] 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. |
| Sep7-04, 08:14 AM | #3 |
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I think the "ground effect" is due to a boundary layer of relatively motionless air - but that's just an educated guess, at best.
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| Sep7-04, 08:30 AM | #4 |
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Ground effect on planes (and cars?)
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. |
| Sep7-04, 11:48 AM | #5 |
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Mentor
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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. |
| Sep7-04, 12:24 PM | #6 |
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-------\____________/------- 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?
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| Sep7-04, 02:10 PM | #7 |
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Mentor
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| Sep7-04, 08:32 PM | #8 |
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| Sep8-04, 05:31 AM | #9 |
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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.
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| Sep8-04, 12:10 PM | #10 |
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I think you mean russ_watters.
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| Sep9-04, 04:17 AM | #11 |
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Ah!
Excuse me. I meant russ_watters. Thank you for your link.
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| Sep16-04, 09:06 PM | #12 |
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| Sep17-04, 03:46 PM | #13 |
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Perhaps this site's explanation will be a bit clearer. It has a better illustration, at least.
And here's a page with many more "Wing In Ground effect" craft (all recreational). |
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