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I have some confusion concerning the ground effect. If we take two applications: Aircraft and Cars, they seem to have completely opposite effects.

Aircraft

From Ground effect (aerodynamics), we can read:

During normal flight, the upper wing surface experiences reduced static air pressure and the lower surface comparatively higher static air pressure. Flying close to a surface increases air pressure on the lower wing surface, nicknamed the "ram" or "cushion" effect, and thereby improves the aircraft lift-to-drag ratio. The lower/nearer the wing is vs. the ground, the more pronounced the ground effect becomes.

I can understand this, the air gets 'trapped' between the wing and the surface (and as consequence the pressure on the underside of the wing goes up)

Cars

However, from Ground effect (cars) we read:

Designers shifted their efforts at understanding air flow around the perimeter, body skirts, and undersides of the vehicle to increase downforce with less drag than compared to using a wing.

This kind of ground effect is easily illustrated by taking a tarpaulin out on a windy day and holding it close to the ground: it can be observed that when close enough to the ground the tarp will be drawn towards the ground. This is due to Bernoulli's principle; as the tarp gets closer to the ground, the cross sectional area available for the air passing between it and the ground shrinks. This causes the air to accelerate and as a result pressure under the tarp drops while the pressure on top is unaffected, and together this results in a net downward force. The same principles apply to cars.

The cross sectional area decreases, causing a higher velocity. This higher velocity decreases the pressure, causing the tarpualin to go down.

This confuses me, as it seems that the same situation (a plate in the vicinity of a surface) causes two completely opposite effects.

Both the "trapping air -> pressure goes up" as well as "trapping air -> velocity goes up -> pressure goes down" explanation seem reasonable to me.

This made me wonder:

Why the ground effect has such a different outcome for wings and cars?

If I have a plate under an angle close to the ground, will the air be trapped, creating an air cushion pushing the plate up, or will the air accelerate, causing the plate to be pushed down? Does it have to do with some degree of area decreasing?

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  • $\begingroup$ "How come" is lousy english writing, please change to "why" on all occurrences. $\endgroup$
    – quiet flyer
    Commented Jun 17, 2020 at 16:51
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    $\begingroup$ @quietflyer thanks for the comment. Why is it considered bad english? It seems to be quite common, see this Google Ngram $\endgroup$
    – ROIMaison
    Commented Jun 17, 2020 at 18:54
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    $\begingroup$ @quietflyer I don't see why informal writing should be discouraged on Av.SE. There's no reason to be so pedantic (and if there is, do have the courtesy to use proper grammar and capitalization). $\endgroup$
    – Sanchises
    Commented Jun 17, 2020 at 19:26

6 Answers 6

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It actually has exactly the same outcome! Well, in reverse.

Wings generate lift by forcing air down (one way or another, let's not go into details). When the wings are close to the ground, the air that is forced down can't get out of the way, resulting in a higher pressure.

Cars generate downforce by forcing air up. When air close to the ground is forced up, it can't be replaced by 'new' air easily. This results in a low pressure, sucking the car down.

The ground effect is thus exactly the same in either case: air that is either pushed out or pulled in, cannot be replenished quickly enough due to the ground being in the way. The difference in outcome is because a wing is attached at a positive angle w.r.t the ground, whereas a car is shaped like a very thick wing at a negative angle.

A comparison could be a hovercraft on the one hand, which is lifted off the ground almost exclusively by ground effect (if you were to drop a hovercraft from a plane, it wouldn't matter much whether it was powered up or not). On the other hand, a vacuum cleaner, which is sucked down to the floor exclusively by ground effect (indeed, Feynman's sprinkler experiment will tell you it is not sucked down because of the upwards acceleration of air - only the 'engine' part of your vacuum is forced down by momentum exchange. Test this by holding the vacuum in the air - you won't feel it 'pull' at all).

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    $\begingroup$ Then I have another related question. Let's assume we have a flat plate at the bottom of our car. However, using panels at the side (shielding the tires for example) we create a narrowing channel. This will cause the velocity to rise in the throat, and as an effect the pressure drops, and more downforce is generated. However, no air is forced up or down. If downforce is only generated by moving air up, how come only an increase in velocity will also increase the downforce? $\endgroup$
    – ROIMaison
    Commented Nov 3, 2015 at 14:44
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    $\begingroup$ Sorry I confused you; I meant to explain that the ground effect is not due to momentum exchange but due to pressure increase or decrease. How exactly this pressure is decreased is not addressed; I used forcing air up as an example to stay close to the airplane analogy. $\endgroup$
    – Sanchises
    Commented Nov 3, 2015 at 15:13
  • $\begingroup$ Wings generate lift by being forced up by air, not forcing air down. $\endgroup$
    – ptgflyer
    Commented Nov 5, 2015 at 16:00
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    $\begingroup$ @ptgflyer that's Newton's third law for you. $\endgroup$
    – Sanchises
    Commented Nov 5, 2015 at 16:38
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    $\begingroup$ @ptgflyer I was hinting at the concept of downwash. Newton's 3rd law says that, for the wing to be pushed up, something has to be pushed down - compare the ground denting slightly when you stand on it. $\endgroup$
    – Sanchises
    Commented Nov 13, 2015 at 21:46
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Because they are designed differently. As you've yourself pointed out, while an aircraft produces lift, the racecars produces downforce i.e. lift directed downwards. The following figure shows the design of the wing of an aircraft and the front wing of a racing car.

Ground Effect

Image from Ground Effect Aerodynamics by Erjie Cui and Xin Zhang

Basically, the downforce of the cars is the lift towards the bottom.

  • When the wings are close to the ground , two things happen- the effective span is increased due to the interference of ground with vortices (within a wingspan).

Ground effect vortices

Source: ghdstudios.com

  • Still closer to the ground (around 1/4th span), the air flow between the wing and the ground is compressed to form an air cushion. The pressure on the lower surface of the wing is increased creating additional lift.

The net result is the increase in lift and decrease in (induced) drag.

In case of cars, the same thing happens, with the force in the downward direction. As the air is 'trapped' in the the gap between the suction surface (underside in case of a car) and the ground, it initially accelerates. As the height is reduced, the flow accelerates more, resulting in less pressure and more downforce.

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  • $\begingroup$ I think there's more to it than simply turning the wing upside down (as you do in your first picture). I agree with the wiki when it says: "Starting in the mid-1960s, 'wings' were routinely used in the design of race cars to increase downforce (this is not a type of ground effect)" At end you mentioned that close to the ground the air flow between the wing and the ground is compressed causing an air cushion, why wouldn't this happen at the underside of the car? $\endgroup$
    – ROIMaison
    Commented Nov 3, 2015 at 12:10
  • $\begingroup$ @aeroalias What about normal cars that don't have wings that produce negative lift? They also experience a ground effect. $\endgroup$
    – mezzanaccio
    Commented Nov 3, 2015 at 12:12
  • $\begingroup$ @mezzanaccio The down-force can be produced by the body of the car. In fact, around half of the downforce in a racing car is produced by the body. Wings are used because they are simply more efficient. $\endgroup$
    – aeroalias
    Commented Nov 3, 2015 at 12:20
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    $\begingroup$ @ROIMaison Because wings create a downwash, which is not done by the cars. This basically creates something akin to ram pressure in a wing near the ground, which is absent in the car, which has a flat surface. $\endgroup$
    – aeroalias
    Commented Nov 3, 2015 at 12:25
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Remember that lift means the downward acceleration of an air mass. This bends the aerodynamic force vector backwards, so a small part of the pressure force acts against the direction of motion.

If the ground blocks this bending, less of the aerodynamic force will be felt as drag. This is true regardless of the direction of this force - it can be up or down. The closer the distance between wing and ground is, the more this effect can be observed.

The difference between the ground effect of an aircraft and that of a car is their angle of attack and the resulting direction of lift: Up in case of the aircraft and down in case of the car. The magnitude of the resulting force is not changed much, only its backward tilt is reduced, which results in a dramatic drag decrease.

The car body acts also as a wing, albeit one with a small aspect ratio. All the racecar designers did is to replace the top-mounted wing (which did not feel much ground effect) by the car body itself (which is much closer to the ground). By keeping the entry area under the front bumper smaller than the exit area under the rear bumper, the car body acts like a wing at a negative angle of attack, and ground effect makes its downforce creation quite efficient.

Widening gap between car body and ground in side view

Widening gap between car body and ground in side view (picture source). Skirts make this more efficient by preventing air entering from the side, but in the ever evolving tug-of-war between race car engineers and regulators they and some aspects of car bottom shaping have become less efficient in recent times.

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  • $\begingroup$ But if I follow this explanation, then the example with the tarpaulin does not make a lot of sense to me... If it moves to the ground, does it mean that it accelerates the air upwards? $\endgroup$
    – ROIMaison
    Commented Nov 3, 2015 at 14:02
  • $\begingroup$ Then your understanding of the tarpaulin experiment is not correct. It is a bad example, anyway, because it is floppy while wing and car are stiff. The idea is to hold the windward side of the tarpaulin close to the ground, and then the wind will bend it down. If you move the whole tarpaulin down on all four edges symmetrically, no such predominant motion is visible; instead, it will flap randomly. $\endgroup$
    – Peter Kämpf
    Commented Nov 3, 2015 at 15:00
  • $\begingroup$ I hope you can help me improve my understanding. In the commens below this answerhttp://aviation.stackexchange.com/a/22643/4197, I give an example of a flow that has no vertical momentum exchange, yet provides an upward force. This gives me some issues with the air deflection explanation.. $\endgroup$
    – ROIMaison
    Commented Nov 3, 2015 at 15:19
  • $\begingroup$ I think I understand it now. In the case of the wing, there is a momentum exchange, and this effect is made stronger by the ground effect. In the case of the case, at the bottom plate,(if we assume a straight bottom plate) there is no momentum exchange, but due to the narrowing channel under the car, the flow accelerates, causing downforce. Correct? $\endgroup$
    – ROIMaison
    Commented Nov 3, 2015 at 15:33
  • $\begingroup$ @ROIMaison: The momentum exchange is only part of the process - in the end, the momentum of the air causes a pressure field on the ground. In ground effect, the same happens, but much less air and less motion is involved, because the ground is so much nearer. Your example doesn't create momentum-free lift, just lift which involves less motion of the air. $\endgroup$
    – Peter Kämpf
    Commented Nov 3, 2015 at 22:06
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The main difference between the two cases is that the wing generates lift by inducing a downward wind component. When this downward wind component is close enough to the ground, an increase in lift will occur according to the explanation you posted.

The two effects have the same name, but are explained by different physical principles. Just think of the distances between a wing and the ground (a few meters) and between a car chassis and the ground (a few centimetres). A car is not a lifting body and will not induce a downwind, therefore this will not cause an upward force.

The car ground effect is just "another" ground effect, which is explained correctly by your quote.

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    $\begingroup$ It depends on the design of course, but a car actually is a lifting body. The whole purpose of wings, spoilers, and front air dams is to create downforce to counteract the lift produced by the body at the higher speeds of a race car. I got my '83 Mustang GT up to 120 once and could actually feel the car rising and getting lighter on its tires. Not something you want in the corners! $\endgroup$
    – Michael Hall
    Commented Jul 21, 2019 at 15:40
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There have been some great answers here with awesome information, but I don't think any of them address the premise of the question, which I believe to be incorrect.

The premise is that it is ground effect that is creating a force keeping the car on the ground. This is not accurate. A race car sticks to the ground because it is designed to overcome ground effect.

An airplane in ground effect can still easily be flown into the ground with a sufficiently negative angle of attack, or from lack of lift due to wing stall. A car can be made to stick to the ground with suffient downforce - despite ground effect, not because of it.

If a sufficient flow of air is allowed under the car, with a positive angle of attack, it will absolutely experience the same exact ground effect as an airplane does.

Conversely, an airplane can be driven all the way down the runway at cruise speeds without lifting off if you hold the elevator all the way in, though I wouldn't recommend doing that. Ground effect is not a magic levitation force, it simply reduces drag and it can easily be overcome. If not, landing would be impossible.

The bodywork of a race car is designed to overcome ground effect. The bottom of many race cars are also designed to maximize the advantage of low pressure, but this is not ground effect. This effect can be described anecdotally with the child's experiment of placing newspaper flat on a table on top a ruler, with the end of the ruler off the table, and then striking down on the ruler, which will break the ruler thanks to the air pressure on the top of the paper. When the car tries to lift off the road, the same force helps keep it stuck - but that force is not the "ground effect" that we refer to when flying an airplane close to the ground.

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  • $\begingroup$ +2 if I could. This is the only correct answer to the question, IMHO. $\endgroup$
    – JimmyB
    Commented Nov 4, 2015 at 11:37
  • $\begingroup$ @HannoBinder: Why? Ground effect is mostly about reduced induced drag, not about lift. The direction of lift is secondary. Greg's and your understanding of what ground effect is is different from everybody's else, IMHO. $\endgroup$
    – Peter Kämpf
    Commented Nov 5, 2015 at 20:21
  • $\begingroup$ Odd, that you would think that since I even said the same about reduced drag that you just did. $\endgroup$
    – Greg Taylor
    Commented Nov 5, 2015 at 22:20
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    $\begingroup$ The more I've thought about this the more I think the problem is that ground effect is an overloaded term. Car guy are describing a totally different force when they use the term ground effect. $\endgroup$
    – Greg Taylor
    Commented Nov 5, 2015 at 22:23
  • $\begingroup$ @PeterKämpf I'm pretty sure it's about both; Wikipedia says "improves the aircraft lift-to-drag ratio", reason: less vortexes and more air cushion under the wings, and "While in the ground effect, the wing requires a lower angle of attack to produce the same amount of lift." $\endgroup$
    – JimmyB
    Commented Nov 6, 2015 at 15:26
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I think there is a lot of great information in the answers here but they miss the obvious. It's about the angle of attack.

Even a wing angled to point downwards close to the ground will not generate lift - it will create a lower pressure under/behind the wing (as well as a greater pressure above) and "suck" (and push) it down into the ground. And that is what is desired and designed with cars.

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