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I found How come the ground effect has such a different outcome for wings and cars? and: I am not allowed to comment as a new user because of some weird rule and I do not know enough to answer the question as I searched the web to find the answer to this.

Everyone is explaining that the two types of "ground effects" have the same name but are two entirely different phenomenons, but no one seems to have answered the question properly, at least that I can understand. It is all well and good saying they are different effects for different purposes but that does not explain why the two opposite effects occur in such similar conditions. I was originally searching for the answer to this question strictly in the context of car design - why the car-context ground-effect sucks the car down using the venturi effect yet having a full teardrop shaped car is meant to be impractical for the reason of creating a lifting air cushion underneath, though someone may think that the curve towards the ground would act as a venturi and suck the car down. The question is: "why do the opposite effects happen in such similar conditions". Venturi effect or air cushion? What decides which occurs?

I think aeroalias' answer sort of explained it but not in a clear and direct way; venturi effect occurs until the venturi is so extreme that the flow is "choked" and the venturi effect ceases to operate and you just have an air cushion? Is that the answer?

edit: This might make it clearer as to what I am asking though it is mainly a rephrasing of what I have already asked:

Why isn't an air cushion formed underneath a car (like with the aircraft) when the underside is curved towards the ground? Does the venturi effect occur when the curve is shallow but an air cushion form when it is steeper and flow is "choked"? I think I understand the basic theory of the car ground effect but what causes the aircraft ground effect and what conditions determine whether upward or downward force is produced?

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    $\begingroup$ "Everyone is explaining that the two types of "ground effects" have the same name but are two entirely different phenomenons" My answer argues that they are the same phenomenon with different outcomes. I added a small paragraph which should make the only difference between the two more obvious, although Peter Kampfs answer there gives a more accurate explanation. $\endgroup$ – Sanchises Jun 17 at 6:30
  • $\begingroup$ @Sanchises, thanks! Your added paragraph really helped me understand the similarity $\endgroup$ – ROIMaison Jun 17 at 7:58
  • $\begingroup$ @Sanchises Sorry. I think your answer was arguing differently toa lot of the others. So your answer is basically saying that having an enclosed area over the ground prevents air from either being replaced or escaping when it is sucked or pushed in? So in a sucker car like a chaparral 2J you can keep low pressure underneath but in a hovercraft you can keep high pressure underneath to keep it floating? I think Inunderstand that though I also think that is different to the commonly accepted emaning of "ground effect" for both aviation and cars which don't involved sealed areas or active suction. $\endgroup$ – Elliott Savva Jun 28 at 21:19
  • $\begingroup$ You say "sealed" but neither case is actually sealed. The ground effect does increase very rapidly with smaller gap, so I used extreme examples to give an intuitive feel for the underlying physics. In the end, it's all about creating a barrier so that you need less downward airflow (=kinetic energy 'lost' to the downwash) to generate the same lift. For huge wings, just the ground is more than enough; for cars, a smaller gap is required. $\endgroup$ – Sanchises Jun 29 at 6:16
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venturi effect occurs until the venturi is so extreme that the flow is "choked" and the venturi effect ceases to operate and you just have an air cushion? Is that the answer?

No. Neither answer to your linked question talks about venturi effect at all!

In the aircraft case the wing is still far enough above the ground there there is no significant venturi effect at all. The pressure field around the wing is still similar as in free flight, but because the air can't move down behind the wing, the effective angle of attack is decreased, which reduces the induced drag. It does not even increase the lift all that much, reduction of drag is the most significant effect.

In the car case the bottom of the car is much closer to the ground, but the venturi effect is still not the major part. The underside of the car acts as an inverted wing, which creates upward lift. The ground again prevents the, in this case, upwash, this time by preventing other air to fill in and this again reduces the induced drag. The point is again not creating more force, the point is creating the force with less drag, so the car can go faster with the same engine (most racing classes are limited by engine power or cylinder volume).

Also note that venturi effect would be always downward, so if it was true that venturi effect increases until it becomes “choked”, it would pull the aircraft (which has the wings higher, so definitely no choking) down and possibly (depending whether it would be choked under the car) push the car up, the opposite what is observed.

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  • $\begingroup$ Hmmm... how is it then that it is often stated that ground effect for race car X is Y kg? Yep, guess it should be N in the first place, but anyway, it is often measured in hundreds of kilograms. $\endgroup$ – Jpe61 Jun 15 at 21:52
  • $\begingroup$ Thank you for anwering! I think I need to read up on aerodynamics a bit more to understand most of what you mean. $\endgroup$ – Elliott Savva Jun 17 at 2:16
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    $\begingroup$ @Jpe61, I strongly suspect that is just sloppy wording calling the downward force ‘ground effect’ though it actually is just a downward lift and the ground just allows generating it with much less drag. $\endgroup$ – Jan Hudec Jun 17 at 7:18
  • $\begingroup$ Yea, could be. After all, we are talking about an industry, that has named downward lift generating devices "spoilers" 😃 $\endgroup$ – Jpe61 Jun 17 at 8:44
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    $\begingroup$ @JanHudec Flow choking is a problem in all ducted airflows, and that includes both Venturi ducts and the undersides of cars. If it happens in a turbojet intake the engine "surges". If it happens in the cooling duct for an engine or its radiator, the engine overheats. If it happens under a car, the steering suddenly gets awfully light! $\endgroup$ – Guy Inchbald Jun 17 at 11:00
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It is true that an aerofoil and a fast car both have streamlined and cambered profiles.

A wing in flight generates a high-pressure wave beneath it, which helps support the plane. This wave travels down to the ground and reflects off it. Normally the reflection trails well behind the plane, but very low down it can impact the wing and boost lift. This is ground effect.

When streamlined cars first attained high speeds, the effect was similar to that of a wing; a high pressure wave was created beneath it and reflected back off the road, the steering lightened considerably and the tyres' grip on the road loosened drastically. Ultimately, multiple reflections between ground and chassis could cause such a massive build up or wall of pressure that the flow was choked and drag rose steeply.

Early attempts to counter these undesirable side-effects included upside-down aerofoils at nose and tail. But bolting on fragile palliatives was never going to be a good solution.

Car designers turned to the wind tunnel and aerodynamics text book and there they discovered the Venturi effect. By designing the underside as a Venturi tube and avoiding flow choking, the whole vehicle became the upside-down wing.

In this respect there is really no difference between the upper surface of a wing and the wall of a Venturi tube, in fact it can sometimes be helpful to think of a wing as half a Venturi, or vice versa. Both generate the same horizontal flow acceleration and orthogonal suction via the Bernoulli effect. The trick with fast cars is to make the underside work harder than the top side and to profile the Venturi carefully to avoid the sharp decreases in area which cause flow choking.

The only problem then left was to harden the suspension so it did not bottom out at top speed.

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  • $\begingroup$ So what happens if I have a flat plate at an angle of attack as it approaches the ground? Will it 'support the plane' and push it up or 'act as a venturi' and suck it down? $\endgroup$ – ROIMaison Jun 16 at 10:38
  • $\begingroup$ Would be an interesting experiment. My guess is that at low AoA the lift would reduce due to suction but at high AoA the flow would choke and lift increase. But I could be wrong. It's what wind tunnels are for. $\endgroup$ – Guy Inchbald Jun 16 at 10:50
  • $\begingroup$ Wind tunnels don't explain why something happens :) $\endgroup$ – ROIMaison Jun 16 at 13:06
  • $\begingroup$ @ROIMaison You didn't ask why, you asked what. That is indeed what wind tunnels are for. Can you please stick to comments on the main answer. $\endgroup$ – Guy Inchbald Jun 16 at 17:06
  • $\begingroup$ I am allowed to comment now apparently. Thank you. I feel this is the answer that best answers the question. I think I need to learn a bit more to understand what Jan Hudec is talking about. So it is a case of venturi effect then after all, but only when the angle of attack is shallow enough? I also don't quite understand how a wing is like a venturi? I thought a wing is basic device that creates pressure underneath by forcing air under as it goes forward (if the front is higher than the back) or creates pressure above and therefore downforce by forcing the air over (when the front is lower)? $\endgroup$ – Elliott Savva Jun 17 at 1:57
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the wing on a plane is designed to generate lift while flying in the open air. It happens to generate less drag when flying ~ten feet off the ground, because a "bubble" of air gets squeezed between the bottom of the wing and the surface of the ground and this helps support the wing. this is called "ground effect" for airplanes.

you want a fast car to stay stuck to the ground and NOT turn into an airplane, and for this reason the car's body is designed to work like an airplane wing upside down while always "flying" just a couple of inches off the ground, so it creates downforce instead of lift. To generate downforce while almost touching the ground requires the use of aerodynamic features that don't look much like wings, and the fluid flow properties they exploit are known as "ground effect" for cars.

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  • $\begingroup$ But why isn't an air cushion formed underneath a car (like with the aircraft) when the underside is curved towards the ground? Does the venturi effect occur when the curve is shallow but an air cushion form when it is steeper? I think I understand the basic theory of the car ground effect but what causes the aircraft ground effect and what conditions determine whether upward or downward force is produced? $\endgroup$ – Elliott Savva Jun 17 at 2:03
  • $\begingroup$ @ElliottSavva, the underside of a car is not curved towards the ground, it is curved slightly away from it. $\endgroup$ – Jan Hudec Jun 17 at 7:33
  • $\begingroup$ @PeterKämpf, what I meant was exactly what you said, sorry for my poor wording, will delete the comment. $\endgroup$ – niels nielsen Jun 17 at 16:14
  • $\begingroup$ @JanHudec That's what I always assumed, as that would mean more volume and reduced pressure, but according to the venturi effect/bernoulli's principle the way I understand it, that would increase the pressure on the underside of the car, as the energy pushing the air along would be transferred to the internal energy of the gas (in order to slow the gas down as the wdth of the "tube" is increased) therefore increasing the pressure and temperature? What am I missing? I hear lots of things about high performance cars like Mclaren F1s and Jaguar XJ220s being shaped to accelerate air underneath. $\endgroup$ – Elliott Savva Jun 28 at 21:08
  • $\begingroup$ @ElliottSavva, less volume => higher speed => lower pressure (as internal energy due to pressure is converted to kinetic energy). This is only relevant if the surfaces are very close together though. The other part of the effect is forcing the air to curve upward – just like wings make air curve downward – which reduces the pressure by conservation of momentum (it takes upward force to make the air change direction). This is relevant with any distance between the surfaces, or even no second surface, just having the other surface close reduces the drag from it. $\endgroup$ – Jan Hudec Jun 28 at 22:04

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