Ground effect on planes (and cars?)

[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?

 

[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.

 

[Gokul43201]

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

 

[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.

 

[russ_watters]

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.

 

[Clausius2]

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.

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.

You can do this easily enough by having the front of the car lower than the back

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?

 

[russ_watters]

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.

Actually, I was under a common misconception here. See http://www.aa.washington.edu/faculty/eberhardt/lift.htm site.

Many pilots mistakenly believe that ground effect is the result of air being compressed between the wing and the ground.

To understand ground effect it is necessary to look again at the upwash. Notice in Figure 15 that the air bends up from its horizontal flow to form the upwash. Newton's first law says that there must be a force acting on the air to bend it. Since the air is bent up the force must be up as shown by the arrow. Newton's third laws says that there is an equal and opposite force on the wing which is down. The result is that the upwash increases the load on the wing. To compensate for this increased load, the wing must fly at a greater angle of attack, and thus a greater induced power. As the wing approaches the ground the circulation below the wing is inhibited. As shown in Figure 16, there is a reduction in the upwash and in the additional loading on the wing caused by the upwash. To compensate, the angle of attack is reduced and so is the induced power. The wing becomes more efficient.

The description is a little tough, but it sounds like the ground inhibits airflow under the wing, causing more to flow over the top.

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

You're right. http://www.usairnet.com/encyclopedia/Ground_effects.html is a link:

The basic idea is to create a volume of low pressure underneath the car, which sucks the car to the road. Naturally, to maximize the force one wants the maximal volume at the minimal pressure. Racing car designers have achieved low pressure in two ways: first, by using a fan to suck air out of the cavity; second, to design the underside of the car as an inverted aerofoil so that large amounts of incoming air are accelerated through a narrow slot between the car and the ground, lowering pressure by Bernoulli's principle.

 

[Gonzolo]

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.

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.

 

[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.

 

[Gonzolo]

I think you mean russ_watters.

 

[Clausius2]

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

 

[Ivan Seeking]

Speaking of ground effect

FYI

The Russian Ekranoplan

The Russian Ekranoplan looks like an aircraft but it only flies a few metres above water. Translated the Russian name Ekranoplan means sea skimmer and this is exactly what the Ekranoplan does. The Ekranoplan is what is known as a Ground Effect Vehicle, it operates on the principle of wing in ground effect, where the air gap between a wing and the ground is small enough for the air to be compressed.

Born out of the Soviet Unions Central Hydrofoil Design Bureau it was developed by their chief designer Rostislav Alekseev. Several different Ekranoplan's were developed from 1961 to 1990. Since the fall of the Soviet Union many of the design team from the CHDB have formed a new company with plans to develop new craft for passenger and cargo usage. Also 11 small 5 passenger craft were constructed between 1995 and 1997, they were used in trial commercial operation in USA and the Bahamas...

...The largest Ekranoplan called the KM was built in 1967, it was dubbed the Caspian Sea Monster after the sea it was on when first seen by the west. It was almost twice the length of a Boeing 777-200 but as with all Ground Effect Craft it need only half the wingspan. At over 100 m long, weighing 540 tons fully loaded, the KM could travel over 400 km/h mere meters above the surface of the water. Once moving at speed, the Ekranoplan was no longer in contact with the water, and could move over ice, snow, or level land with equal ease.

The important design principle is that the wing lift reduces the further above the surface of land or sea that the ekranoplan "flies". Thus its dynamically stable in the vertical dimension.

These craft were originally developed by the Soviet Union as very high-speed (several hundred km/hour) military transports, and were mostly based on the shores of the Caspian Sea and Black Sea. The largest
could transport over 100 tonnes of cargo. [continued]

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

 

[LURCH]

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

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