Ground Effect and Wingspan

[Dazzler]

It is said that ground effect generally extends from the surface up to a distance equal to the wingspan of the aircraft.

Why is this the case?

Also - what actually is the definition of the wingspan? Is it the distance from the left wingtip to the right wingtip? Or is it the length of just one of the wings?

[tgrayson]

Originally Posted by Dazzler

It is said that ground effect generally extends from the surface up to a distance equal to the wingspan of the aircraft. Why is this the case?

Great question. I don't think I've ever seen it addressed in any aerodynamics book talking about GE.

My suspicion is that it's related to the theoretical mass of air that a wing operates on; imagine a giant hose with an airplane flying through it, with the hose just wide enough in diameter to accommodate the wings. This is referred to as a "stream tube". In theory, the wing doesn't act on air outside this tube and therefore the ground can't affect the airflow around the airplane until the ground gets inside the tube. Since the diameter of the tube is equal to the wingspan, it will only occur within one wingspan. Remember, speculation only.

A "span" is wingtip to wingtip. The portion of the wing from a tip to the fuselage is called a "semi-span".

[B767Driver]

Originally Posted by Dazzler

It is said that ground effect generally extends from the surface up to a distance equal to the wingspan of the aircraft.

Why is this the case?

Also - what actually is the definition of the wingspan? Is it the distance from the left wingtip to the right wingtip? Or is it the length of just one of the wings?

Induced drag is directly proportional to wing downwash. For whatever reason, the magnitude of the downwash vector is not strong enough to extend past one span length. I suspect this is a fairly typical ratio across various wings. Of course, the CsubL across various airfoils really doesn't change all that much. .20 on the low side to .70 on the high.

[tgrayson]

Originally Posted by B767Driver

Of course, the CsubL across various airfoils really doesn't change all that much. .20 on the low side to .70 on the high.

For landing? A Cessna 172 has a landing CL of 2.0 with full flaps, 1.6 clean. Your airplane probably hits at least 3. I haven't seen anything in the 4's; maybe with some active boundary layer control.

[B767Driver]

Originally Posted by tgrayson

For landing? A Cessna 172 has a landing CL of 2.0 with full flaps, 1.6 clean. Your airplane probably hits at least 3. I haven't seen anything in the 4's; maybe with some active boundary layer control.

That would be for clean airfoils. I would assume the ratio stays somewhat consistent across similar flap settings.

[tgrayson]

Originally Posted by B767Driver

That would be for clean airfoils. I would assume the ratio stays somewhat consistent across similar flap settings.

I'm still not sure what you're saying. .7 Cl is very low for any airfoil, clean or dirty.

[B767Driver]

Originally Posted by tgrayson

I'm still not sure what you're saying. .7 Cl is very low for any airfoil, clean or dirty.

Actually, in the above discussion I meant CL to be CD numbers. Does that make more sense?

[fish314]

The explanation that I've always heard for this has to do with induced drag and wingtip vortices. Of course, this explanation is simplified and in layman's terms only.

It works like this: If you imagine a wing with infinitely long wings, then lift is created on such a wing by the air across the top having a lower pressure than the air across the bottom. People argue about "bernoulli" lift, vs. "newtonian" lift, but it all basically boils down to that simple statement. The air across the bottom as higher pressure than the air across the top. The higher pressure on the bottom surface pushes the wing UP more than the lower pressure on the top surface pushes the wing down, and VOILA-- Lift!

In an infinite wing, there are no wingtips- because the wing goes on forever. Obviously such a wing can only exist in theory, but it's useful for doing the mathematics. Anyways, in an infinite wing once the air hits the wing, the air that travels along the bottom can't get to the top, and vice versa. In a "real" wing, however, (which doesn't have infinite length) near the wing tip some of the air on the bottom tries to move around the side of the wing due to the higher pressure on the bottom of the wing than the top. This "spilling over" is what produces wing tip vortices. And these wing tip vortices are BAD. The reduce lift and increase drag.

Anyways, the airflow pattern around the wing depends on the shape and SIZE of the wing. This pattern includes the vortex pattern at the tips of the wings. That vortex pattern is also proportional to the size of the wing (and other things, too).

It turns out that when the wing is about 1 wingspan above the ground, that vortex pattern starts to hit the ground in it's attempt to go from the bottom of the wing to the top of the wing. This starts to break up the wing-tip vortex a little bit, and prevent some of that loss of lift and increase in drag. This is what we call "ground effect"

Anyway, I've heard ground effect described as beginning 1 wingspan in altitude, and I've also heard people say that it's 1/2 wingspan in altitude. Probably a question of how much of an effect there has to be before people say that it's a "SIGNIFICANT" effect.

[tgrayson]

Originally Posted by B767Driver

Actually, in the above discussion I meant CL to be CD numbers. Does that make more sense?

Immensely. It's amazing what difference one letter makes.

[tgrayson]

Here's an image from a book by Darrol Stinton, I think "Anatomy of an Airplane." His first picture shows the cross section of the stream tube I mentioned. The second shows the stream tube being squashed against the ground.

[MikeOH58]

Today I was told by someone that ground effect occours because PSI of 14.5 in the atmosphere....The Air is more dense above the ground which creates all of the effects and causes of ground effect....I've never heard it explained this way, and I don't know if its right or not...

thoughts?

[tgrayson]

Originally Posted by MikeOH58

I've never heard it explained this way, and I don't know if its right or not...thoughts?

It's not. During constant descents, you will continually be encountering higher pressure air, but that doesn't produce any ground effect. Similarly, at high altitude airports, you won't have anything close to 14.5 psi, yet ground effect still exists.

It's caused purely due to ground proximity.

[Chewie von Nubbins]

I just watched a show about secret Russian military equipment from the Cold War the other day, and they did well over 30 minutes on the life work of Rostislav Alexeev and the Ekranoplan. The premise of the Ekranoplan was to make an airplane that never leaves ground effect and could ellude radar and carry nuclear weapons to the shore of the US, without detection. This was the ghetto looking airplane that somebody on here uses in their avatar. This thing:




If you are looking for probably the most hard-core studies and facts on ground effect, I would look here. Here is the link from wiki:

http://en.wikipedia.org/wiki/Ground_effect_vehicle

And BTW, one of the questions for the CFIA written in the question bank says that ground effect is half the wingspan of the aircraft. But, we all know how reliable some of those questions are.

[tgrayson]

Originally Posted by Chewie von Nubbins

If you are looking for probably the most hard-core studies and facts on ground effect,

While true, remember that GEV's wings are much closer to the ground than an aircraft's wing and the goal of the aircraft is different. Many of the GEV articles focus on increased lift in ground effect, which can give the reader the wrong idea about aircraft operations in the same regime. What is really meant is that a smaller angle of attack can provide the lift that would require a larger angle of attack outside ground effect. For all vehicles, lift must equal the weight, or they would be zooming upwards. This leaves the reduced drag as the most notable symptom.

Moreover, these articles talk about chord-dominated effects, which I have never seen in any book talking about aircraft operations. Assuming the effect is real (I'm leery of stuff found on the web, but not in books), an aircraft's wing is probably far enough above the ground to make the effect minor, unless you're landing gear up.