How ground effect works?

[stepwilk]

For an article I'm doing for Aviation History Magazine on Soviet ekranoplans, I'm trying to come up with a simple but complete explanation of how ground effect works. It's not as easy as it was back when the old "cushion of air" theory held sway. (As an aerodynamicist friend wrote to me recently, "You're right that 'cushion of air' has gone out the window, which is too bad in a way, because like creation myths it offered a simple, easily memorized way to get people to stop asking unanswerable questions...")

Question: can anybody help me out by suggesting a brief explanation that will be understandable to people who are familiar with aircraft (though they aren't necessarily pilots) but who -aren't- aerodynamicists?

[bookworm]

OK, I'll bite. In the freestream the air beneath the wing would have a velocity downwards. In ground effect, that's not possible, because the ground gets in the way. So the pressure of the air below the wing is increased, because slower air is higher pressure air (Bernoulli).

[bingofuel]

I also think the proximity of the ground can restrict the wingtip vortices forming which would reduce induced drag.

[stepwilk]

I'm afraid bookworm's is the old "cushion of air" theory. And as for the wingtip vortices, it's a misapprehension--common enough--that they "create drag." They don't. They are the -result- of drag. They have already departed the airplane, and they can't create anything but wake turbulence.

[mad_jock]

The lift fairys get scared and they don't fall off cause they can see the ground

[osmosis]

Stepwilk, I believe if you bury your head in too much theory you're going to soon forget what you're there for. The acceptance and then rejection of one new theory after another doesn't alter what happens in the real world. I suggest you closely observe some agricultural spraying operations down on the deck by both fixed and rotary wing.

Also, the following link displays plans for a very simple and surprisingly effective wing in ground effect. I have engaged children in making and test flying these models and they know nothing of vortices or the numerous qualifiers of drag; indeed the lift fairies are much more easily understood by little people. You may just be surprised.

Ram: a model Wing-in-Surface-Effect Craft (http://triton.naoe.t.u-tokyo.ac.jp/akimoto/WISES_old/WISE/ramodel.html)

[stepwilk]

Well, it's a little like explaining how wings fly by sticking your hand, palm down, out a car window at 60 and saying, "See? That's how." For decades, we all believed that airfoils created lift because the air flowing over the top had to go faster to rejoin at the trailing edge the air going under the bottom, until somebody said, "Why? What law requires one air molecule to exactly meet another?"

What this discussion has so far shown me is that we're all baffled by how ground effect works. We haven't progressed far beyond a military flight instructor a friend of mine had back in prop/piston days, who cautioned him to not add power harshly during a close-to-the-runway go-around, because he'd "blow the lift out from under the wings."

[osmosis]

The first paragraph in your last post sums up what I am trying to point out: That we can formalise various theories until the cows come home but...

...the wing still flies regardless of the theory in current use.


...we're all baffled by how ground effect works...
Would we be a little less baffled and a little less critical of bookworm's theory about the ground getting in the way if we were to SEE what was going on? Again, watch some low level spraying, you'll soon see what happens.

Further, take a ride in a fabric covered biplane. Compare the fabric of the top of the lower wing to the bottom of the upper wing. There's something going on there.

[24Carrot]

Some things just aren't simple!

For my money Ground Effect has to do with how horizontal the air is as it approaches the wing or rotor. This affects how vertical the Lift Vector is, and so how big the induced drag is. But the actual details are significantly different for aircraft wings, helicopter rotors, and aerofoils spanning wind tunnels, even though the end result is similar.

Vehicles specifically designed for exclusive GE operation could also show effects that we don't even notice in normal aviation.

[rotorfossil]

I suspect that there is an slightly increased pressure below the wing which moves the neutral point at the leading edge, diverting and speeding up more airflow over the top surface, increasing the effective circulation.
Alternatively to use Newtonian theory, a wing works by displacing air downwards, equal and opposite force upwards is lift. If the air can't be displaced, the upwards reaction is increased.

[24Carrot]

Rather than leave these ideas dangling...

I think the simplest explanation of a wing in ground effect is via "Lifting Line Theory", although nobody would claim that was simple. The trick is to put a mirror image vortex system "underground", which simulates the ground by eliminating all vertical airflow at ground level.

The mirror image "bound vortices" (i.e. mirroring those along the wing) reduce airspeed above and below the wing, i.e. increase pressure above and below the wing, with a slight reduction in lift. There is no cushion of air.

The mirror image "trailing vortices" (i.e. mirroring those in the wake) reduce downwash behind the wing, but increase upwash ahead of the wing. This makes the lift vector more vertical, reducing induced drag.



A hand-waving physical explanation, (which does at least predict these effects), is:

Air can't pass through an aircraft so it has to go around it. In sub-sonic flight the air starts getting out of the way some way ahead of the aircraft. It is reacting to a pressure wave ahead of the aircraft.

The most important thing to note here is that the pressure wave starts many chord lengths ahead of the aircraft.

At height, there is plenty of room above and below the aircraft for the air to get out of the way, and each route takes about half the air. The pressure wave is symmetrical vertically. The airflow approaches the aircraft more or less horizontally and (after the wake vortices have cancelled themselves out) we see a residual flow deflected downwards.

In effect the wing has 'bent' horizontal air downwards slightly to achieve lift. The lift vector must be tilted backwards, and we have induced drag.

In Ground Effect, most of the air must pass above the aircraft (the pressure wave is shaped more like a giant flattened hemisphere), and so the wing 'bends' rising air back down again. The lift vector is more vertical, with less induced drag.

Another (small) Ground Effect is that the smaller one-sided pressure wave has to be twice as strong as the two-sided version, and so the air approaching the aircraft has a slightly lower airspeed, reducing lift slightly.

Ground Effect is all about reduced drag. It is often perceived as extra lift, because losing drag means you are soon faster than you would have been, which does allow you more lift.

[rotorfossil]

Yet another idea. The situation is similar to a helicopter operating in ground effect. The reduced induced flow means that for a given angle of attack, more lift is produced. You don't need more lift if the objective is to maintain height, so pitch is reduced. Thus the same lift is produced at a reduced angle of attack and the resultant of the lift and drag vectors is angled further forward, ie less drag and less power demand. In a ground effect vehicle, the whole wing operates at more efficient lift/drag ratio. I'm not sure whether the parallel of the bad news with helicopters where operating in ground effect enhances the tip vortices and tends to reduce some of the benefits. Perhaps the tip vortices in ground effect vehicles also have enhanced tip vortices.

Incidentally there does appear to be a small increase in pressure under a rotor disc as you come into ground effect (in the absence of wind) and is observable by a reduction of indicated height on the altimeter.

[stepwilk]

I think you've pretty much got it. What I'm going to write, since this is a nontechnical audience, is that frankly, ground effect is a complex, difficult-to-explain phenomenon best left to textbooks, but suffice it to say that an airplane flying very close to water or level ground such as a runway experiences less induced drag. The downwash angle is also reduced, and this rotates the lift vector forward, thus allowing an aircraft to stay aloft using less power and less fuel. Or, in the case of ekranoplans, carrying more weight than they could otherwise lift outside of ground effect.

[Ascend Charlie]

Gentlemen, and particularly Rotorfossil, you need to read Nick Lappos' comments on this in the Rotorheads section. Just do a search on "pressure bubble" or "helicopter urban myths" and sift through Nick's answers. He shoots down most of the comments in this thread, and gives the real reasons. In good instructional manner, I will not spoon-feed you, it is your job to look it up.

For those not familiar with him, Nick was Sikorsky's chief test pilot for many years.

[stepwilk]

Rotorfossil's explanation comes closest to current theory re. ground effect, even if he wasn't Sikorsky's chief test pilot. I'm always suspicious of the "Chuck Yeager Effect..."

[whyisitsohard]

Strewth, EVERYONE knows this.

As the plane gets closer to the earth the shadow scare all the little insects which then all flap their wings in unison. The resultant draught keeps the plane up longer! Come on guys, its easy!:cool:

D

[john_tullamarine]

I am always amazed at the knowledge one can pick up on PPRuNe -

These I shall tuck away for future training anecdotes -

(a) the lift fairies get scared and they don't fall off cause they can see the ground

(b) who cautioned him to not add power harshly during a close-to-the-runway go-around, because he'd "blow the lift out from under the wings."

(c) as the plane gets closer to the earth the shadow scare all the little insects which then all flap their wings in unison. The resultant draught keeps the plane up longer

If there are any other gems I've missed I'm sure I'll catch them on a re-read.

A bit like a lot of stuff in aerodynamics - we can observe it, measure it, usually predict it. As for why it really happens the way it does .. mmm ? Lots of stories for different levels or need, some of which go into lots of sums but not necessarily elemental understanding across the board.

Generally, we leave agonising over the wheres and whys to the pure theoretical physicists .. the average engineer just wants to get the beast designed, tested, certificated, manufactured and delivered to the customer.

[mad_jock]

John I hope you have also got the lift fairy explanation for stalling.

Which is when the wing reachs the critical angle of attack the lift fairys are unable to hold onto the wing any more and slide off the upper surface of the wing to hang onto the trailing edge where they bounce around giving a rumbling noise. When you reduce the AoA they can climb up again.

Vortex generators give them something to hang onto.

Its quite scary how much you can explain with "lift fairies" theory.

[TheBigD]

Ground effect reduces wing tip vortices, which in turn reduces induced drag. Think of it as there is more wing area available to produce lift since wing tip vortices are affecting a smaller portion of the wing due to the interference of the ground with air flow patterns. Hence, lift is increased in ground effect for that reason.