Better not hiccup....

[DaveBB]

Except Chuck Yeager, Bud Anderson, and a few others....

[colmbo]

He's in ground effect.  Not sure how it effects the P-51D, but some planes have to be forced to lose altitude once in ground effect (B47 for example).

Not really an accurate statement.  Ground effect reduces drag..with a clean airplane such as a B-47 you need to be right on speed wise to avoid a long float.  Drag devices are your friend.

They have to be forced to slow down....then the going down is automatic.

[DaveBB]

Ground effect increases lift.  Air is literally reflected off the ground and back onto the wing.  The only way it may decrease drag is by decreasing the angle of attack needed to maintain the required lift.

[pembquist]

Reflected off the ground? I don't understand what that means. I thought the big part of ground effect was that circulation is interrupted by the ground,(no wing tip vortices.)

[DaveBB]

http://youtu.be/xr8N0Z4Cl0U?t=2m33s

Here is where I got my information.

[colmbo]

Here's a definition on wiki:

When an aircraft is flying at an altitude that is approximately at or below the same distance as the aircraft's wingspan or helicopter's rotor diameter, there is, depending on airfoil and aircraft design, an often noticeable ground effect. This is caused primarily by the ground interrupting the wingtip vortices and downwash behind the wing. When a wing is flown very close to the ground, wingtip vortices are unable to form effectively due to the obstruction of the ground. The result is lower induced drag, which increases the speed and lift of the aircraft.

Primary effect is a reduction in drag.

[earl1937]

Reflected off the ground? I don't understand what that means. I thought the big part of ground effect was that circulation is interrupted by the ground,(no wing tip vortices.)

Don't want to disagree with anyone on this subject, but will tell of an experience I had one time in the B-29. One of the things about the "fowler" type flaps on the 29 is the ground effect is very noticeable on hot days, like down at McDill at Tampa in the summer time. We found that in order to shorten "float" time and roll out distance, just as we crossed the "numbers' and about 4 or 5 feet high, just bleed off about 10% and the old bird would settle right down on the runway. We had to go to Thule, Greenland on a training exercise one week, and on final approach, snow and ice on runway to some degree, but not totally covered, as we flared over the numbers, we didn't even have time to raise the flaps any as the bird set right down on the runway as soon as we reduced power all off.
Any paved runway is warmer than what I call normal and produces "heat" thermals, which affect ground effect on aircraft. In any cold weather climate which I flew in, seems as though we always touched down sooner than expected, so I would think the cool or cold runways doesn't produce as much ground effect, which makes me think the theory of small heat thermals affecting aircraft on landing might be the granddaddy of ground effect. I know that when you land on a grass or dirt runway, I have never noticed any ground effect.
Just bend over and look down a paved hi-way someday and you will see, even in winter time, small heat thermals rising from the surface.

[pembquist]

DaveBB I skimmed the video but couldn't find the part about reflections except in the graphic that showed the ground effect plane vs the hovercraft, can you tell me approximately where the explanation is in the video.

The graphic that I saw represented the wing trapping a cushion of air below it. I believe this only happens very very close to the surface and is not the cause of lift in an akranoplan.

I'm a little confused about the argument of lift vs drag in ground effect because they seem like two sides of the same coin. My understanding is that inhibiting circulation from the high pressure bottom of the wing to the low pressure top has the effect of virtually lengthening the wing span so that the wing can fly at a lower angle of attack for the same amount of lift thereby reducing the amount of drag for a given amount of lift. This is why they put those cool little winglets on airliners.

The way I understand ground effect vehicles is that they can only fly in ground effect because out of ground effect the wing has to be at the angle of attack at which it stalls.

I would say that a better way of thinking about the whole thing might be to describe a wing in ground effect as developing lift more efficiently than a wing out of ground effect. This means that there is more power available for acceleration and conversely that an airplane in ground effect does not slow down as fast as one out of it.

DaveBB I don't know if you have done much reading on aerodynamics so I apologize if I am stating the obvious but, most of the lift generated by a wing is done by the top of the wing not the bottom. This means among other things that you can hang all kinds of stuff below the wing and it still works really well.

[DaveBB]

It starts at 2:35 and last for 30 seconds.

"The wing in ground effect occurs when an aircraft flies low and distance between the wing and land is small.  The incoming air reflected by the wing hits the land and returns back.  Thus, an additional lifting power appears.  Such effect is most complete when the flight altitude is less than the wing's width."

The graphics literally show air being deflected by the wing, striking the ground, and bouncing back to the undersurface of the wing.

Also, why did you skim the video? I had it set to start at the presentation of the ground effect?  Did it not start at 2:35?

[pembquist]

Video started at the beginning title sequence. Looked at the bit you just pointed out and while I'm not an aerodynamicist it doesn't make any sense to me from the reading on subsonic aerodynamics that I have done. I just looked at the Wikipedia page on the ekranoplan and the first paragraph of the article basically reiterates my understanding with the exception of a late reference to a "cushion of air" which seems to contradict the beginning.

The graphic and description in the video just seem like a bad intuitive guess on the part of the film producers, maybe they didn't have the money to make an animation complicated enough to explain what's going on.

[earl1937]

http://youtu.be/xr8N0Z4Cl0U?t=2m33s

Here is where I got my information.

I am not sure if these guys who made this video know what they are talking about or not, but, Have you ever watched a wind tunnel test on a aircraft wing, using smoke trailers to record event? Sure doesn't look like the info in this video is supported by visual facts. Take a Cessna 172, high wing, and a Piper PA-28 Cherokee, low wing and I don't see where there are much difference, if any in touch down from power off, to impact with the runway! If they wing did in fact produce some sort of cushion of air, the PA-28 would float a lot longer on landings than a Cessna 172. They are both about the same weight, so that wouldn't have much effect if any. I still maintain that heat thermals, refected off the warm surface, is more responsible for ground effect. The amount of "float" in this game is not reflected in the RL.

[pembquist]

I don't know about the thermals but if you have an engine out and there is a ditch running through the field you've picked to land on, I can say with confidence that ground effect kicks into turbo mode.

[colmbo]

Thermals off the runway and ditches across a field are not ground effect.  Here is a pretty good definition.

The info that was in the video posted by davebb stated the effect was when within 1/2 chord width of the ground.  That close to a surface I can see where you could get some "cushioning" effect.  Practically you won't see that in aircraft because you just can't get that low.  The drag reduction seen when in ground effect occurs within one wingspan and is most pronounced within 1/2 wingspan of the surface.

Mooney aircraft have a reputation for floating on landing.  It is a clean design, low wing with short gear legs, it may be one airplane that can get within 1/2 chord width from the surface.

Along with floating on landing another place you can get into trouble if you don't understand ground effect is taking off, especially when hot/high.  Due to the decreased drag the aircraft may be able to lift off but is then unable to climb out of ground effect due to decreased performance from the density altitude.

Ground effect can be useful for getting off a short/soft field (without significant obstacles) because you can get in the air when very slow, then accelerate while in ground effect before starting a normal climb.  I've done this off gravel/sand bars -- flaps up as you begin roll to limit drag, pop FULL flaps and rotate when speed is right (on my 182 that was usually just when the airspeed indicator started to wiggle, well below normal book numbers).  Airplane comes off the ground, you don't try to climb you just fly along easing the flaps up and letting the airplane to accelerate to climb speed.

[curry1]

This is linked to 2:35 into the video it explains the ground effect quite well.  I recommend watching the entire video.  This series is truly excellent.

http://www.youtube.com/watch?v=xr8N0Z4Cl0U&t=2m35s

[pembquist]

I think there is confusion about what is meant by "ground effect".  If we are talking about aeroplanes it is mainly the reduction of induced drag. If we are talking about ground effect vehicles then we are probably talking about stagnation of air under the vehicle providing a high pressure cushion. I still don't think the reflections in the video are plausible.