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relationship of ground effect and wing chord

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relationship of ground effect and wing chord

Old 02-01-2003, 05:09 AM
  #1  
mikenlapaz
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Default relationship of ground effect and wing chord

Can any one direct me to or furnish info on the relationship between plane size (chord) and when that chord starts interacting with the ground surface.
Is their a factor that can be used to calculate when GE comes into play? If there is a relationship is it the same for landing and takeoff?
It seems that with a greater distance between the two it would lessen the landing 'float' of some models if the landing gear were 'taller'.
Thanks for the assist.
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Old 02-01-2003, 05:35 AM
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Default relationship of ground effect and wing chord

It is a rough rule that ground effect begins to have an effect at 1.5 times the wingspan of the aircraft. Im not sure what effect chord has.
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Old 02-01-2003, 06:47 AM
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Default relationship of ground effect and wing chord

I always heard it was in relation to wingspan too. Nothing about chord. 1.5 spans sounds about right. But personally I'd say it's only really obvious at one span and less.
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Old 01-03-2009, 02:21 AM
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Default RE: relationship of ground effect and wing chord

wingspan only eg wingspan 20feet ground effect starts at 20 feet
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Old 01-03-2009, 08:45 AM
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Default RE: relationship of ground effect and wing chord

At typical model speeds and sizes - I would go for one CHORD as a more accurate number
Why? low speed approaches show hardly any if any real ground effect.
speed has a HUGE effect on pressure build up.
Air ALWAYS takes the path of least resistance and in small sizes and low speeds there isn't much pressure build up.
The best ground effect setup has the most chord.
Almost like the Space Vehicles.
Funny that -
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Old 01-03-2009, 06:14 PM
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Default RE: relationship of ground effect and wing chord

Indeed, as Dick says, with models there is no real 'ground effect'. I have one model which I can detect a definite effect with when landing, but it's a 48" delta, with around a 36" centre chord, and a couple of inches off the ground when landing, and with an AOA unapproachable by 'normal' airplanes. Don't sweat it, just learn to land at your models 'stall + 2mph' and you too won't notice any 'effect'.
Evan, WB #12.
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Old 01-20-2009, 06:47 AM
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Default RE: relationship of ground effect and wing chord

There is no relationship between wing chord and ground effect. The relationship is between wingspan and aircraft speed. The ground effect area is within the wingspan of the aircraft. RC aircraft usually accelerate rapidly through the ground effect region on takeoffs. On landings if the engine idle is set to high, it’s possible to get into ground effect on landing and watch the plane “float” down the field into the tall grass. All planes (large and small) fly through ground effect. It’s a matter of how fast one flies through it that matters. The following is an excerpt from Airplane Flying Handbook FAA-H-8083-3A, Chapter 5 Takoffs and Departure Climbs.
GROUND EFFECT ON TAKEOFF
Ground effect is a condition of improved performance encountered when the airplane is operating very close to the ground. Ground effect can be detected and measured up to an altitude equal to one wingspan above the surface. [Figure 5-6] However, ground effect is most significant when the airplane (especially a low-wing airplane) is maintaining a constant attitude at low airspeed at low altitude (for example, during takeoff when the airplane lifts off and accelerates to climb speed, and during the landing flare before touchdown).
When the wing is under the influence of ground effect, there is a reduction in upwash, downwash, and wingtip vortices. As a result of the reduced wingtip vortices, induced drag is reduced. When the wing is at a height equal to one-fourth the span, the reduction in induced drag is about 25 percent, and when the wing is at a height equal to one-tenth the span, the reduction in induced drag is about 50 percent. At high speeds where parasite drag dominates, induced drag is a small part of the total drag. Consequently, the effects of ground effect are of greater concern during takeoff and landing.
On takeoff, the takeoff roll, lift-off, and the beginning of the initial climb are accomplished in the ground effect area. The ground effect causes local increases in static pressure, which cause the airspeed indicator and altimeter to indicate slightly less than they should, and usually results in the vertical speed indicator indicating a descent. As the airplane lifts off and climbs out of the ground effect area, however, the following will occur.

• The airplane will require an increase in angle of attack to maintain the same lift coefficient.
• The airplane will experience an increase in induced drag and thrust required.
• The airplane will experience a pitch-up tendency and will require less elevator travel because of an increase in downwash at the horizontal tail.
• The airplane will experience a reduction in static source pressure as it leaves the ground effect area and a corresponding increase in indicated airspeed.

Due to the reduced drag in ground effect, the airplane may seem to be able to take off below the recommended airspeed. However, as the airplane rises out of ground effect with an insufficient airspeed, initial climb performance may prove to be marginal because of the increased drag. Under conditions of high-density altitude, high temperature, and/or maximum gross weight, the airplane may be able to become airborne at an insufficient airspeed, but unable to climb out of ground effect. Consequently, the airplane may not be able to clear obstructions, or may settle back on the runway. The point to remember is that additional power is required to compensate for increases in drag that occur as an airplane leaves ground effect. But during an initial climb, the engine is already developing maximum power. The only alternative is to lower pitch attitude to gain additional airspeed, which will result in inevitable altitude loss. Therefore, under marginal conditions, it is important that the airplane takes off at the recommended speed that will provide adequate initial climb performance.
Ground effect is important to normal flight operations. If the runway is long enough, or if no obstacles exist, ground effect can be used to an advantage by using the reduced drag to improve initial acceleration. Additionally, the procedure for takeoff from unsatisfactory surfaces is to take as much weight on the wings as possible during the ground run, and to lift off with the aid of ground effect before true flying speed is attained. It is then necessary to reduce the angle of attack to attain normal airspeed before attempting to fly away from the ground effect area.

Frank
"Takeoffs are optional. Landings are mandatory"
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Old 01-20-2009, 07:37 AM
  #8  
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Default RE: relationship of ground effect and wing chord

The only models I've flown that show any hint of ground effect are low wingers. And I'm not sure the high wing loaded ones showed anything at all. My Tiger60 shows it. Sometimes. I think. Maybe.

It seems to happen at about halfspan or about 3XC (3 chords) above the ground. But the model doesn't pick up speed (from the theoretical loss of drag) and since that's the most emphasized symptom in the flight text books, I'm guessing it doesn't actually happen. What does, is that sometimes the plane seems to not settle. And that could be other things. I've never noticed the text books say the plane will float and not settle. Just from landing performance, I'd vote that we don't experience ground effect with our models. And I don't know what or if the text books say anything about ground effect on landing. Anybody got a quote from a text that mentions landing?

BTW, the text quoted mentions 25% and 1/10 span for the meaningful effects it chose to describe. I would guess they did that because their perceived symptoms don't show up significantly and usefully outside that range. 1/10 span is probably about the height most landing gears keep the wing away from the ground. And above 25% span, a smaller reduction that 25% in induced drag doesn't do much for you or to your airplane.

We don't have instruments. Nor can any of us accurately tell the difference in airspeed that will happen with such small reductions in induced drag. And our model's extremely light wing loadings (in comparison to full scale) predispose our models to have almost no induced drag to worry about compared to full scale. So I'm voting with the guys who say our models don't show ground effect. I think they might, but I'm convinced none of us could see what the texts say happens.
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Old 01-20-2009, 08:57 AM
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Default RE: relationship of ground effect and wing chord

The FAA exerpts are fine -if you are flying a Cessna 150 - or other underpowered overweight lightplane .
the relevance to our stuff tho -is HIGHLY speculative.
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Old 01-20-2009, 12:31 PM
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Default RE: relationship of ground effect and wing chord

The ground effect is present irrespective of the size of the aircraft. Problem is that very seldom do we fly low enough with our model aircraft to notice it. Ground effect depends on wing span as well as chord and in a somewhat simplified model the effect can be separated into a span-dominated part and a chord-dominated part.
The span-dominated part of the ground effect is associated with a decrease of the induced drag of the wing. What happens is that that the wing-tip vortices resulting from the pressure difference between the upper and lower surfaces are displaced outwards, away from the wing. This results in a decrease of the induced drag of the wing similar to what would happen if the aspect ratio of the wing is increased. In most cases the drag reduction is accompanied with an inrease of the slope of the Cl vs. alpha curve. Experimental as well as theoretical work has shown that as the wing approaches ground, this effects is gradually introduced. The effect becomes significant when the height above ground is less than approximately one wingspan. In order to obtain a 20% decrease in induced drag the height above ground of the wing must be appoximately 1/3 of the wingspan or less.
The chord-dominated part of the ground effect is associated with a change of the maximum lift coefficient of the wing. For flat bottomed airfoils or airfoils with some camber, the maximum Cl may increase somewhat if the flying altitude is small compared to the chord of the wing. I don't have figures at hand but I may be able to dig some experimetal figures out from my archive, given enough time. From memory I recall that for this effect to be readily noticed, the height of the wing above the ground should be less than one wing chord. If the airfoil has very little camber, i.e., approaches a symmetrical shape, and/or the angle of attack is small the maximum lift coefficient may be reduced. In these cases the wing lower surfce and the ground casues a venturi effect, reducing the the dynamic pressure on the lower surface of the wing.

Edit: Tried to improve the spelling and grammer somewhat.
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Old 01-20-2009, 01:27 PM
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Default RE: relationship of ground effect and wing chord

Da Rock,
I agree that as RC pilots ground effect is hard to perceive but is still there all the same. Try taking off at half throttle or something less. When you reach the point that you can fly within a wingspan of the ground but cannot fly higher without adding power you are in ground effect. What you describe “that sometimes the plane seems to not settle.” is due to ground effect. Re:
“If, during the landing phase of flight, the aircraft is brought into ground effect with a constant AOA, the aircraft experiences an increase in CL and a reduction in the thrust required, and a “floating” effect may occur. Because of the reduced drag and power-off deceleration in ground effect, any excess speed at the point of flare may incur a considerable “float” distance. As the aircraft nears the point of touchdown, ground effect is most realized at altitudes less than the wingspan.” Reference Pilot’s Handbook of Aeronautical Knowledge Handbook Chapter 4 Page 4-11.
[link]http://www.faa.gov/library/manuals/aviation/pilot_handbook/[/link]

Regards
Frank

"Takeoffs are optional. Landings are mandatory."
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Old 01-20-2009, 01:45 PM
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Default RE: relationship of ground effect and wing chord


ORIGINAL: dick Hanson

The FAA exerpts are fine -if you are flying a Cessna 150 - or other underpowered overweight lightplane .
the relevance to our stuff tho -is HIGHLY speculative.
There's nothing "speculative" about aerodynamics that can be voted away. I've flown "underpowered overweight" C172s in ground effect during soft field takeoffs and I've also flown scale 76" overpowered .60 J3 cubs and other scale planes in ground effect. The aerodynamics are the same. Its just how we get there that's different. As RC pilots the more one attempts more scale like takeoffs and landings the more you will come into contact with ground effect.

Regards
Frank
"Takeoffs are optional. Landings are mandatory"
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Old 01-20-2009, 01:47 PM
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Default RE: relationship of ground effect and wing chord

Well said Red B.

Regards
Frank

"Takeoffs are optional. Landings are mandatory."
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Old 01-20-2009, 04:57 PM
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Default RE: relationship of ground effect and wing chord


ORIGINAL: phlpsfrnk


ORIGINAL: dick Hanson

The FAA exerpts are fine -if you are flying a Cessna 150 - or other underpowered overweight lightplane .
the relevance to our stuff tho -is HIGHLY speculative.
There's nothing "speculative" about aerodynamics that can be voted away. I've flown "underpowered overweight" C172s in ground effect during soft field takeoffs and I've also flown scale 76" overpowered .60 J3 cubs and other scale planes in ground effect. The aerodynamics are the same. Its just how we get there that's different. As RC pilots the more one attempts more scale like takeoffs and landings the more you will come into contact with ground effect.

Regards
Frank
"Takeoffs are optional. Landings are mandatory"
Frank I can't buy it .
I have too much flying time in models ranging from 5 ounces to 45 pounds .
The largest ones ar still so smal that the change in pressure beneath the model is "minute " to say the least
Now take a 767- those wings
capture a lot of air and you don't need any education in aerodynamics to see the huge plumes of dust /vapor moving under and aft the wings
Mother nature simply takes th easiest path and IF the wing is big enough and travelling fast enough there is a definite change in pressure beneath
but little stuff really can't restrict the air movement and make pressure differences like large stuff can
sure "theory"is fine but unles it cn be practically applied - it is worthless.
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Old 01-20-2009, 05:04 PM
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Default RE: relationship of ground effect and wing chord


ORIGINAL: phlpsfrnk

Da Rock,
I agree that as RC pilots ground effect is hard to perceive but is still there all the same. Try taking off at half throttle or something less.
Never said it wasn't there. Said that with our models what it does isn't worth worrying about.

And I start all my takeoffs way under half throttle. The airplane is flying way before there would be the possibility to get it airborne too soon as described in the Primary Flight Training manual excerpt. Our models simply have powerloadings and wingloadings that create a flight envelope that's nowhere near full scale.

Like I said, our models don't show it enough to see or for it to matter. And I'm betting that only very few very large low wingers actually experience anything close to significant.
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Old 01-20-2009, 05:41 PM
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Default RE: relationship of ground effect and wing chord


ORIGINAL: phlpsfrnk


ORIGINAL: dick Hanson

The FAA exerpts are fine -if you are flying a Cessna 150 - or other underpowered overweight lightplane .
the relevance to our stuff tho -is HIGHLY speculative.
There's nothing "speculative" about aerodynamics that can be voted away. I've flown "underpowered overweight" C172s in ground effect during soft field takeoffs and I've also flown scale 76" overpowered .60 J3 cubs and other scale planes in ground effect. The aerodynamics are the same. Its just how we get there that's different. As RC pilots the more one attempts more scale like takeoffs and landings the more you will come into contact with ground effect.

Regards
Frank
"Takeoffs are optional. Landings are mandatory"
Frank I can't buy it .
I have too much flying time in models ranging from 5 ounces to 45 pounds .
The largest ones ar still so smal that the change in pressure beneath the model is "minute " to say the least
Now take a 767- those wings capture a lot of air and you don't need any education in aerodynamics to see the huge plumes of dust /vapor moving under and aft the wings (pressure differentials)
Mother nature simply takes the easiest path and IF the wing is big enough and travelling fast enough there is a definite change in pressure beneath
but little stuff really can't restrict the air movement and make pressure differences -as large stuff can
sure "theory"is fine but unless it can be practically applied - it is worthless.
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Old 01-21-2009, 02:59 AM
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Default RE: relationship of ground effect and wing chord

It can be pretty comical to see a low time Q-500 pilot get caught in ground effect and watch the dead stick plane maintain a 6 foot altitude for more than a quarter mile past the intended landing area. No way do these planes have such a flat glide by any other means..
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Old 01-21-2009, 08:43 AM
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Default RE: relationship of ground effect and wing chord


OK guys,
To answer Mike's post point by point.
ORIGINAL: mikenlapaz

Can any one direct me to or furnish info on the relationship between plane size (chord) and when that chord starts interacting with the ground surface.
Is their a factor that can be used to calculate when GE comes into play? If there is a relationship is it the same for landing and takeoff?
It seems that with a greater distance between the two it would lessen the landing 'float' of some models if the landing gear were 'taller'.
Thanks for the assist.
mikenlapaz
"Can any one direct me to or furnish info on the relationship between plane size (chord) and when that chord starts interacting with the ground surface." The relationship is more wing span than cord as described by Red B.'s post and its one wingspan.
"Is their a factor that can be used to calculate when GE comes into play?" Not sure this answers the question but speed to maintain flight in ground effect is less than the normal wing stall speed. There's probably a formula for calculating that speed but I don't know it.
"If there is a relationship is it the same for landing and takeoff?" Yes!
"It seems that with a greater distance between the two it would lessen the landing 'float' of some models if the landing gear were 'taller'." Your statement is true.

For dick & da Rock,
I'm not trying to have a major debate here but here's my opinion. If you have ever taken off from wet tall grass and kept the nose down after rotation to gain speed, you have used and flown through ground effect. If you have ever attempted a landing at near stall speed and watched the plane "float" the entire length of the field to the tall grass, you have experienced ground effect. You do not have to worry about ground effect if you can launch your plane vertically and accelerate in a climb. You do not need to worry about ground effect if you fly your plane onto the ground at some speed far greater than the stall speed of the wing. Is a 40% Extra 300, 60% less affected by ground effect? At what scale can we disregard the effects of this aerodynamic fact of life? Has understanding this made me a better pilot? I'd like to think so.

Regards
Frank
"Takeoffs are optional. Landings are mandatory"
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Old 01-21-2009, 10:02 AM
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Default RE: relationship of ground effect and wing chord

Unless the craft can cause a pressure change under and ahead of it's self
what is it riding on?
a ground effect has to be SOMETHING which contributes to bouyancy -if not what is it?
Proper use of speed and rate of climb is another issue
"the speed needle has to increase faster than the rate of climb needle or you quickly return to starting point. I disregard nothing but also don't accept anything I can't proove.
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Old 01-21-2009, 10:44 PM
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Default RE: relationship of ground effect and wing chord

I'm certainly not having a debate here. Just sharing experiences and observations.

For example, there is testimony of a Q500 model floating on ground effect at 6' altitude for over a quarter mile. Here is an observation and opinion of that: The texts say it's effects are significant a heck of a lot closer to the ground than 6' for a model like the Q500s. So that sucker ain't even close to being in effective ground effect at roughly 1.5X it's wingspan above the ground. Since it's a racing plane with a highpitch, small diameter prop and a hot engine, my opinion of the cause differs greatly. That Q500 pilot had the idle too high. If ground effect could hold off a model plane about twice it's wingspan above the runway, it'd be so effective once the sucker actually got within half a wingspan, the sucker couldn't get through that cusion. And our models would never touch down. We'd have to walk out and pick 'em out of the air.

In my experience, every modeler who ever asked me to help them solve the "floater" problem got help that included adjusting their idle or their prop selection. Lots of them had lousy lowspeed needle settings. Probably three quarters of them had high idles for whatever reason. The rest were running high pitch props. Once we got the engine/prop at idle to quit pulling the airplane along at flying speed the ground effect no longer held the airplane off.

You know, there is more than one basic truism in full scale piloting. There is also the rule about controlling your altitude with the throttle. There is no mention about that rule no longer applying in ground effect.
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Old 01-22-2009, 02:49 AM
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Default RE: relationship of ground effect and wing chord

Da-Rock, NPRA pylon planes land dead stick, the engines are killed at a safe distance after the heat is over. Their glide paths at altitude look like what you might expect from a 3.5 pound plane with 500 sq inches, it's pretty good but not "glider-like". Once they get a certain distance from the ground, if the pilot hasn't taken measures to bleed off enough speed, then the glide does appear to become "glider-like". It could be an illusion, but it is an illusion shared by many.
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Old 01-22-2009, 03:04 AM
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Default RE: relationship of ground effect and wing chord

Yep, I've seen it and experienced it myself. Had a Q500 with a .25 on the nose as a sport model. If I didn't take care to keep the model nose high so it was keeping the wing operating at a more draggy and higher lift coefficient the darn thing just would not come down on it's own. And rather than actually drive it into the ground I would just give up, power up and go around. I got to be pretty good at holding it comfortably near a stall. When flown that way it would settle in pretty nice.

But I don't consider that ground effect so much as it's poor glide path control where the pilot comes in with too much airspeed for such a relatively lightly loaded plane and with the low drag airframe it just takes the full length of the field to bleed off the extra speed. CP, I'll bet a lot of the overshoots look like they want to land by the end of the runway but by that time there's not enough left and it just goes into the weeds.
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Old 01-22-2009, 07:13 AM
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Default RE: relationship of ground effect and wing chord

Yeah, add poor glide path control to the list. I was going to suggest pilot skill (basically the same thing from a different viewpoint) but felt it was too obvious. In fact, poor glide path control isn't always pilot skill. Sometimes inadequately balanced/trimmed planes won't do what's needed. Or can't do it.

But that's gawdawful bodaceous ground effect if it's working at 1.5X the wingspan to float the model. Methinks adding better elevator effectiveness to the list of solutions would be suggested for those deadstick Q500s that are caught by that amazingly strong ground effect while none of the other pilots are. They of course proved to have used their elevators to establish a pitch that actually slows the plane in it's lowspeed end of the envelope. And their elevators had sufficient deflection to work for them. I'd suggest those Q500s that floated over the horizon at 6' altitude need some trimming. CG and elevator deflection would be worth a look.
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Old 01-22-2009, 12:48 PM
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Default RE: relationship of ground effect and wing chord

Watch the Shuttle clear all the sand off the runway just before it touches down!
And the similar fiddle-shaped SR-71 is a known floater.
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Old 01-22-2009, 04:12 PM
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Default RE: relationship of ground effect and wing chord

The methods and skills that are needed to land these planes at your feet are common knowledge and coached by the experienced hands. Even the most experienced, expert pilots will have an occasional landing SNAFU. I don't think the force that prevents 2 objects from slapping sides in water or air due to a pressure rise is the sole force that prolongs a Q-500s glide.....I think rising air from the hot tarmac that we typically fly over might be more intense [less diluted] at ground level. If so, this is still a ground effect, just not what the doubters here have considered.
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