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Cessna v. Piper trainers

Old 07-27-2009, 01:04 PM
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I’ve instructed for 7 years, have over 1700 hrs of instruction given in the Cessna and about 2400 hrs in Piper’s. My experiences from watching students in the aircraft are as follows

Pros Cessna’s

Two Doors
Slightly easier to learn to land
Spot landings are a cinch
Landing gear is more forgiving
Better Ventilation
Good Sightseeing airplane
More stable (easier in bumpy IMC)
Mixture control
Bigger rudder, more control on cross wind landings
Real slips
A descent stall break

Con’s Cessna’s

Fueling (drag that ladder out)
De-Icing
Cheaper feel to A/C
Reliability issues
Cessna ARC radios (ughhhh)
Carb icing (at least you learn about it)

Pro’s Piper

Nicer looking
Throttle quadrant feels and looks more jet-ish
Better reliability
Sturdier landing gear (more leeway with student’s oops)
Better heater
Commercial maneuvers are just plain fun in a low-wing A/C
Ground Effect = If you botched a power off 180 and are going to be short you can dive it into ground effect and float it to your point.

Con’s Piper

Single Door
Ventilation
Climbing over the wing
Pathetic rudder
Students get sloppy about crosswind inputs during taxing.
Updraft carburetor (very little carb icing so you get lazy about carb heat)
Updraft carburetor – improper starting procedure can lead to fires.
Ground Effect – Pipers will float a long time if you come in fast, they don’t just settle like a Cessna will.

***NOTE: There is some misinformation about ground effect posted here.

When an airplane’s wing is flown close to the ground, wingtip vortices's are unable to form very well. This results in a huge reduction of induced drag and a slight increase in the speed of the air-flow over the outer part of the wing, which results in a slightly MORE lift. However it is the reduction of induced drag that causes the low wing A/C to float and float and float down the runway if excessive speed is carried into the flare.

Ground Effect
Ground effect in aircraft - Wikipedia, the free encyclopedia

My personal advice is to fly both aircraft and pick the one that fits you better. If your school only has one or the other, don’t worry about it. They are both fine trainers and good A/C to learn in.

As a side note, I find that piper made it extremely easy to trade up to a twin. When you look at a Seminole it’s nothing more then an Arrow with 2 engines, all the controls are familiar and transition time is usually quicker for the student.
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Old 07-27-2009, 03:04 PM
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Originally Posted by 3664shaken View Post

***NOTE: There is some misinformation about ground effect posted here.

When an airplane’s wing is flown close to the ground, wingtip vortices's are unable to form very well. This results in a huge reduction of induced drag...
Correct. But how it does it you are not quite clear on, because you claim there is an increase in lift, not so. There is a reduction in lift while in ground effect. This is why low wing airplanes tend not to bounce, although they do tend to float.

... and a slight increase in the speed of the air-flow over the outer part of the wing, which results in a slightly MORE lift...
Not at all correct... first of all, have you ever noticed that designers prefer a high-wing configuration for short field and STOL airplanes? Why, if there is so much more lift in ground effect would they do this? The reason is along with a reduction in drag, ground effect reduces lift. It's all part of the same principle which is known in academics as circulation. Circulation around a wing is one of the key elements responsible for creating lift.

Don't take it from me, try this: Denker on Ground Effect.

--------------------------------
3.12.5 Bound Vortex


Let’s not forget about the bound vortex, which runs spanwise from wingtip to wingtip, as shown in figure 3.27. When you are flying in ground effect, you are influenced by the mirror image of your bound vortex. Specifically, the flow circulating around the mirror-image bound vortex will reduce the airflow over your wing. I call this a pseudo-tailwind.17 Operationally, this means that for any given angle of attack, you need a higher true airspeed to support the weight of the airplane. This in turn means that a low-wing airplane will need a longer runway than the corresponding high-wing airplane, other things being equal. It also means – in theory – that there are tradeoffs involved during a soft-field takeoff: you want to be sufficiently deep in ground effect to reduce induced drag, but not so deep that your speeds are unduly increased. In practice, though, feel free to fly as low as you want during a soft-field takeoff, since in an ordinary-shaped airplane the bad effect of the reflected bound vortex (greater speed) never outweighs the good effect of the reflected trailing vortices (lesser drag). As a less-precise way of saying things, you could say that to compensate for ground effect, at any given true airspeed, you need more coefficient of lift. This explains why all airplanes – some more so than others – exhibit “squirrely” behavior when flying near the ground, including:
  • Immediately after liftoff, the airplane may seem to leap up a few feet, as you climb out of the pseudo-tailwind. This is generally a good thing, because when you become airborne you generally want to stay airborne.
  • Conversely, on landing, the airplane may seem to drop suddenly, as the pseudo-tailwind takes effect. This is unhelpful, but it’s not really a big problem once you learn to anticipate it. It does mean that practicing flaring at altitude (as discussed in section 12.11.3) will never entirely prepare you for real landings.
  • The wing and the tail will be influenced by ground effect to different degrees. (This is particularly pronounced if your airplane has a low wing and a high T-tail, but no airplane is entirely immune.) That means that when you enter or exit ground effect, there will be squirrely pitch-trim changes ... in addition to the effects mentioned in the previous items. Just to rub salt in the wound, the behavior will be different from flight to flight, depending on how the aircraft is loaded, i.e. depending on whether the center of mass is near the forward limit or the aft limit.
During landing, ground effect is a lose/lose/lose proposition. You regret greater speed, you regret lesser drag, and you regret squirrley handling.
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Old 07-27-2009, 04:46 PM
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A few more comments. I spend a lot of time thinking about aerodynamics.

1. The AvWeb article by Ms. Pendleton is a pretty good article in-as-much it is designed to be, but it does not get very deeply into the subject. A thorough treatment of ground effect would have to mention circulation which it does not. Also, I do not see in the article where she claims that ground effect increases lift. She says quite accurately that it decreases drag.

2. This Wikpedia article does not have any source citations and it also is a poorly written article. One of the moderators at at Wikipedia even tagged it as such. Bear in mind, just because something is posted to Wikipedia implies nothing about its accuracy, correctness, authenticity, or scholarship. Anyone can go and put their two cents worth on a Wikpedia entry and the only thing that will happen is the moderators will tag it as problematic in some way. That's fine, it's just a wiki article, which is a community based free-access knowledge-sharing forum. But we as readers need to read the source citations and see who wrote the articles. Are they scholars on their subject? And what are their qualifications?

In this case a non-authority wrote the article, the article is very poorly researched, is full of misconceptions, and has no citations. Even so, I do not see where they claim that ground effect increases lift. He says ground effect reduces drag (correct) and as an indirect result the airplane skims along faster over the ground, and this increases the lift as faster speed affects the lift equation. The author does not say the extra lift is directly connected to ground effect in any aerodynamic way. The exact opposite is indeed the case: ground effect nullifies lift to some extent by slowing circulation around the wing. Less circulation = less lift AND less drag because it is what causes both. See the Denker website listed above for a fairly thorough treatment suitable for non-engineers. Another good read for non-engineers is Aerodynamics for Naval Aviators.

The source of confusion on this is that effective angle of attack increases when the downwash vector is removed by the action of ground effect. This makes the wing seem to produce more lift than it was, by giving back the amount of kinetic energy that was being wasted in forming the wingtip vortices. This occurs when the induced drag is removed by removing the downwash behind the wing. Many textbooks just stop at this point and say "the wing produces more lift in ground effect" which is not really true. Out of ground effect the wing was producing more lift, but there was a huge loss in kinetic energy to the vortices felt by the wing through downwash. One thing to remember there is that wingtip vortices are not isolated to the ends of the wings, they are an accumulated result of vortex action across the entire wing.

Look at it this way. If I am paying 25% of my paycheck as taxes then I get a tax break, I am not really making more money, I have just stopped losing so much money to taxes. In the case of ground effect I actually am making less money overall, but getting a lot more of it back in the form of tax breaks so you could say I am making more money, but this is not really true. Hope this helps.

Last edited by Cubdriver; 07-27-2009 at 09:24 PM.
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Old 07-28-2009, 04:32 PM
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Cubdriver,

You are confusing and conflating several issues, trying to use them to back up the false belief that lift decreases in ground effect. (I’ll address the secondary issues in my next post, but let’s hit the main one here.)

Your second replies seems to indicate that you wish to dispute sources and argue from what you feel is an authoritative source. No problem there, I only had a netbook and the internet but a full Google search will reveal the numerous other cites that you can visit that agree with me on this. If that is not authoritative enough, I am home now and I looked up some more sources for you.

I suggest the following books that all explain why lift increases in ground effect.

Fundamentals of Aerodynamics – John Anderson
The Illustrated guide to Aerodynamics – H. C. “Skip” Smith
Low Speed Aerodynamics – Joesph Katz & Allen Plotkin
An Introduction to Aircraft Performance – Mario Asselin
Theory of Wing Section – Ira Abbott & A. E. von Doenhoff

You suggested Aerodynamics for Naval Aviators

An excellent book that also agrees that lift increases in ground effect.

If you still don’t believe that the Navy knows then read the NASA report about F-15’s and increased lift in ground effect.

http://www.nasa.gov/centers/dryden/pdf/88337main_H-1999.pdf

Now it is true that NASA hasn’t tested pipers or Cessna, but they did test low-wing transport jets and they also saw an increase in lift in ground effect.

http://www.nasa.gov/centers/dryden/pdf/88010main_H-1273.pdf

Of course that is what the physics and computer models all predicted and has been known for quite some time as the reports allude to.

This is also a good link that explains it very well with some good diagrams and pictures.

http://www.se-technology.com/wig/html/main.php?open=aero

From An Illustrated Guide to Aerodynamics

“Not all of the floating tendency can be attributed to drag. A reduction in downwash also changes the orientation of the relative airstream so that the effective angle of attack increases. This is an increase in angle of attack resulting from changing the direction of the airstream rather than the direction of the chordline, as we normally think of it. The effect is the same because angle of attack is the angle between the airstream and the chordline. Increased angle of attack leads to increased lift; therefore, even thought it is slight, an additional amount of lift is experienced in ground effect as well as decreased drag. Both effects, of course, act to keep the airplane flying.”

From Gliem CFI Toolbox:
http://www.cfitoolbox.com/cfitoolbox_pp_example.pps#10

“The reduction of the wingtip vortices alters the spanwise lift distribution and reduces the induced angle of attack and induced drag. Thus the wing will require a lower angle of attack in ground effect to produce the same lift coefficient, or, if a constant angle of attack is maintained, an increase in the lift coefficient will result.”

(wow even Gliem got it right!)

From An Introduction to Aircraft Performance

“As the aircraft flies closer and closer to the ground, the lift coefficient increases for a given AoA. . .
Another important effect for an aircraft flying IGE is a large reduction of induced drag. The increase in the lift coefficient and the decrease in drag coefficient, for a given AoA, translate into an increase in lift-to-drag ration. This drag reduction will reduce the aircraft’s rate of deceleration (and maybe even increase the velocity if the drag becomes smaller then the thrust available). Combined with the increased lift coefficient, the aircraft will tend to float and refuse to touchdown. . . .”

From the NASA report:

For the F-15 aircraft, the change in the lift coefficient because of ground effect doubled from approximately 0.05 to 0.10 as the sink rate decreased from approximately 6.5 to 0.7 ft/sec.”

As far as John Denker goes, he is highly controversial and I would not use him as a primary source. I think the vast amount of literature by noted authors, educators, scientists, NASA, and a plethora of other sources in aerodynamics is overwhelming in this case.

Could they all be wrong? Maybe but the empirical data that NASA has along with all of the computer models and wind tunnel tests all seem to say the same thing. I’ll put my money and my beliefs behind those guys instead of John Denker.
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Old 07-28-2009, 05:29 PM
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Originally Posted by Cubdriver View Post
This is why low wing airplanes tend not to bounce, although they do tend to float.


Hmmmm, I have seen many bounced landings in low wing A/C, including the worst one ever in a Extra 300L.

There are many reasons an airplane bounces upon landing.

Too much airspeed.
Improper elevator application by the pilot.

Some landing gear is more prone to bounces then others. Piper uses oleo struts which dampen botched landings and result in less bounces. Cessna flat spring steel gear legs are particularly bounce prone while the tubular steel gear legs are less, they are still more bounce prone then oleo struts.

The other reason Cessna's may bounce more is the approach profile that is flown by them vs the Piper's approach.

In the Piper most CFI's instruct flying into ground effect, leveling off and then increasing attitude as the plane begins to fall out. The drawback with this method is nose gear first touchdowns in the beginning.

With Cessna's a round out is usually started at about 20 feet and when you enter ground effect you are already pitching for the flare. The drawback is if you are not managing speed and sink rate a higher than usual sink rate may occur, couple that with springy landing gear and the tendency for the student to pull back even harder and viola you have more bounces.

There is also one more reason why a low wing airplane tends to settle down and stick and this is due to their design and ground effect. Downwash normally reduces the angle at which the airstream hits the horizontal stabilator, when the airplane enters ground effect, downwash is reduced. This reduction in downwash increases the angle and gives the stabilator a higher (or less negative) AoA. This result is a small nose down pitching moment that decreases lift and gives the feeling of the low wing A/C sticking onto the runway. Of course the obvious resultant minor loss of lift also decreases the chances of a bounce.

Originally Posted by Cubdriver View Post
Not at all correct... first of all, have you ever noticed that designers prefer a high-wing configuration for short field and STOL airplanes? Why, if there is so much more lift in ground effect would they do this?


There are a plethora of reasons that they choose high-wing aircraft but none of them have to do with ground effect, although there is one benefit but it is incidental.

First of all back-country A/C are conventional gear, low wing conventional gear would be suspect to hitting, rocks, shrubs, etc. Ever seen the flaps of a Piper, Mooney, Beech, etc, that has been landed on dirt strips, they are dinged up, hence you want/need your wings up high.

#2 the wing of a high -wing A/C is a complete lifting surface, from tip to tip therefore you can have a smaller wing for equal amount of lift, highly important in small spaces

#3 Forward and Down visibility is extremely important while scouting out the landing strip, nailing the landing and then taxing afterwards.

There are many more reasons, but ground effect is not one of them.

However, one good thing about a high wing A/C is that if they are overloaded they usually don't fly until they are able to. We have all seen videos or heard of stories about low-wing A/C struggling to get out of ground-effect when overloaded, or in high density altitude situations.

http://www.youtube.com/watch?v=gOaAZ1i2gNA

I could only imagine more of these incidents happening on backcountry strips if low-wing A/C were flown into them.
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Old 07-28-2009, 05:46 PM
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The mathematical side:

Cl = L / qA

Cl = Coefficient of lift
L = lift force
q = dynamic pressure
A = wing area

Dynamic pressure is a function of fluid density (air which will remain unchanged) and velocity of that fluid (also will not change).

Lift force in its plain form is 1/2 (p V2 A Cl)

p = air density
V = velocity

Well again air density won't change and velocity won't ? If non of these go up, how does Cl go up when in ground effect?

These two formulas, often given as the be all end all in production of lift, leave one thinking lift cannot go up in ground effect. They leave out advanced lift ideas like, circular flow and pressure changes from the lack of suction of tip vortices, out of the picture (as they well should for pilots).

These all have a small effect which increases the lifting force when in ground effect, causing an increase in lift coefficient. My apologies, I am not an engineer so I do not know how to apply all the formulas for a demonstration. Point is, Cl does go up in ground effect which increases lift.


Edit: Cubdriver, there are truths in what you posted, the aircraft exhibits a flow pattern that acts as a form of suction pulling the aircraft downward. I however don't understand how it works and that article doesn't help much. It was just in my flight dynamics notes, noted as advanced airflow. Anyone have more insight?

Last edited by shdw; 07-28-2009 at 05:57 PM.
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Old 07-28-2009, 07:05 PM
  #37  
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Originally Posted by 3664shaken View Post
...I suggest the following books that all explain why lift increases in ground effect.

Fundamentals of Aerodynamics – John Anderson
The Illustrated guide to Aerodynamics – H. C. “Skip” Smith
Low Speed Aerodynamics – Joesph Katz & Allen Plotkin
An Introduction to Aircraft Performance – Mario Asselin
Theory of Wing Section – Ira Abbott & A. E. von Doenhoff...[/COLOR]

Fundamentals of Aerodynamics by John Anderson resides on my bookshelf and I used it extensively for aerodynamics in college. I used to borrow Theory of Wing Sections by Ira Abbott & A. E. von Doenhoff from a professor, and as a classic work on the subject it
predates high-speed aerodynamics but it is useful. As for the other ones, I have equivalents: Low Speed Aerodynamics by Joesph Katz & Allen Plotkin is equally treated in Aerodynamics for Engineers by John J. Bertin. I personally studied under one of the contributors to the latter, a Mr. Robert J. Englar. An Introduction to Aircraft Performance by Mario Asselin is equally treated in Aircraft Performance and Design by John D. Anderson. I always like Anderson's writing, he is a very good writer in addition to a leading scholar. He is getting on in years now, so we may lose him at any time. These are all great books. Do you have an AE degree? Seems like maybe you do having a book list like that. Engineering texts are pretty expensive.

So we both have good books. But you are not reading them very well on this particular subject. You missed the section on circulation when it explains that in ground effect the wing vortices “see” a mirror image of themselves underground and the effect is felt all the way up to the wing. It is a mathematical fact although obviously there is nothing going on under the ground. The net effect is a reduction in downwash and an increase in angle of attack in ground effect. I never said otherwise- by the time I wrote my last post I was no longer saying without qualification that a wing produces less lift in ground effect, I was saying that when you account for the change in effective angle of attack caused by ground effect there is no increase in lift. I also extended it to an explanation of why low wing airplanes settle faster, but I admit I do not have any data on that it was just my practical experience. The theory is correct, however.

I read your links by NASA article and SE Technology and they do not contradict this in the least. They simply do not mention that the extra lift is from a change in effective angle of attack and that effective angle of attack is what counts bar none. You want me to contradict all those sources but they are not speaking to this issue at all. If you reduce the
angle of attack the amount the reduction in downwash does to decreases it you have a wing that is producing less lift.

And why do STOL airplanes hate to use a low-wing configuration?
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Old 07-28-2009, 07:31 PM
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Originally Posted by shdw View Post
The mathematical side:

Cl = L / qA

Cl = Coefficient of lift
L = lift force
q = dynamic pressure
A = wing area

Dynamic pressure is a function of fluid density (air which will remain unchanged) and velocity of that fluid (also will not change).

Lift force in its plain form is 1/2 (p V2 A Cl)

p = air density
V = velocity

Well again air density won't change and velocity won't ? If non of these go up, how does Cl go up when in ground effect?

These two formulas, often given as the be all end all in production of lift, leave one thinking lift cannot go up in ground effect. They leave out advanced lift ideas like, circular flow and pressure changes from the lack of suction of tip vortices, out of the picture (as they well should for pilots).

These all have a small effect which increases the lifting force when in ground effect, causing an increase in lift coefficient. My apologies, I am not an engineer so I do not know how to apply all the formulas for a demonstration. Point is, Cl does go up in ground effect which increases lift.


Edit: Cubdriver, there are truths in what you posted, the aircraft exhibits a flow pattern that acts as a form of suction pulling the aircraft downward. I however don't understand how it works and that article doesn't help much. It was just in my flight dynamics notes, noted as advanced airflow. Anyone have more insight?


Out of ground effect the wing "sees" Vr, which is the air coming at it shifted downwards by the "w". "w" is caused by the downwash behind the wing. In ground effect however, this "w" is removed and the angle of the airstream the wing "sees" is much closer to "Vo". It is a simple geometric phenomenon. Notice that when you go from air at angle Vr to angle of Vo, you have a much greater angle of attack. This is what causes the extra lift we all know from ground effect.

To extend the theory a little farther- downwash is the result of circulation around the wing. Wingtip vortices and circulation are one and the same thing. Ground effect interrupts this effect and there is less circulation. If there is less circulation, then there is less drag and there is less total lift. I did not say there was less net lift.

To revisit an analogy I used earlier- if you are being taxed less you cannot really say that you make any more money. You are simply getting more of what you already made. Same thing here- the wing is not making any more lift, it is actually making a bit less. But it is losing a lot less to downwash behind the wing. You can say it is making more lift but this is not really true. It has simply stopped losing so much kinetic energy to wingtip vortices. The net lift is greater although the total lift is less.

Last edited by Cubdriver; 07-28-2009 at 07:57 PM.
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Old 07-28-2009, 08:39 PM
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Originally Posted by cubdriver
angle of the airstream the wing "sees"
Your mistaken, the resultant vector cannot be confused with the point where the force is applied. The wing doesn't see anything, just like the airplane doesn't feel anything. The wings AOA does not change from the resultant vector, it is based on the mean point where the force (relative wind) is applied.


Originally Posted by Cubdriver View Post
Wingtip vortices and circulation are one and the same thing. Ground effect interrupts this effect and there is less circulation..
Downwash is reduced, down wash is not the be all end all in flow patterns/circulation. Look up Coanda and Magnus effects, there are many advanced flow patterns happening around your wing.


If you want, here is another source.




Figure 8: Ground effect changes drag and lift.

Source: Google Image Result for http://www.free-online-private-pilot-ground-school.com/images/ground_effect_drag_lift.gif

Sorry bud, even the random internet google searches are right here.
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Old 07-28-2009, 08:42 PM
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Originally Posted by Cubdriver View Post
Fundamentals of Aerodynamics by John Anderson resides on my bookshelf and I used it extensively for aerodynamics in college. I used to borrow Theory of Wing Sections by Ira Abbott & A. E. von Doenhoff from a professor, and as a classic work on the subject it predates high-speed aerodynamics but it is useful. As for the other ones, I have equivalents: Low Speed Aerodynamics by Joesph Katz & Allen Plotkin is equally treated in Aerodynamics for Engineers by John J. Bertin. I personally studied under one of the contributors to the latter, a Mr. Robert J. Englar. An Introduction to Aircraft Performance by Mario Asselin is equally treated in Aircraft Performance and Design by John D. Anderson. I always like Anderson's writing, he is a very good writer in addition to a leading scholar. He is getting on in years now, so we may lose him at any time. These are all great books. Do you have an AE degree? Seems like maybe you do having a book list like that. Engineering texts are pretty expensive.

So we both have good books. But you are not reading them very well on this particular subject. You missed the section on circulation when it explains that in ground effect the wing vortices “see” a mirror image of themselves underground and the effect is felt all the way up to the wing. It is a mathematical fact although obviously there is nothing going on under the ground. The net effect is a reduction in downwash and an increase in angle of attack in ground effect. I never said otherwise- by the time I wrote my last post I was no longer saying without qualification that a wing produces less lift in ground effect, I was saying that when you account for the change in effective angle of attack caused by ground effect there is no increase in lift. I also extended it to an explanation of why low wing airplanes settle faster, but I admit I do not have any data on that it was just my practical experience. The theory is correct, however.

I read your links by NASA article and SE Technology and they do not contradict this in the least. They simply do not mention that the extra lift is from a change in effective angle of attack and that effective angle of attack is what counts bar none. You want me to contradict all those sources but they are not speaking to this issue at all. If you reduce the angle of attack the amount the reduction in downwash does to decreases it you have a wing that is producing less lift.

And why do STOL airplanes hate to use a low-wing configuration?

Figured I throw in my 2 cents of worthless currency......

Just to get everyone on the same track here (for us dumber folks)...

A wing in the ground effect behaves like a wing with a greater aspect ratio - because of the reduced vortices. There is a lower induced velocity (=downwash) associated with this as well, leading to a lower induced AoA (Ai) and lower induced drag coefficient (C/di).

- If a constant AoA is maintained prior to and into the ground effect, the lift coefficient will increase.
- Specific thrust required is reduced in the ground effect


From my limited understanding - it isn't that total lift increases, it is that the C/L-AOA curve shifts to the left (and sharper) in the ground effect... basically you encounter higher lift conditions at a lower AoA, as the C/Lmax shifts to a lower alpha.... the stall shift to a lower alpha as well.

C/Lmax in not increased, however C/L is increased sharper to C/Lmax


The reduction in downwash in what gives the increase in C/Lmax... however there is less energy in the flow patterns giving the critical AoA shift.

Last edited by ryan1234; 07-28-2009 at 09:33 PM.
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