Explaining Ground Effect

[uwochris]

Hey guys,

I am looking for some advice on the best way to explain ground effect to a new student. (i.e. why induced drag decreases, why stability is impacted, etc).

Any tips or advice would be great. I would like to be able to explain it in a simple manner; however, I do not know an easy way to describe it.

Thanks in advance,

Chris.

 

[falcon20driver]

http://www.dauntless-soft.com/PRODUCTS/Freebies/Library/index-flighttraining.html

Click the chapter 17 link and there is a ground effect page to click on.

 

[Typhoon1244]

Tell them it has to do with El Nino.

 

[chawbein]

Magic.

 

[ThomasMore]

See How It Flies, by John Denker

 

[jasonwb]

ground effect

falcon20driver is right . that flight training handbook is awesome.
if you dont have it try to find one it is very helpful.

when i would explain ground effect i would do all of the little rules about 1/2 the wingspan, bigger the craft the bigger the ground effect, therefore the bigger they are the easier they are to land. and all of that good stuff.
i have seen a lot of instructors leave out using ground effect in the takeoff phase to really let the student feel it try to push it
away from the runway and make them hold it in ground effect.
it is a weird thing but it is very useful for take offs and landings.

in the classroom i would take a small pillow and set it on a table and put a small plastic airplane on it and pull the pillow across the table with the airplane on it to kind of symbolize the ground effect holding the plane aloft.

good luck and hope this helps you out.

 

[ShawnC]

Magic.

I tell people PFM.

How does the transponder work? PFM.

How does DNS work? PFM.

It's the answer to everything.

 

[pa28cfi]

It's hard to illustrate here but I used to explain it with a picture, I would make a real simple drawing of the plane looking at it from the rear, then I would make a couple of circles showing the wingtip vortices curling over the top of the wing creating induced drag, then I would draw a big fat line underneath to simulate what happens to this airflow when you put the ground in the way. Kind of like the fried egg commercial: this is your airplane on induced drag, this is your airplane on ground effect.

 

[apdsm]

There is no such thing as ground effect. It is a lie put out by the government.

 

[pilotman2105]

At the risk of making a mistake, here it goes...

*May be helpful to have a drawing surface to explain this

The creation of lift due to Bernoulli's principal requires that the airflow over the top of the surface moves faster than the air underneath. At the back of the airfoil, the faster moving air "overrides" the slower moving and and causes "downwash."

Ground effect interrupts this downwash forcing it to go horizontal before it wants to. This reduction in downwash leads to a reduction in relative wind, or upwash (think of a rock in a stream. The water starts moving out of the way before it reaches the rock. Same idea with the upwash. It is "compensating").

With the reduction of upwash (relative wind), there is an increase in the angle of attack, which produces more lift. To maintain a constant angle of attack, you have to decrease the pitch slightly (nearly unnoticeable).

That is how I plan on explaining ground effect if I ever get the chance to use this new CFI ticket. We'll see how it works out

 

[falcon20driver]

You're a little confused on what ground effect is, reread this http://www.faatest.com/books/FLT/Chapter17/GroundEffect.htm

Ground effect allows the plane to fly slower and climb with less power (thrust) only because of a reduction in "induced drag" caused by "wingtip vortices"

 

[falcon20driver]

I don't have a copy of "Stick and Rudder" but I'm sure there is an excellent explanation of ground effect in it also.

 

[pa28cfi]

Boy pilotman you've got me totally confused. I read your post 3 times and I still don't understand it.

I know I'm going to regret getting sucked into this but here goes:

Induced drag is caused by the "planing effect" the wing has on the air like a wake board moving through water (this is right out of the FTH), circular vortices are created that curl up over the top of the wing, the resulting force that acts opposite the direction of the flight path is called the rearward component of lift, this is what causes induced drag. When the ground is in the way it disrupts the circular flow of these vortices therefore there is no "induced drag" when close to the ground.

 

[pilotman2105]

You're a little confused on what ground effect is, reread this http://www.faatest.com/books/FLT/Chapter17/GroundEffect.htm

Ground effect allows the plane to fly at a reduced AOA only because of a reduction in "induced drag" caused by "wingtip vortices"

I left out the induced drag part. Thought I may be missing something in there.

That aside, wingtip vorticies are not the only things that are contributing to the reduction in downwash. My understanding is that downwash is created by both wingtip vorticies (which I neglected to mention in my previous explanation) and the difference in velocity of the two airflows (underneath and over the top of the wing). The article you posted seems to dance around this subject a bit.

Thoughts?

 

[falcon20driver]

It's real easy to explain, an airplane climbs because thrust is greater than drag, if your 172 becomes airborne at 50 knots at an angle of attack of 20 degrees and full power and you climb to 10 feet and the plane levels off and quits climbing, it's because you have just increased drag to the point that it know is equal to thrust (by climbing out of ground effect and allowing the wingtip vortices to come to life and increase induced drag).

 

[pa28cfi]

I left out the induced drag part. Thought I may be missing something in there.

There is no other "part", you're bringing in elements that have nothing to do with induced drag or ground effect. Ground effect is defined as a reduction of induced drag when close to the ground. Angle of attack and downwash are only relevant in that they create Lift (and induced drag, a by-product of lift). There is no change in relative wind created by "upwash". I don't know how else to explain it.

 

[falcon20driver]

A little more.

Most lift is produced by the downwash from above the wing (reference FAA Flight Training Handbook chapter 17) not from lower pressure. The equal and opposite reaction to the downwash from above the wing is "an upward push on the wing"

When flying out of ground effect the wingtip vortices push the downwash farther down at the wingtips and tilt the total lift vector aft, this aft component of lift is induced drag.

When in ground effect the wingtip vortices are restricted and the vertical component of the lift vector is greater, so two things happen in groud effect:

1. The plane is essentially lighter because it can fly at a lower speed because of the greater vertical component of lift.

2. The plane can climb with a lower power setting because of less drag.

 

[VNugget]

Most lift is produced by the downwash from above the wing, not from lower pressure

Why do you make those sound mutually exclusive? Isn't that like saying "a car is propelled mostly by the friction between the tires and the road, not the torque produced by the engine," or some such?

Two complementary concepts.

 

[falcon20driver]

I guess I didn't get that quite right, here is how it's written in the FAA's Flight Training Handbook

"Herein lies the key to flight. The fact that most lift is the result of the airflow's downwash from above the wing, must be thoroughly understood in order to continue further in the study of flight."

The low pressure does not create the downwash, the low pressure is created by the increase in the speed of the air as it travels over the wing and changes direction to a downward flow, the downward flow of air creates the upward (lift) force on the wing, not the low pressure. I don't see how they complement each other. The downwash of air forces the wing up like a fireman is forced backwards as the water is shooting out the fire hose.

 

[VNugget]

The low pressure does not create the downwash, the low pressure is created by the increase in the speed of the air as it travels over the wing

That's a non-sequitur, you are trying to show me what the low pressure creates (or, in our case, doesn't create) by telling me what creates the low pressure.


The downwash of air forces the wing up like a fireman is forced backwards as the water is shooting out the fire hose.

Of course, action-reaction. So what causes the downward flow?

 

[falcon20driver]

That would be the curved upper surface of the wing, or in a symetrical wing its the Angle of Attack that changes the direction of the airflow and creates the downwash, by the way the change in direction of the airflow is why its speed increases and the pressure decreases.

I've posted several links to the FAA Flight Training Handbook, read chapter 17 and all of this will make sense.

 

[pa28cfi]

Of course, action-reaction. So what causes the downward flow?

Simple, the wing is flown at an angle to the flightpath (AOA) so it deflects the air downward. Also because of this angle there is positive pressure underneath the wing like when you stick your hand outside the car window the positive pressure of the air pushes your hand up. There's an old saying that you could make a dinner plate fly if you had enough thrust, the camber of the airfoil only makes things much more efficient. The positive pressure in front of the wing contributes about 25% to the total lift while the downward deflection accounts for the other 75% (those numbers are out of the american flyers cfi manual). I wish I could post the picture that goes with it.


P.S. This has been a great discussion, I'm currenty in the process of studying for my cfi reinstatement so I'm relishing this opportunity to test my knowledge.

 

[VNugget]

Nowhere does it say that there is any lift not accounted for by downwash. Think about it, The down-force to the air is the same as the lift to the airplane (Newton's 3rd law), and that downforce is equal to the mass of the air moved * its acceleration (F = ma, Newton's 2nd law). No other factors involved. Saying that there is some lift not accounted for by downwash is saying that F does not = ma, no?

As I'm tyipng this, I just re-read your post and realized I missed you were talking abou "downwash from above the wing," and not downwash in general, creating most of the lift, which is true. Oops.

But it still seems nonsensical to compare downwash "versus" pressure differentials.

 

[pa28cfi]

I'm not talking about pressure differential like you're thinking. I'm simply talking about the force of the air smacking against the underside of the wing as it moves forward and pushing it up just like a kite or your hand outside a car window. It is referred to in the book I have as "positive pressure" and contributes to overall lift. The "pressure" underneath the wing doesn't change it is the same as the ambient pressure it's only "higher" relative to the pressure on top of the wing. You get about 25% of the overall lift from the the impact of the air forcing the wing up from underneath and the other 75% from the downwash created by the air moving over the top of the wing being forced doward at the trailing edge.

 

[VNugget]

Er... my post was a reponse to falcon20driver... I didn't realize you posted between us until just now.

 

[fulcrum]

INduced drag

See It has to do with Local Relative Wind on the aerofoil

INduced drag is caused by the lift vector pointing slightly backwards ----------ie and it is directly proportional to the angle of attack

but as soon as the wing comes close to the ground the movement of air around the wing is affected it is forced to move paralell to the ground thus changing the local relative wind

remember lift is perpendicular to the (Local) relative wind

And of course there is a reduction in wingtip votices , just because the ground interferes with thier formation just like Wing tips or Increased Aspect Ratio.

My 2 cents
Fulcrum
if you want to know more you can attend my class

http://www.flight-school.us/aerodynamics.htm

 

[Airpiraterob]

show your student some pics and any film you can on the russian Ekranoplan (caspian sea monster) ground effect airplane. thats the only way it flys and is a good starting point to begin with.