Explaining Ground Effect

[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.