Lift Coefficient to AoA Graph, Question

[Bjarnum]

During an interview I was shown a 'Lift of Coefficient to AoA' Graph and questioned about it. Then the interviewer drew a line at the end of the Lift line and asked what this meant. (Graph image link below)

I didn't know and so he briefly said it had something to do with "Flat Plate" effect.

Here is the graph with the added Lift line at the end, I indicated such with an arrow pointing to it:

MobileMe Gallery (http://gallery.me.com/nexusmedia#100022/Image202&bgcolor=black)

Any comments, answers or further explanations of what this 'added section of line' means, would be greatly appreciated.

Cheers

[Mad (Flt) Scientist]

That kind of shape - the one drawn is a bit exaggerated or idealised of course - might occur where the first peak - the traditional CLmax - occurs where the airflow separates from the wing and the wing stalls. The increase in CL with AOA that is added later can be due to two things:

1. The body or other non-wing component is now lifting enough at the higher AOA to compensate for the loss of wing lift. Some fighter types see this kind of behaviour.

2. If the wing is highly swept, there may now be vortices being generated along the LE of the wing which cause it to start to lift again. Crudely, Concorde did something like this, except without such pronounced peaks and troughs I believe.

A third possibility is in the presentation. If you are careless with definitions you can end up with the drag force being accounted into apparent lift. This can occur if you define the lift force as "up" in aircraft body terms (sometimes called "Cz") instead of the more usual "up in terms of direction of flight" that is usually used. But I'm assuming it's one of the first two.

[Bjarnum]

Thank you Mad (Flt) Scientist. This really helps out, especially the 1st explanation. Are there any graphs out there that you know of that show the characteristics of a particular plane achieving this phenomena.

Anyone else have any other explanations?

[kharmael]

I believe the flat plate effect is when the aerofoil aspects of the wing are no longer taken into account but it's simply the airflow exerting a force on the bottom of the wing (sticking your hand out of a car window).

Correct me if I'm wrong.. :\

[Mad (Flt) Scientist]

Thank you Mad (Flt) Scientist. This really helps out, especially the 1st explanation. Are there any graphs out there that you know of that show the characteristics of a particular plane achieving this phenomena.

Anyone else have any other explanations?

Figure two of this reasearch paper contains a lift-curve slope of roughly the shape you are describing:
http://ftp.rta.nato.int/public//PubFullText/RTO/MP/RTO-MP-011///$MP-011-17.PDF

If you concentrate on the "steady" (solid) line you'll see a small recovery in lift with alpha when increasing past 30 degrees. I can't see it stated anywhere what the actual aerofoil or model was, though.

[M33]

100% with kharmael, the basic lifting force of the wing is based on the reduced pressure above due to the increased velocity, but also slight increase in pressure on the lower surface.

However as the critical angle is reached the sparation point on the upper surface moves forward and the reduction of pressure on the top surface decreases.

The flat plate theory, is based upon the idea that lift is created in a combination of low pressure above, (now gone), but also the high pressure below, hence as the AoA increases the high pressure below the wing plays a large role, and Cl is able to see slight improvement.

Paper aeroplane theory! No real aerofoil but yet can glide very well!

My 10c anyhow.