Mitek,

The cl is the 2D lift coefficient (infinite wing model). CL is the 3D lift coefficient (finite model) and takes into account the air that moves from underneath the wing around the wing tips and dumps out on top of the wing (this is the wing tip vortices you may have heard of). The problem is the votices does two things: 1. creates drag. 2. The "span wise flow" reduces the lift that would normally be produce in this area of the wing which reduces the performance of the whole wing....so the CL will always be less than the cl. On the NASA sim page you have a AR correction BUTTON this is taking the model from the infinite to the finite model. In general the lower the Aspect ratio(AR) the larger percent of the wing that will be impacted by the vortices's.....building infinite wings or even very long slender wings are impractical, so AR from about 5 to 10 are the most practical to build (at least for large planes).

If you display the CL vs AOA plot graph on the NASA page and activate the STALL MODEL you will see a curve although it is not real noticeable at 10 degree absolute AOA the wing starts to stall. What this means is that the air (boundry layer), starting at the trailing edge and progressing to the leading edge, is separating from the wing. This causes drag but it also means that less of the wing is doing the work. At around 15 degrees AOA the amount of increased lift due to higher AOA is off set by the amount of lift being lost due to separation so this is the max CL (the highest point on the graph, the top of the hill). At around 19 degrees the air flow over the top of the wing become completely detached (STALL AOA). So the reason the CL vs AOA curve kind of looks like a hill is it is showing graphically the porgressive detachment of the boundary layer.

I will look around.... I use to have a very good link to a web-site that show the progressive stall of a wing with some pictures and some drawing. One thing that is a little confusing is the idea of smooth air flow(laminar), turbulent flow, and flow separation. Might be something to do a search on if your interested......have you ever seen the smoke coming from a cigar, etc....right after it leaves for a few inches the smoke is very uniform (laminar) at some point the smoke starts swirling and bubbling...turbulent flow. A foil can have either laminar flow or turbulent flow but not be stalled and sometimes foils are designed to force the flow to turbulent in order to keep the boundary layer from seperating(stalling).(see the princeton link with the tennis ball... the wire is creating a turbulant flow which allows the boundry layer to stay attached longer) Stall is caused from 'boundary layer separation' of the air flowing over the wing. I may be getting off track here but if you interested you might look for pictures that show drag....and flow...tuft picture(little pieces of string that indicate flow) Bluff body pictures etc.

Here are a few links to get you started:
[link]http://www.grc.nasa.gov/WWW/K-12/airplane/boundlay.html[/link]
[link]http://www.princeton.edu/~asmits/Bicycle_web/blunt.html[/link]


Happy Flying!!!!!