Originally Posted by
BOAC
I'm pretty sure that the flow is NEVER 'completely attached', so we are talking levels of 'separation'.'.
Exactly.
Except maybe for an ideal laminar flow wing. This exists only on paper though.
In reality it is pretty safe to assume you are correct.
The polars @dimsp81 is probably referring to are usually for an infinite and ideal wing without twist and taper, sweep, slats and other geometrical properties or also physical properties like rivets or dents or other imperfections of a real wing which are significantly influencing separation behaviour.
So this critical AoA thing is nice but to which section in which condition of a real wing does it refer?
Due to these mentioned geometrical features the separation will be a progressive process with increasing AoA but starting at very low AoA.