Ilikeplanes

The description that Ollie gave works the same for a flat plate at an angle of attack. What happens with these (while at a small angle of attack) is the following: The forward stagnation point is once again slightly below the leading edge, just like a normal airfoil. That means from just above the stagnation point, air is trying to move forward and around the leading edge, then over the upper surface of the airfoil. However, the leading edge on a flat plate presents a sharp corner, which the air cannot navigate. The air separates locally, and form a little bubble of separated flow just behind the leading edge on the upper surface of the plate. Air approaching from in front of the airfoil and above will now flow over this bubble, reattach behind the bubble on the upper surface of the plate and flow to the trailing edge the same way as on a normal airfoil. The overall effect is a flow field that looks almost the same as for a normal airfoil; the flow still accelerates significantly over the top of the airfoil and especially over the leading edge region, and lift is still produced in the same way as Ollie described. The change in momentum due to the downwash is also still present, resulting in a force according to Newton's laws.

However, as the angle of attack is increased, the separated region increases quickly in size until the flow does not reattach anymore behind it and the plate stalls. This happens at a much lower angle of attack than a "proper" airfoil, which makes the flat plate a less efficient shape for producing lift than a normal airfoil.

It is possible to curve the plate a little, giving it camber which will increase the maximum angle of attack a little but now the lower part of the airfoil will stall again due to separation on the bottom of the airfoil at low angles of attack. In general, thin airfoils tend to have a smaller angle of attack range than thick airfoils.

I highly recommend the website Ollie pointed out - it is a very good introduction to aerodynamics.

Regards,
Ben