Originally Posted by
jhurditch
When lowering flaps however, whilst the camber is increased, the change of shape occurs abruptly at the rear of the wing due to the flap slab. That is to say, the mean camber line is increased only at the rear of the wing.
The change of pressure gradient with flap extended is abrupt rather than smooth and laminar for a clean wing. For this reason, flow seperation occurs at a lower angle of attack than with a clean wing.
If you cosider the AoA as Angle between the air stream and the line from leading to trailing edge (i.e. the TE of the flap) that is not exactly true.
It is true if you consider the Angle between the air stream and the main plane of the wing (i.e. the wing without the flap).
Btw. the slotting rather postpones flow separation.
Let's have an example in order to make it clearer.
Let's assume the flap is 20% of the chord length.
If you now deflect the flaps 20° down this tilts the angle of the overall chord (including flap) by 4°, i.e. if the pitch of the aircraft remains constant, the AoA is increased by 4°.
Or inversely in order to keep aerodynaimc AoA constant you would have to pitch 4° Nose down. Your AoA instrument would then indicate an AoA decreased by 4° although aerodynamically in reality it is still the same as initially.
The more cambered profile (including the flap) might have an AoAmax increased by 1,5°. This would mean that the AoA displayed at the stall would decrease by 2,5° as the probe keeps the aircraft as reference plane and not the (changing) chord of the wing.
Edit:
As you normally refer the AoA to the main plane (it would be absolutely impractical to do otherwise and for no benefit) the wing will stall earlier as the air will be accelerated stronger as without flaps deflected thereby increasing the lower pressure on the upper side, making it more susceptible for separation.