For an aerofoil to create lift, the passing airflow needs to be relatively smooth and streamlined. As angle of attack increases, the smooth air flowing over the top of the aerofoil struggles to turn downwards and follow the upper surface. The airflow then becomes progressively less uniform and more turbulent as angle of attack is further increased.
Once the critical angle of attack is exceeded the airflow over the wing separates from the upper surface towards the trailing edge. The airflow beyond the separation point is significantly more turbulent. The lift generated sharply decreases and drag increases. This is known as a stall.
The angle of attack beyond which a stall occurs is called the critical angle of attack. When the critical angle of attack is exceeded, there is a dramatic decrease in lift and a continuing increase in drag.
For typical light aircraft, the critical angle of attack is roughly 16°.
As angle of attack changes, the air pressure distribution around the wing changes. Starting with a relatively low angle of attack, the pressure differential between the top and bottom of the wing is relatively low.
As angle of attack increases, the pressure differential between the top and bottom surface increases, and the centre of pressure moves forwards.
The point along the leading edge where the airflow splits above and below the wing is called the stagnation point.
The stagnation point is always on the lower surface, near the leading edge. As angle of attack increases, the stagnation point moves aft.
Most aircraft incorporate some kind of artificial stall warning device that sounds a horn and/or illuminates a warning light. These stall warning devices are designed to activate about 5 – 7 knots above the stall speed (i.e. it is a warning of approaching the stall, not necessarily an indication that the aircraft has stalled).
The stall warning devices found on typical light training aircraft are activated by a change in the stagnation point. Since the stagnation point moves aft as angle of attack increases, the position of the stagnation point can be used as an indicator of angle of attack.
When the stagnation point reaches a position that indicates the wing is approaching the critical angle of attack, the stall warning device is activated. This is usually by means of a small tab that is forced upwards when the stagnation point moves behind the tab on the leading edge.