12.68 Stalling and Spinning

12.68.2 Describe the stalling angle of attack, with reference to:

(a) disruption of streamline flow over the upper surface of the aerofoil;

The stalling angle of attack is the critical angle at which the airflow over the aerofoil can no longer remain smooth and attached to the upper surface.

At low angles of attack, the airflow follows the curvature of the wing, remaining attached (streamlined flow). This produces efficient lift.

As the angle of attack increases:

• The airflow begins to slow and lose energy on the upper surface
• The boundary layer separates from the surface
• Flow becomes turbulent rather than smooth

At the stalling angle of attack:

• There is significant separation of airflow over the upper surface
• Lift decreases rapidly
• Drag increases sharply

This disruption of streamline flow is the primary cause of the stall, not airspeed alone.

Summary:
A stall occurs when increasing angle of attack causes airflow separation over the upper surface, resulting in a sudden loss of lift and increase in drag.

(b) reduction of lift and increase of drag.

As the angle of attack (AoA) increases, lift increases up to a maximum point (CLmaxC_{L_{max}}CLmax​​). Beyond this point—the stalling angle of attack—the airflow over the upper surface separates significantly.