Gradual Stall

This is a desirable pattern and occurs when separation is initiated at the trailing edge of the aerofoil; the remainder maintains the pressure differential. As the separation moves slowly toward the LE, the aircraft approaches stall gradually, giving the pilot enough time to take corrective action. The forgiving and gentle nature of this stall is ideal for an ab initio trainee pilot. The type of aerofoil that experiences this type of stall has a generously rounded LE, providing smooth flow negotiation but not necessarily other desirable performance characteristics.

3.7.1 Abrupt Stall

This type of stall invariably starts with separation at the LE, initially as a small bubble. Then, the bubble either progresses downstream or bursts quickly and catastrophically (i. e., abruptly). Aerofoils with a sharper LE, such as those found on higher-performance aircraft, tend to exhibit this type of behavior.

Aircraft stall is affected by wing stall, which depends on aerofoil characteristics. Section 3.19 addresses wing stall (see Figure 3.40).