Stalling, Spinning and High-Angle-of-Attack Aerodynamics
The stall
The stall occurs at the critical angle of attack, not at a particular speed. Any combination of speed, weight, load factor and configuration that reaches the critical angle will stall. Candidates must know that stall speed rises with the square root of load factor, that flaps lower stall speed but may raise the nose-down pitching tendency at the break, and that ice, rain and contamination raise stall speed. Buffet, aileron snatch and reduced pitch authority are warning signs; the stall warning device typically operates a few knots or a few degrees before the break.
Spins
A spin requires both a stall and yaw. The incipient stage precedes the fully developed steady-state spin, and recovery follows the standard sequence: power idle, ailerons neutral, full opposite rudder, positive forward elevator, then recovery from the dive. Candidates must understand that the spin is a high-angle-of-attack condition in which one wing is more deeply stalled than the other, and that the autorotation is sustained by the differential lift and drag.
Traps and a worked example
Traps: believing a stall can be avoided by speed alone; assuming the stall speed is fixed for a type; and thinking ailerons should be used to lift a dropped wing in a spin (this can aggravate autorotation). Another trap is confusing the spin with a spiral dive, in which the airspeed rises rapidly.
Rule of thumb: stall speed varies as the square root of load factor; at 2 g it is about 1.41 times the 1 g value.
Worked example: an aircraft stalls at 50 kt at 1 g. In a 60° banked level turn the load factor is 2, so the stall speed is 50 × √2 ≈ 71 kt. In a 75° bank, load factor is about 3.9 and stall speed about 99 kt.