Stall

Stall — Flow Separation

Stall occurs when the angle of attack α exceeds the critical value α_stall and the boundary layer on the upper wing surface massively separates. Result: dramatic lift loss and strong drag rise.

Mechanism

At small α: flow follows the airfoil cleanly. Pressure top < bottom → lift.

At critical α (α_stall, about 14–18°): adverse pressure gradient on the upper surface becomes too large → the boundary layer lacks energy to follow the gradient → separation.

Above α_stall: massive flow separation → large vortex region above airfoil → CL drops abruptly, CD rises sharply.

Symptoms of imminent stall (stall warning)

1. Stall warning horn (acoustic, mechanical, or electrical).
2. Buffet (vibration in yoke and fuselage) — vortex shedding felt in the structure.
3. Mushy controls — ailerons respond less precisely.
4. Nose drops despite pitch-up — pilot must pull more yoke.
5. Airspeed falls below Vs.

What is NOT the cause?

Important: stall is NOT caused by low airspeed. It is excessive angle of attack.

You can stall at any airspeed if α gets too high:

• Accelerated stall: in a turn at high bank (load factor n → higher CL required).
• Pull-up from dive: high pitch rate, momentarily high α.
• Wind shear: sudden IAS loss with high α.

Stall speed Vs

Vs = √(2·W / (ρ·S·CL_max))

Vs depends on:

• Weight W: Vs ∝ √W → 21% more weight → 10% more Vs.
• Load factor n: in bank/pitch manoeuvres n applies instead of 1 → Vs(n) = Vs · √n.
• Configuration: flaps, gear, icing.
• Altitude: via IAS, Vs stays constant (ρ cancels, but IAS is normalised to ρ₀).

Cessna 172 example:

• Vs0 (full flaps) = 48 KIAS (typical POH).
• Vs1 (flaps up) = 51 KIAS.
• In 60° bank turn (n = 2): Vs = 51 × √2 ≈ 72 KIAS.
• At MTOM → 100% Vs; at 80% weight → Vs ≈ 51 × √0.8 ≈ 46 KIAS.

Stall recovery — standard procedure (FAA, EASA)

Detection

• Stall warning horn / buffet / nose drops → act immediately.

Recovery (FAA Airplane Upset Recovery Training Aid Rev. 3)

1. Pitch — yoke FORWARD (reduce α) — most important step.
2. Power — full (throttle FULL).
3. Roll — wings level (ailerons coordinated with rudder).
4. Lose altitude if needed (short descent acceptable, priority is exiting stall).
5. Regain lift: only once α < α_stall.

Important: in many GA aircraft, power is secondary — the primary step is yoke forward. Consult POH.

Spin (next lesson)

If one wing stalls earlier than the other (asymmetric), autorotation can develop → spin.

Wing design for benign stall behaviour

• Washout (wing twist nose-down at tip) → root stalls first, aileron control at tip retained.
• Stall strip at root — provokes early root stall.
• Washout + cambered LE for trainers like the C172.

Stall recovery in flap configuration

• Flaps up at stall — stall speed higher but more stable.
• Full flaps at stall — Vs lower, recovery requires care (flap-flow dependence).
• POH-specific practice.

Pilot training (FCL.235 skill test)

Stall exercises are mandatory in PPL training:

• Power-on stall (cruise → stall).
• Power-off stall (approach → stall).
• Approach stall (full flaps).
• In bank stall (coordinated turn → stall).

Each with correct recovery demonstration.