Stall & high-lift devices

081 Principles of FlightChapter 3

Stall fundamentals

Key fact

Stall occurs at critical angle of attack — fixed value affected by wing design and aspect ratio only (not bank, not turn). Higher AR → lower critical AoA. After critical AoA: CL reduces sharply (not to zero), drag increases massively.

Formula — Stall speed relationships

VS new = VS old × √(W new / W old)
VS new = VS old × √(1 / cos bank)
VS new = VS old × √n     where n = load factor

Key fact

Stall speed increases with: load factor (√n), mass, forward CG, icing, decreasing thrust, sweepback. Decreases with flaps/slats. IAS stall speed approximately constant at lower altitudes; increases at higher altitudes due to compressibility/Mach effects on TAS.

Stall characteristics by planform

Planform	Stall pattern	CP movement	Recovery
Rectangular	Root→tip	CP moves aft	Nose-down tendency, good buffet warning
Elliptical	Whole wing at once	CP aft	Poor warning
Tapered (unmodified)	Tip→root	CP aft	No warning — worst
Swept (unmodified)	Tip first	CP moves forward	Pitch-up at stall — very dangerous

Key fact

Super/deep stall: stable stall, swept wings highest risk. Negative tail stall: tailplane stops producing downforce → uncontrollable pitch down (icing on stabilizer).

Stall warning & protection

Key fact

Stall warning vane: stagnation point moves down → vane lifts (installed below leading edge). Stick shaker: activates at lower AoA than stick pusher, at speeds above VS; inputs = AoA + rate of change. Stick pusher: forces stick forward at/beyond limit AoA. Stall strips: induce root stall first. Wing fences: reduce spanwise flow.

Exam trap

Slats increase critical AoA; flaps decrease critical AoA. Slats outboard (tips); flaps inboard (root). Slats have greater effect on stall speed than flaps at any setting.

High-lift devices — trailing edge flaps

Key fact

Clean config: highest CL/CD. Flaps 30°–45° adversely affect ratio. Deploying flaps: CLMAX ↑, VS ↓, critical AoA ↓, CP moves aft → nose-down tendency (tailplane downwash increases → pitch-up moment opposes). Retracting flaps → tendency to sink.

Key fact

Maintaining level flight, constant IAS with flaps deployed: CL eventually same — must lower nose and increase thrust. With flaps retracted: AoA must increase.

Flap type	Effect
Plain	Increases camber
Slotted	Camber + re-energises flow through slots
Fowler	Most effective — moves aft, increases area + camber

Slats & other devices

Key fact

Slat: increases CLMAX more than yawing moment; large stall speed reduction with relatively less drag; postpones stall by energising boundary layer at suction peak. Krueger flap: lower surface leading edge hinged forward. Vortex generators: delay separation, re-energise boundary layer.

Spoilers & speed brakes

Key fact

Roll spoilers: reduce lift on one wing; local drag increase suppresses adverse yaw. Symmetric spoilers: speed brakes — increase drag, steeper descent. Constant FL & speed: CD ↑, CL unaffected. Constant AoA deployed: CD ↑, CL ↓. Spoiler extension increases stall speed.

Spin recovery

Key fact

PARE: Power idle, Ailerons neutral, Rudder opposite rotation, Elevator forward. Both wings stalled in a spin. Incipient spin: rudder toward raised wing; no ailerons.

Icing & frost

Key fact

Icing: CLMAX ↓, drag ↑, critical AoA ↓, VS ↑, controllability ↓. Most critical at rotation. Landing distance up 40–50%. Frost: lift ↓, drag ↑; forms over whole wing. Stall warning calibrated without ice — stall can occur before warning activates.