Washout (Wing Twist)

Washout is a twist of the wing such that the angle of attack at the root is greater than at the tip. It is a design measure for safe stall behaviour.

Two types

1. Geometric washout

Chord tilted nose-down at the tip (negative twist).
Typical value: −2° to −4° between root and tip.
Example: root α = 4°, tip α = 1°. As AoA increases, root reaches α_stall first.

2. Aerodynamic washout

Airfoil varies along the span — tip has an airfoil with higher α_stall.
Rarely used alone; often combined with geometric washout.

Purpose — benign stall behaviour

On tapered wings, the tip stalls first (lift distribution higher there). When the tip stalls, the ailerons also lose effectiveness — pilot cannot correct bank → spin risk.

With washout: root stalls first, tip remains in normal flow → ailerons effective → pilot can stabilise bank and recover.

Stall sequence with washout

Pilot pulls yoke → α rises everywhere.
Root reaches α_stall first (AoA there higher).
Lift at root drops → lift centre shifts outboard.
Pitching moment nose-down (toward root) → benign stall development.
Tip remains flow-stable → ailerons effective.
Pilot can recover by pitch-down.

Quantitative significance

Typical C172 (Cessna 172):

Span 11 m.
Root chord 1.5 m, tip chord 1.12 m (slightly tapered).
Washout about −1.5° to −2°.
α_stall (root) reached at effective α = 16°, α_stall (tip) at effective α = 16°+1.5° = 17.5° → root stalls first.

Other methods for root-first stall

Stall strip: small triangular wedge on the leading edge near root → provokes early flow separation.
Leading edge slats only at tip: tip has higher α_max.
Wing fence: blocks flow from drifting toward tip.
Airfoil variation: tip uses airfoil with higher α_max.

Wash-in (next lesson)

Wash-in: opposite of washout — tip α > root α. Very rare in normal aircraft, sometimes used in:

High-aspect-ratio gliders: to optimise lift distribution.
Propeller-effect compensation (slipstream gives right wing slight extra lift — wash-in on left).

Manufacturing realisation

Directly in the spar: spar slightly twisted across span.
Varying airfoil: airfoil changes along span.
Authoritative: certification specification (CS-23 for GA), load cases, stall behaviour.

Consequences

Effective AR slightly reduced (tip carries less lift).
Stall speed at root = effective Vs of the aircraft.
Benign stall behaviour for PPL trainers essential (CS-23 certification relevant).

Other names

Washout (English): standard.
"Schränkung" (German): traditional word, common in glider engineering.
"Twist" or "Geometric twist": engineering terminology.