Washout is a characteristic of aircraft wing design which deliberately reduces the lift distribution across the span of an aircraft’s wing. The wing is designed so that the angle of incidence is greater at the wing roots and decreases across the span, becoming lowest at the wing tip. This is usually to ensure that at stall speed the wing root stalls before the wing tips, providing the aircraft with continued aileron control and some resistance to spinning. Washout may also be used to modify the spanwise lift distribution to reduce lift-induced drag.
Washout is clearly visible in this image of a CF-18 Hornet. Note the angle of the Sidewinder missile on the wingtip rail as compared to the angle of attack of the fuselage. The Hornet has approximately 4 degrees of washout.
In fluid dynamics, a stall is a reduction in the lift coefficient generated by a foil as angle of attack increases. This occurs when the critical angle of attack of the foil is exceeded. The critical angle of attack is typically about 15°, but it may vary significantly depending on the fluid, foil, and Reynolds number.
Airflow separating from an airfoil at a high angle of attack, as occurs at a stall.
A Schweizer SGS 1-36 being used for deep-stall research by NASA over the Mojave Desert in 1983.
