Washout refers to a feature of wing design to deliberately reduce the lift distribution across the span of the
wingof an aircraft. The wing is designed so that lift is strongest at the wing roots and decreases across the span until reaching zero at the wing tip. This is usually to ensure that, at the stall, the wing root stalls before the wing tips, providing the aircraft with some resistance to spinning. Washout may also be used to modify the spanwise lift distribution to reduce lift-induced drag.
Washout is commonly achieved by designing the wing with a slight twist, reducing the
angle of incidencefrom root to tip, and therefore causing a lower angle of attackat the tips than at the roots. This is sometimes referred to as structural washout, to distinguish it from aerodynamic washout.
Wingtip stall is unlikely to occur symmetrically, especially if the aircraft is maneuvering. As an aircraft turns, the wing tip on the inside of the turn is moving more slowly and is most likely to stall. As an aircraft rolls, the down-going wing tip is at higher incidence and is most likely to stall. When one wing tip stalls it leads to wing drop, a rapid rolling motion. Also, roll control may be reduced if the airflow over the
ailerons is disrupted by the stall, reducing their effectiveness. Thus, a pilot may be unable to maintain control, perhaps leading to an unrecoverable spin.
On aircraft with
swept wings, wing tipstall also produces an undesirable nose-up pitching momentwhich hampers recovery from the stall.
Washout may be accomplished by other means e.g. modified aerofoil section,
vortex generators, leading edge wing fences, notches, or stall strips. This is referred to as aerodynamic washout. Its purpose is to tailor the spanwise lift distribution or reduce the probability of wing tipstall.
Winglets have the opposite effect to washout. Winglets promote a greater proportion of lift being generated near the
wing tips. (This can be described as aerodynamic wash-in.) Winglets also promote a greater bending momentat the wing root, possibly necessitating a heavier wing structure. Installation of winglets may necessitate greater aerodynamic washout in order to provide the required resistance to spinning, or to optimise the spanwise lift distribution.
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