Stall strips

A stall strip is a fixed device employed on the leading edge of fixed-wing aircraft to modify the aerodynamic characteristics of the airfoil. Stall strips are used to initiate flow separation at chosen locations on the wing during high-angle of attack flight, so as to improve the controllability of the aircraft when it enters stall. They are typically employed in pairs, symmetrically on both wings. On aircraft where wing airflow is affected by asymmetrical propeller wash, a strip may be used on a single wing to reduce risk of entering a spin.
Operation.
A stall strip alters the wing’s stall characteristics and ensures that the wing root stalls before the wing tips. This is usually as a result of initial aircraft flight testing which shows that the existing stall characteristics are unacceptable for certification.
In some cases, such as the American Aviation AA-1 Yankee, stall strips are planned to be used on the wing from the start. In the case of the AA-1 the left and right wings were identical, interchangeable and built on a single wing jig, thus the more traditional use of washout in the wing design was not possible.
Stall strips typically consist of a small piece of material, usually aluminium, triangular in cross section and often 6-12 inches (15–30 cm) in length. It is riveted or bonded on the point of the wing’s leading edge, usually at the wing root. At high angles of attack, it trips the still-laminar boundary layer, initiating flow separation earlier than at other positions along the wing. This has the effect of causing the wing root to stall before the outer portions of the wing, ensuring a progressive outward stall, minimizing the risk of spinning and giving maximum aileron control throughout the stall.
They are usually factory-installed or, on rarer occasion, an after-market modification.