Winglets reduce the tip vortex thus reducing the effective tip AOA
just to add a little on the mechanism for this--by reduction or vortex strength you reduce the induced drag
because the span wise flow once it reaches the end of the wing meets up and twist around produces a drag force due to the acceleration created by the change in the direction of air circulation around the wing---a change in velocity vector =acceleration
this can be imagined kind of like a rope attached to the wing tips... this drag is the result of producing lift in steady state and since fundamentally less drag produced results a lower induced angle of attack---and a lower alpha effect as
BOAC and
Bearfoil have mentioned
however the overall effects of winglets on stall qualities is probably relatively insignificant due to being overridden by the much more influential effects of twist [to meet stability requirements] as to not make much of a difference...also with just one winglet most aircraft only exhibit a minor handling difference wrt to trim..so they don't seem to cause very large aerodynamic changes---my guess
note winglets are mainly used to control aspect ratio a major player in the induced drag coefficient[ Cdi ]since most airplanes don't exhibit a rectangular planform---- Aspect Ratio [AR or A] is defined for a non-rectangular wing as the ratio of span squared/wing area or b^2/S
so, although span is not increased by much the wing area 'S' is increased thereby lowering the induced drag coefficient and consequently the induced aoa-
