Tip Stalling
The wing of an aircraft is designed to stall from the root to the tip.
This:
a. induces buffet over the tail surface thereby providing advice of the stall,
b. retains aileron effectiveness up to the critical angle,
c. avoids the large rolling moment that would arise if the tip of one wing stalled before the other;
d. reduces the downwash behind the root and may provide a stable nose down moment.
A rectangular wing will usually stall from the root because of the reduction in effective angle of attack at the tips caused by the wing tip vortex. If washout is incorporated to reduce vortex drag, it also assists in delaying tip stall. A tapered wing on the other hand will aggravate tip stall due to the lower RN at the wing tip.
Wing tip stalling can be prevented by:
a. Washout. A reduction in incidence at the tips will result in the wing root reaching its critical angle of attack before the wing tip.
b. Root Spoilers. By making the leading edge of the root sharper, the airflow has more difficulty in following the contour of the leading edge and an early stall is induced.
c. Change of Section. An aerofoil section with more gradual stalling characteristics may be employed towards the wing tips. i.e. increased camber
d. Slots and Slats. Used on the outer portion of the wing will increase the stalling angle of that part of the wing.
Effect of Sweepback on Stalling. When a wing is swept back, the boundary layer tends to change direction and flow towards the tips. This outward drift is caused by the boundary layer encountering an adverse pressure gradient and flowing obliquely to it over the rear of the wing. Initially when the boundary layer flows rearwards from the leading edge it moves towards a favourable pressure gradient, i.e. towards an area of lower pressure. Once past the lowest pressure however, the component at right angles to the isobars encounters an adverse pressure gradient and is reduced. The component parallel to the isobars is unaffected, thus the result is that the actual velocity is reduced (as it is over an unswept wing) and also directed outwards towards the tips
Since the outboard sections of the wing trail the inboard sections the outboard suction pressures tend to draw the boundary layer toward the tip. The result is a thickened low energy boundary layer at the tips, which is easily separated.
Slots, slats, and fences tend to reduce spanwise flow.
(I know a picture is worth a thousand words but I couldn't paste the accompanying picture)