Plap;
At the wing tip the pressure difference between the upper and lower surface MUST be equal to zero, this means that there is a varying pressure distribution along the wing span, hence the formation of the trailing vorticies. Consequently, the flow has a velocity component along the spanwise direction (towards the fuselage on the upper surface) therefore the local flow direction at the wing tip is NOT perpendicular to the mean free stream flow but acutally flows inboard. Winglets, or wingtip sails, attempt to take advantage of the fact that the local flow direction at the wingtip is not perpendicular to the free stream flow. Acting as small aerofoils, there is an effective angle of attack between the local flow direction and the winglet. This leads to a resultant aerodynamic force with a component in the direction of motion - hence the 'thrust' or reduction in drag.
Hope this helps,
Regards
ASI
[This message has been edited by ASI (edited 08 March 2001).]