How do winglets work?

[Wing Nut]

I understand they help with stability, but I get the impression they do more to add to the economy of the plane.  Anyone know?

[Craig.]

from what i understand, they prevent the air that passes over the top side of the wing from falling over the side of the wing, thus maximising the lift created by the wing, meaning less power is needed from the engines to keep the lift up, saving fuel, the efficiency is not that much in the short term but it does help over time,
i might be wrong here but thats how i understand them to work

[Rifleman]

Very good Craig....here is the actual lowdown from an aerodynamic specialist........

A winglet is a wing-like surface attached to the tip of the wing that is used to improve fuel efficiency and climbing capability. It accomplishes this by acting like a small sail with a lift component that generates a traction force, draining energy from the tip vortices. Consequently, it reduces the vortex strength, lowering induced drag. "Induced drag represents 30-40 percent of the total drag of a transport air-plane at the cruise condition, so it has a big impact on fuel consumption," explains Bento Silva de Mattos, senior engineer at Embraer. "Because the induced drag coefficient is proportional to the square of the lift coefficient, a reduction in drag improves climbing capability as well, because the lift coefficient is high at this condition."

Despite the fact that winglets have been in use for many years, optimization of winglet design had been difficult because the complex airflow patterns around these wing-mounted airfoils could not be understood through wind tunnel testing. The Reynolds number has a big effect on the flow structure; f light tests always showed larger drag reductions than the ones from wind tunnel data. Using simulation results as a guide, the engineers fine-tuned the winglet geometry to further reduce induced drag, improving fuel consumption.

There is also a great deal more info on low speed applications for winglets, here ...http://www.soaridaho.com/Schreder/Construction/Masak_Winglets.htm