Why are the wing tips on modern passenger jets upturned?

Answer 1 : 

The small upturned fins on the end of the wing stop the formation of wing-tip vortices. Forming these vortices takes quite a lot of energy, and so leads to increased drag. On planes that do no have the wing-tip fins, one can sometimes see the vortices during take-off and landing.

Large birds have special separated flight feathers at their wing tips that achieve the same effect..

Answer 2 : 

Not surprisingly, arguments about whether winglets are a `a good thing' are not clear-cut. When they were first introduced, it appeared that improvements in efficiency of about 10 per cent could be produced.

For newer wing designs the improvements were smaller. In 1980, Boeing engineers found that winglets were unlikely to prove commercially attractive for their new 747 jumbo jet derivative.

In a new design, increasing the wingspan generally proves to be a better alternative to winglets if the vortex drag is too high.

But, there are limitations on wingspan imposed by airport operations. If these apply, it can be worthwhile to design the wing with winglets.

The benefits are not large, say 2 or 3 per cent in efficiency, but even these benefits can prove important in airline economics.However, aircraft are not bought solely on the basis of performance. Appearance also counts.

This is particularly true of business jets, so if an aircraft has narrative, efficient-looking winglets it can be hyped. It is then only necessary for the winglets to be harmless, rather than beneficial.

Source : The Hindu