Why wingtips?

Winglets, those upward turning ends of aircraft wings ,boost the performance by reducing drag. this is accomplish by reducing the strength of the wingtip vortices form by the difference between low pressure on the upper surface of the wing. these vortice represent an energy loss and increase in drag. in essence ,the winglet provides an effective increase in the aspect of the wing without extending the wingspan. winglet come in variety of styles- the airbus 320 has a very small upper and lower winglet :
A320
the boeing 747-400 has a conventional ,vertical upper winglet ;
B747-400
and the boeing 737 has an eight-foot winglet with curving transition from wing to winglet.

B737-800
since the airflow around the winglet is quite complicated, the winglet must be carefully designed and tested for each aircraft.in the past winglet is likely to be retrofitted to existing wings, but new airplane are being designed wit winglets from the start.(adapted from fundamental of fluid mechanic text book ) this subject is my lovely subject coz always deal with the formation and principles of flight. i cant score it well in last semester but i must do it well in this semester .