Perhaps an aerodynamicist can correct me, but isn't the purpose of winglets to reduce the tip vortices caused by the high pressure air underneath the wing rushing round at the wingtip to the low pressure region above the wing? In doing so, a spinning vortex is generated which causes drag?
Also, why are the Boeing winglets huge, but the Airbus A300/320/321 ones tiny? - presumably they have different performance?
U