Winglets are not an optimal solution, they are an average solution. The 4% benefit is a marketing tool, and is not over the entire flight.
Enroute, with the AoA changing, the effective efficiency of the winglets is not for all phases of flight. That is why when looking at a retrofit, the airline must look at the intended use for payback. If the ac is doing short hops, there is no payback.
Again, there is NO reduction of wake vortex associated with the winglets. That early marketing assumption has been negated by NASA, Volpe, and DLR research, as has the assumption that the vortices is generated by the wingtips.
The wake vortices change throughtout the phases of flight with the AoA of the wings and reduction of weight. On Approach, the flap settings have a significant effect on the wake generation. The wake from the same 737-800 with flaps 30, is much different with flaps 40.
Notice the evolution of winglets, sharklets, and xwings?
Wingtip?
