Dead-on MD11Nut. I just wanted to add a note. There are many aerodynamic tricks that can be used to save fuel, not just winglets. Just look at the raked wingtips on the B767-400ER.
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Photo © Thomas Millard
The wing structure required to support the loads a winglet places on the wing is costly and is not easily recovered by the puddle jumper operation of short and mid-range aircraft. Even long-range aircraft like the 767 have obvious design related cost trade-offs that drove Boeing to decide to rake the wingtip instead of putting a winglet on it.
If there was only one right way to do things in this business, all aircraft would have wing mounted engines, fowler flaps, and cabin doors that retract inside the fuselage ceiling.