Folded wingtips look awfully similar to the winglets you see on modern jets and they fly fine! "Winglets, as the name applies, are miniature wings, and as such

Folded wingtips look awfully similar to the winglets you see on modern jets and they fly fine! "Winglets, as the name applies, are miniature wings, and as such they generate their own tip vortices that degrade their performance. Winglets reduce drag, but they also generate lift at the tip, increasing the bending moment at the wing root and requiring the structure to be beefed up, adding weight." Folded wingtips don't produce a smooth surface so my intuition would be that it would increase drag.