How do wing tip vortices that interact with the wing surface affect drag?
MEng Aeronautics and Astronautics student at Southampton uni here. We've been studying this in some detail recently and I've found that MIT has released a set of lecture notes on finite wing theory, including a really useful document focussing on tip vortices and their impact on aerodynamic performance (http://web.mit.edu/16.unified/www/SPRING/fluids/Spring2008/LectureNotes/f05.pdf). This explains the induced drag acting on a wing/other lifting surface which is caused by tip vortices generating downwash, which in turn alters the wing's geometric angle of attack.
Hope this helps, or at least gives you a starting point for further research :)