All Those Wingtips
Mel's experience is pretty common, except he's more honest than some owners who changed wingtips. Over the last 50 years I've seen a few scientific wingtip fads swoosh by. Hoerner, upswept, downswept subsoilers, cut off flat, flat plates, raked tips, raked the "wrong" way tips, all that. Reported results range from spectacularly life-changing to nothing at all. Usually the latter. In a lot of cases, it seemed to me that when a guy pays big $ for a mod, it dang well works, even if no change can be proven or measured. My late professional aerodynamicist buddy always said extra wingspan was the way to get the improvements those majick wingtips were striving for. He also said there was very little improvement to be had until the wing was flying at high angles of attack. In level cruise, the wing is not working very hard, hence very little wingtip vortex to try to cancel. Closed course pylon racers that constantly turn hard and airplanes flying at very high altitudes benefit the most, but in the end, span seems to matter more, leaving aside the shape of the wingtips. I remember an article by the late Ken Willard in R/C Modeler magazine back in the 70s where he tried different wingtips on a model sailplane. Trick was, he only changed one tip, then flew the model. He tried several shapes and reported that he never found much of a turning tendency with any of those shapes. I did 29 years with one of the the big gummint airframers, fairly often working with aero guys. (I built simple wind tunnel models) What I always seemed to hear was "Yeah, that's a pretty nice wingtip, but gimme more span, I gotta have more span..." Designers admitted they just rotated the top and bottom airfoil surfaces 90 degrees and blended them in CATIA to make a tip. Oh well. So here I loop back to the OP... when styling an airplane, probably the third most visually important thing is the wingtips, right after the vertical tail and the cowl. Aren't those Zip Tips spectacular?