Bob
The EVO wing kit comes with a flat aluminium wing tip. Aesthetically I wanted a different shape, something that would go with my camouflage paint scheme. I modeled a set after the wing tips of a 337 Cessna and they looked great. After flying the plane for a while I started to wonder if the tips were having any affect on speed so I took one off and installed one of the supplied flat tips.
With a flat tip on the right wing and my beautiful tip on the left side I went for a flight. As soon as I levelled off I could actually feel a difference. The plane was yawing and correcting continuously. The right wing would move ahead, or more correctly the left wing would lag until the plane got to a point where either the dihedral effect, or the fuselage drag caused the plane to bump back to straight flight. It was quite interesting to experience and it was obvious to me that my tips were causing more drag than I had expected. I removed the left tip and installed the other supplied flat tip. I would estimate, based on my three leg gps trials, that I gained between one and two knots. After telling a few people about my test I was informed that Steve Wittman used to do exactly this type of testing on his wing designs.
I may have given up a bit of low end lift and a better look, but that amount of speed loss was too much for me to pass on so the flat tips have remained. Now that I have gotten used to them I kind of like the look and it has made me wonder about the glass tips on the tail surfaces?.