You are speaking of the world famous YNOT wing tip mod.

I don't know a dam thing about wind tunnels or calculating lift, however I know 3D.

The tips are great for stability and you don't want the plane more stable for 3D.

On the thick wing type planes, 3D/fun fly/profile, the tips make a huge difference. With them removed, the plane does not couple so much in knife edge and rolls faster.. (Hype, Flip, UCD, Harrier, Madness, Prozac, MOJO, Hovering Cobra, Switch Blade, Kat's, ect)

On the scale aerobatic planes it does not seem to make a difference either way. (Extra, Edge, Cap, Funtana, Vision. etc)

I figured it out on a Madness and on every other thick wing 3D plane I have owned since, I have removed the wing tips and found the exact same thing. On a couple of Flips, I left the ailerons the same and just removed the wing tip, so the aileron extended past the wing tip, funny looking, stupid roll rate.

Here are a couple of links that this is talked about. One is 4 years old.

http://www.rcuniverse.com/forum/fb.a...3&key=wingtips

http://www.rcuniverse.com/forum/fb.a...7&key=wingtips

http://www.rcuniverse.com/forum/fb.a...8&key=wingtips

Some pics of 3D friendly wing tip pics.