It seems more and more planes are turning up with winglets. Just exactly what do these things do? Do they have any real affect on aerobatic planes and if so, what ?
Are there any guidelines on their shape or size (relative to the wing size)?

Thanks in advance.
Bill

"Side area". Enhances the rudder authority.

Also gives you a place to showcase your favorite RC vendors!

They help the plane track in more of a stright line in flight, and seriously reduce wing rock in a harrier. Its kind of like a non electronic gyro. Holds a knife edge rock solid.

Big advantage for reducing wing rock on harriers.

Side force generators make a huge difference in knife edge, and high alpha flight. True winglets, like in the pictured Edge 540 Hamilton scheme, not so much. This plane flew great as a racer or for cruising around upright. Inverted it was a handful. At first I was not impressed at all with the flight characteristics as it was extremely unstable while inverted. I removed the winglets and it was a completely new plane. My other planes I almost always have the SFG on.

On full-sized planes, winglets may only increase airfoil performance by (perhaps) 5%.
Side force generators are much larger in area than full-scale winglets, and so probably will have a lot more effect on flight.