Hello Hal:

I've got a simpler answer that explains nothing at all. Some planes require differential (Edge 540), some don't (Ultimate).

Go out and fly your plane without differential. If it seems to yaw and everything else is set right, try it the next time with 15-20% diff. It will work, or it won't.

Can't figure out why biplanes never seem to need differential though. Must be too advanced a design concept.

Good night all,
Silversurfer