Originally Posted by fnnwizard
Then I found markdrela and read everyone of his posts. I can't find the exact post now, but I remember he wrote something like using diff is using drag to counter the yaw and using rudder is using lift so using rudder, drag is smaller than using diff. Something like that.
If I understand the aerodynamics of aileron inputs correctly, the aileron going down is the one that has more induced drag and the aileron going up (although it travels more) sees less airflow and therefor has less drag in comparison to the down aileron. Also, I believe the aileron that goes up should be sufficient in and of itself to roll the plane and counter adverse yaw, HOWEVER the roll rate would be really bad. So we include a bit of down aileron (even with it's higher drag component) on the opposite wing and we gain back the roll rate. So the idea of aileron differential is to maintain as much roll capability as we need with as little down aileron input as possible.
I do agree with your comments that in a coordinated turn the rudder does act as a lifting surface, but if I have to move it twice as much to keep the plane in a coordinated turn that's got to be twice as much induced drag, no other way around it.
I haven't read all of Mark's writings (although I've read many of them, but not the one you've posted), but I do see in his Supra setup, he doesn't use aileron differential??? I don't know how specific that is to Mark's Supra and if all Supra pilots fly theirs that way or not. I do know that in near dead air no lift conditions when I stuck my stick in the corner and held it there, and held it there, and held it there, my Xplorer didn't fall out of the sky (tip stall) it just slowly decreased in altitude. I also know that going into a normal turn the fuse tracked very nicely with the turn radius (no adverse yaw). These two "tests" said to me (in my young experience) this plane is setup right (or very close to) relative to aileron differential and aileron/rudder coupling. Of course there's more to learn and tinker with...