how do you lessen the roll coupling of a bipe(ultimate), when the rudder is applied- as in knife edge flight? i haven't built it yet (Sig's profile ultimate enlarged), so i was wondering if i could raise or lower the stab to lessen the coupling. right now it is located on the thrust line. i know it won't be totally eliminated, but was hoping to lessen it. on my 44" bipe it couples pretty good. the big one will be 88". i wonder if the bigger size alone will help in reducing the effect.

You can do a little, but not a lot. You can move the rudder aerodynamic center downward to be in line with the plane's CG (or as much as you can); do this by moving the rudder as-is downward or fattening up the portion of the rudder below the plane's CG. This will address the direct rolling action of the rudder, but what you cannot account for is the indirect rolling you will get- with the plane yawed in one direction (as when you are in a knife-edge), the wing that is more forward will produce more lift than the lagging wing (which is somewhat hidden from the wind by the fuselage), and your plane will roll in the same direction as yaw.

Moving the stab will not do anything.

If you have a computer radio, you might create a switchable mix of a little opposite aileron with rudder (which you can activate when you want to knife-edge, for instance).

No matter what you do, the plane cannot be set up to have no roll coupling in all regions of the flight envelope (e.g., fast and slow, heavy and light, etc.), so you will have to practice using that aileron to keep the plane still when yawing, whether knife-edging, doing a hammerhead, crabbing a landing, or doing a rudder turn.

Note that some planes will have more of an effect than others, and I believe the primary variables are the position of the wing relative to the fuse, fuse size, and the aspect ratio of the wing.

To get rid of the coupling, you need wing anhedral. In other words, the wings will have to sag towards the bottom of the plane. Another option is to tilt the outer cabane struts inward at their tops.

I cannot tell you how many degrees it will take of each to totally eliminate the coupling. It would have to be a trial an error affair.

What you were planning to do with the stab will affect the pitch coupling, but would not really change the roll coupling.

George Hicks has a forum, and is very knowledgable. He does this stuff for a living. If anyone can give you an idea of how many degrees anhedral to start with, he would be the one..........

CJ