I built a Zenith 40 sometime back, which was a plans built 40 size pattern style airplane. I remember the designer stating that he had to redesign parts of the airplane to eliminate the roll coupling. In other words, the airplane would not roll when rudder was applied. My current flyer, the RCM Hot Dawg, also has very little roll coupling. Other airplanes, such as the GP Decathlon, have a lot of roll associated with the rudder. So much so that I almost lost it on takeoff a few times trying to maintain the heading on the ground with rudder.

My questions - What quality in an airplane design either introduces roll coupling or reduces it? What do designers do to eliminate it? How can you put an airplane into a stall/spin where the rudder will not drop a wing?

Mervin Friesen
http://mefriese.hsd.ca

The rudder does not drop the wing --

roll couple -basically is a setup where the rudder produces a force which rolls the the plane --as it yaws the plane
this can be in the same direction as the rudder is directed OR the roll may be in opposite direction.
The way the plane is layed out determines this.
high wing planes such as Piper Cub tend to roll in the direction of the rudder application.
low wing planes with no dihedral tend to roll in the other direction (right rudder=left roll)
There are many methods of changing the direction and amount of roll couple .
Basically - the force of the rudder is overcoming the stability of the wing
So the wing needs more (or less) dihedral to counter this force (from the rudder) -which is acting as a rolling force as well as a force which yaws the plane.
Maybe you want some lengthy tech explaination -- but this is the short version.

Well said, Dick!

And my Stearmann pitches down with rudder!
Makes heading corrections when landing very "interesting" !