Originally Posted by High Wing Drifter
Just to add that the only reason the ball isn't centred is because you have aileron induced drag on the up wing, so therefore you use rudder when you use aileron.
No. Something has to make the aircraft yaw.
To make a point mass go round a turn, all you have to do is apply a force at right angles to its direction of motion. But an aeroplane isn't a point mass. It has a nose and a tail, and it has not only to change its direction of motion like the point mass, but also its heading.
A conventional aeroplane with a fin applies yaw damping, i.e. it applies a yawing moment in the opposite direction to any yaw rate. Thus if you want a constant yaw rate, you must apply an into-turn yawing moment yourself. You can either do this with rudder, or you can allow the aircraft to slip out of the turn, which provides an into-turn yawing moment.
Thus there is a small residual requirement for rudder, even without aileron input.