Rudder is used in normal level turns to correct adverse yaw. Adverse yaw is caused by the downward moving aileron producing more drag than the upward moving aileron. This causes a yaw (about the vertical axis) away from the direction of the bank and intended turn. In other words an intended turn and bank to the left will cause the nose to swing to the right.

Normally in full scale for coordinated flight the aileron and rudder is used together in the same direction to compensate for this yaw. Aileron differential applied either mechanically or electronically which lets the upward moving aileron move further up than the downward moving aileron moves down will help to minimise the adverse yaw.

Aileron 'to' rudder mix in which when the ailerons are moved will move the rudder also with a smaller percentage of throw will help to coordinate the turns. Note this mixing in full scale aircraft is common indeed and the fact is most modern general aviation aircraft does have some form of aileron to rudder mix. Usually done in the form of springs mechanically. Taxi most modern general aviation aircraft with the pedals and hand off the column and watch the ailerons respond when making sharp turns with your feet. Yep that is the mechanical mix.

John