It should turn with just rudder, regardless of Dihedral, dihedral will enhance it though. a plane turns on its rudder because when the rudder is deflected it pivots the aircraft, speeding up the relative wind over one wing, this causes more lift over that wing, which in turn causes it to rise up. the other wing will have a lower relative velocity, and thus will generate less lift, causing it to drop. The combination of these two factors causes the plane to bank, which in turn accelerates the turn.