What was found
DEPARTURE, FLIGHT, WITH AIRSPEED CHANGES, AIRCRAFT YAW CHANGES NOTICEABLY. AFTER RUDDER IS RETRIMMED AIRCRAFT DUTCH ROLLS A FEW TIMES IN A JERKING MANNER. WITH INCREASING SPEEDS MORE RT TRIM IS REQUIRED. CREW RETURNED TO DEPARTURE AIRPORT, WHERE A LANDING WAS MADE WITHOUT INCIDENT. REMOVED AND REPLACED LOWER YAW DAMPNER COUPLER. YAW DAMPNER AND RUDDER OPS CHECK AND INDICATES NORMAL ON GROUND TEST. AIRCRAFT WAS RETURNED TO SERVICE.