A rudder is a control surface, usually attached to the upright tail (or vertical stabilizer) which allows the pilot to control the aircraft in the yaw axis. Depending on the model, the rudder is not used to effect turns—the ailerons are used for that—but the rudder is necessary to correctly balance the various acting forces in a turn. Not all radio controlled aircraft have rudders; some steer via ailerons only (see: ParkZone P-51D Mustang) while some smaller park flyers steer with rudder only. On a model with both rudder and aileron control, the rudder (and in some instances, the nosewheel for ground control as is the case with most full-sized aircraft) is operated with lateral motion of the transmitter's left stick. Rudder-only models steer via the right stick.
By convention, the rudder on full-sized civil aviation aircraft is controlled with foot pedals, usually coupled to bellcranks on the rudder via wire cables. Airliners, jets and fighter aircraft have a hydraulically-controlled rudder.