A rudder is a device used to steer a ship or other watercraft. In its simplest form, a rudder is a flat sheet of material attached with hinges to the ship's stern. A tiller - basically, a stick or pole - is attached to the top of the rudder to allow it to be turned in different directions.
Invention of the rudder
The world's oldest known depiction of a rudder can be seen on a pottery model of a Chinese junk dating from the 1st century CE. The West's oldest known rudder can be found on church carvings dating to around 1180, until when ships used oars for direction. The invention of the rudder in the West may have been independant, although it is most likely that the innovation was transmited through trade with the East.
Also, many junks incorporated "fenestrated rudders" (rudder with holes in them, allowing for better control), an innovation adopted in the West in 1901 to increase the manoeuvrability of torpedo boats.
Aircraft rudders
The tail of a KC135 Stratotanker, with the rudder marked
On an aircraft, the rudder is a control surface, usually attached to the fin (or vertical stabiliser ) which allows the pilot to control the aircraft in the yaw axis. It 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. By convention, the rudder is controlled with foot pedals, usually coupled to bell cranks on the rudder via wire cables. In some (rare) aircraft, there are no rudder pedals as the rudder is controlled with springs associated with the roll control. In some designs, an all-moveable fin is used instead of a rudder.
See also