Large, vertical, moveable, flaplike devices attached to the vertical stabilizers on most aircraft, or movable vertical fins on a missile.
An aircraft’s or a missile’s rudder, a flap or a wing-shaped surface mounted at or near the craft’s rear, serves a purpose similar to that of a rudder on a ship. When the rudder is deflected to one side or the other, it produces a force and a resulting moment, or yaw, about the vehicle’s center of gravity. The force rotates the vehicle in the same direction as the deflection of the rudder.
Because rudders have been used for centuries to steer ships, early airplane designers naturally assumed that they could be used to steer airplanes. However, these designers often failed to anticipate the roll of the aircraft that resulted from the use of the rudder. When the rudder causes an airplane to yaw, it causes one wing to travel slightly more quickly through the air than the other and, hence, to produce more lift, which subsequently causes the airplane to roll in the direction of the turn. This roll was a problem with early airplanes, which flew very close to the ground, and required the use of ailerons and similar devices to control the resulting roll. Through experimentation, early aviators learned that the most successful turns are coordinated turns, made using a combination of rudder and ailerons.
On wingless missiles, the rudder is the only device used to make the vehicle turn. A missile’s rudder yaws the missile such that it flies at an angle to the airflow and develops a side-force on its body, or fuselage. This side-force produces the needed acceleration along the turn radius to carry the missile through the desired turn.
Airplane turns are more complex and require more than the use of a rudder. As noted above, when the rudder is deflected, the fuselage yaws, and the wings develop different lift forces. The wing on the outside of the turn develops a larger lift than does the wing pointing into the turn. The difference in lift between the wings results in a roll of the fuselage, which tilts or rotates the lifting force of the wings into the direction of the turn. Because the lifting force of the wings is much greater than the forces on any other part of the airplane, it is the tilted lift that provides the force to turn the airplane. When the turn is properly coordinated, the combination of yaw caused by the rudder, roll caused by the ailerons, and the slight increase in thrust will produce just the right amount of lift to balance the weight of the aircraft, so that the aircraft can make the turn without losing altitude.
The rudder must also be used to keep the airplane from yawing or turning when a multiengine airplane loses one of its engines. When a multiengine plane encounters an engine-out situation, the rudder must be used to produce enough yaw to counteract the effect of having more thrust on one side of the airplane than on the other. For this reason, multiengine airplanes have much larger rudders than do single-engine airplanes.
Another common use of the rudder is to cross-control an airplane, especially in its approach to landing. In an ideal landing, the atmospheric wind would be blowing straight down the runway. In the real world, the wind is often at an angle to the runway and, when landing or taking off, the pilot must adjust the flight of the plane to account for the crosswind. On takeoff, this is done by allowing the plane to yaw into the wind as soon as it leaves the ground and by flying away in a straight line extending from the runway centerline with the airplane turned somewhat into the wind in a slightly sideways motion. The approach to landing can be made in the same manner, with the plane yawed into the wind; at some point, the pilot must align the fuselage with the runway before the wheels touch down, so the aircraft can be properly controlled on the ground. To do this, the pilot uses the rudder to yaw the airplane until it is parallel to the runway and uses the ailerons to keep the wings level. This use of rudder and aileron is the opposite of that used in a turn and is referred to as cross-control.
The rudder is controlled on most aircraft by cables or hydraulic lines connected to pedals on the floor of the cockpit. The pilot presses the right rudder pedal to move the rudder and, thus, the nose of the aircraft, to the right, or presses the left rudder to rotate left. Modern airliners and fighters use power-augmented hydraulic or electrical systems to connect the rudder pedals to the rudder, and the rudder is often connected to an automated control system which will allow control of the airplane by a computer system.
Barnard, R. H., and D. R. Philpott. Aircraft Flight. 2d ed. Essex, England: Addison Wesley Longman, 1995. An excellent, nonmathematical text on aeronautics. Well-done illustrations and physical descriptions, rather than equations, are used to explain virtually all aspects of flight. Docherty, Paul, ed. The Visual Dictionary of Flight. New York: Dorling Kindersley, 1992. A profusely illustrated book showing the parts and the details of construction of a wide range of airplane types, old and new. An outstanding source of information about what airplanes and their parts really look like. Stinton, Darrol. The Design of the Airplane. London: Blackwell Science, 1997. An outstanding reference on the design of all types of aircraft. Slightly technical but well written and illustrated.
Ailerons and flaps
Flight control systems
Forces of flight
Roll and pitch