A rudder is a primary control surface used to steer a ship, boat, submarine, hovercraft, airship, or other vehicle that moves through a fluid medium (usually air or water). On an airplane, the rudder is used primarily to counter adverse yaw and p-factor and is not the primary control used to turn the airplane. A rudder operates by redirecting the fluid past the hull or fuselage, thus imparting a turning or yawing motion to the craft. In basic form, a rudder is a flat plane or sheet of material attached with hinges to the craft's stern, tail, or afterend. Often rudders are shaped to minimize hydrodynamic or aerodynamic drag. On simple watercraft, a tiller—essentially, a stick or pole acting as a lever arm—may be attached to the top of the rudder to allow it to be turned by a helmsman. In larger vessels, cables, pushrods, or hydraulics may link rudders to steering wheels. In typical aircraft, the rudder is operated by pedals via mechanical linkages or hydraulics.
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Yaw (rotation) in the context of Empennage
The empennage (/ˌɑːmpɪˈnɑːʒ/ or /ˈɛmpɪnɪdʒ/), also known as the tail or tail assembly, is a structure at the rear of an aircraft that provides stability during flight, in a way similar to the feathers on an arrow. The term derives from the French language verb empenner which means "to feather an arrow". Most aircraft feature an empennage incorporating vertical and horizontal stabilising surfaces which stabilise the flight dynamics of yaw and pitch, as well as housing control surfaces.
Many early aircraft that lacked a stabilising empennage were virtually unflyable, despite having other effective control surfaces. Even so-called "tailless aircraft" usually have a tail fin (usually a vertical stabiliser). Heavier-than-air aircraft without any kind of empennage (such as the Northrop B-2) are rare, and generally use specially shaped airfoils whose trailing edge provide pitch stability, and rearward-swept wings, often with dihedral to provide the necessary yaw stability. In some aircraft with swept wings, the airfoil section or angle of incidence may change radically towards the tip.
Yaw (rotation) in the context of Aircraft principal axes
An aircraft in flight is free to rotate in three dimensions: yaw, nose left or right about an axis running up and down; pitch, nose up or down about an axis running from wing to wing; and roll, rotation about an axis running from nose to tail. The axes are alternatively designated as vertical, lateral (or transverse), and longitudinal respectively. These axes move with the vehicle and rotate relative to the Earth along with the craft. These definitions were analogously applied to spacecraft when the first crewed spacecraft were designed in the late 1950s.
These rotations are produced by torques (or moments) about the principal axes. On an aircraft, these are intentionally produced by means of moving control surfaces, which vary the distribution of the net aerodynamic force about the vehicle's center of gravity. Elevators (moving flaps on the horizontal tail) produce pitch, a rudder on the vertical tail produces yaw, and ailerons (flaps on the wings that move in opposing directions) produce roll. On a spacecraft, the movements are usually produced by a reaction control system consisting of small rocket thrusters used to apply asymmetrical thrust on the vehicle.