The empennage ( or ), 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 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.
In spite of effective control surfaces, many early aircraft that lacked a stabilising empennage were virtually unflyable. 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 rearwards 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.
Structure
Structurally, the empennage consists of the entire tail assembly, including the tailfin, the tailplane and the part of the fuselage to which these are attached. On an airliner this would be all the flying and control surfaces behind the rear pressure bulkhead.
The front (usually fixed) section of the tailplane is called the horizontal stabiliser and is used to provide pitch stability. The rear section of the tailplane is called the elevator, and is a movable aerofoil that controls changes in pitch, the up-and-down motion of the aircraft's nose. In some aircraft the horizontal stabilizer and elevator are one unit, and to control pitch the entire unit moves as one. This is known as a stabilator or full-flying stabiliser.
The vertical tail structure has a fixed front section called the vertical stabiliser, used to control yaw, which is movement of the fuselage right to left motion of the nose of the aircraft. The rear section of the vertical fin is the rudder, a movable aerofoil that is used to turn the aircraft's nose right or left. When used in combination with the ailerons, the result is a banking turn, a coordinated turn, the essential feature of aircraft movement.
Some aircraft are fitted with a tail assembly that is hinged to pivot in two axes forward of the fin and stabiliser, in an arrangement referred to as a movable tail. The entire empennage is rotated vertically to actuate the horizontal stabiliser, and sideways to actuate the fin.
The aircraft's cockpit voice recorder, flight data recorder and emergency locator transmitter (ELT) are often located in the empennage, because the aft of the aircraft provides better protection for these in most aircraft crashes.
Trim
In some aircraft trim devices are provided to eliminate the need for the pilot to maintain constant pressure on the elevator or rudder controls.
The trim device may be:
a trim tab on the rear of the elevators or rudder which act to change the aerodynamic load on the surface. Usually controlled by a cockpit wheel or crank. an adjustable stabiliser into which the stabiliser may be hinged at its spar and adjustably jacked a few degrees in incidence either up or down. Usually controlled by a cockpit crank. a bungee trim system which uses a spring to provide an adjustable preload in the controls. Usually controlled by a cockpit lever. an anti-servo tab used to trim some elevators and stabilators as well as increased control force feel. Usually controlled by a cockpit wheel or crank. a servo tab used to move the main control surface, as well as act as a trim tab. Usually controlled by a cockpit wheel or crank.
Multi-engined aircraft often have trim tabs on the rudder to reduce the pilot effort required to keep the aircraft straight in situations of asymmetrical thrust, such as single engine operations.
Tail configurations
Aircraft empennage designs may be classified broadly according to the fin and tailplane configurations.
The overall shapes of individual tail surfaces (tailplane planforms, fin profiles) are similar to wing planforms.
Tailplanes
The tailplane comprises the tail-mounted fixed horizontal stabiliser and movable elevator. Besides its planform, it is characterised by: Number of tailplanes – from none (tailless or canard) to three (Lockheed Constellation) Location of tailplane – mounted high, mid or low on the fuselage, fin or tail booms. Fixed stabiliser and movable elevator surfaces, or a single combined stabilator or "[all]-flying tail". (General Dynamics F-111 Aardvark)