Empennage

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 "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.
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 unite 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:
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:
Some locations have been given special names:
Fins.
The fin comprises the fixed vertical stabiliser and rudder. Besides its profile, it is characterised by:
Twin fins may be mounted at various points:
Unusual fin configurations include:
V, Y and X tails.
An alternative to the fin-and-tailplane approach is provided by the V-tail and X-tail designs. Here, the tail surfaces are set at diagonal angles, with each surface contributing to both pitch and yaw. The control surfaces, sometimes called ruddervators, act differentially to provide yaw control (in place of the rudder) and act together to provide pitch control (in place of the elevator).
Outboard tail.
An outboard tail is split in two, with each half mounted on a short boom just behind and outboard of each wing tip. It comprises outboard horizontal stabilizers (OHS) and may or may not include additional boom-mounted vertical stabilizers (fins). In this position, the tail surfaces interact constructively with the wingtip vortices and, with careful design, can significantly reduce drag to improve efficiency, without adding unduly to the structural loads on the wing.
The configuration was first developed during World War II by Richard Vogt and George Haag at Blohm & Voss. The Skoda-Kauba SL6 tested the proposed control system in 1944 and, following several design proposals, an order was received for the Blohm & Voss P 215 just weeks before the war ended. The outboard tail reappeared on the Scaled Composites SpaceShipOne in 2003 and SpaceShipTwo in 2010.
Tailless aircraft.
A tailless aircraft (often tail-less) traditionally has all its horizontal control surfaces on its main wing surface. It has no horizontal stabiliser - either tailplane or canard foreplane (nor does it have a second wing in tandem arrangement). A 'tailless' type usually still has a vertical stabilising fin (vertical stabiliser) and control surface (rudder). However, NASA adopted the 'tailless' description for the novel X-36 research aircraft which has a canard foreplane but no vertical fin.
The most successful tailless configuration has been the tailless delta, especially for combat aircraft.