empennage

Empennage (pronounced /ˌɑːmpɨˈnɑːʒ/ or /ˈɛmpɨnɪdʒ/) is an aviation term used to describe the tail portion of an aircraft. The empennage is also known as the tail or tail assembly; all three terms may be interchangeably used. The empennage gives stability to the aircraft and controls the flight dynamics of pitch and yaw. In simple terms the empennage may be compared to the fletching of an arrow, colloquially, "tail feathers".

Structure

The empennage of a Boeing 747-200.

Structurally, the empennage consists of the entire tail assembly, including the fin, 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 stabilizer and is used to balance and share lifting loads of the mainplane dependent on centre of gravity considerations by limiting oscillations in pitch. The rear section is called the elevator and is usually hinged to the horizontal stabilizer. The elevator is a movable airfoil that controls changes in pitch, the up-and-down motion of the aircraft's nose.

The vertical tail structure (or fin) has a fixed front section called the vertical stabilizer, used to restrict side-to-side motion of the aircraft (yawing). The rear section of the vertical fin is the rudder, a movable airfoil that is used to turn the aircraft in combination with the ailerons.

In every empennage, some arrangement is made for the provision of trim to allow minor adjustment of airflow over the control surface and to unload the pilot from the need to maintain constant pressure on the elevator or rudder controls. The trim may take the form of trim tabs on the rear of the elevators or rudder which act to force those surfaces in the desired direction.

The aircraft's 'black box' (cockpit voice recorder and flight data recorder) are often located in the empennage, because the aft of the aircraft better survives the destructive forces (in most crash scenarios).

Trim

For trim, a stabilizer may be hinged at its trailing edge, forward of the elevator and adjustably jacked a few degrees in incidence either up or down. Early aircraft had a spring in the control circuit which provided an adjustable preload in the desired direction. Multi-engined aircraft always have trim tabs on the rudder when asymmetric forces would impose unusual loads on the pilot's rudder controls and sophisticated light aircraft may incorporate rudder trim also, though more usually, a ground adjustable trim tab is used to elimintate undesired yaw in cruising flight.

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:

• No of tailplanes - from 0 (Tailless or canard) to 3 (Roe triplane)
• 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 flying tail.^[1] (General Dynamics F-111)

Some locations have been given special names:

• Cruciform: mid-mounted on the fin (Hawker Sea Hawk)
• T-tail: high-mounted on the fin (Gloster Javelin)

Fuselage mounted

Cruciform

T-tail

Flying tailplane

Fins

The fin comprises the fixed vertical stabiliser and rudder. Besides its profile, it is characterised by:

• No. of fins - from none (McDonnell Douglas X-36) to three (Lockheed Constellation).
• Location of fins - on the fuselage (over or under), tailplane, tail booms or wings

Twin fins may be mounted on:

• The tailplane (Avro Lancaster)
• Twin tail booms (de Havilland Vampire)
• The wings (F7U Cutlass)^[2]

Tailplane mounted

Twin tailboom

Wing mounted

Unusual fin configurations include:

• Triple fins (Lockheed Constellation)
• Ventral fin, often in addition to a conventional fin (Dornier Do 335)

Triple fins

Ventral fin

Other tail configurations

An alternative to the fin-and-tailplane approach is provided by the V-tail. Here, two angled tail surfaces act differentially to provide yaw control (in place of the rudder) and together to provide pitch control (in place of the elevator).

The Pelikan tail is an all-flying variation on the V tail. It was proposed for the Boeing X-32 but abandoned.

V-tail

Pelikan tail

See also

• Fuselage
• Trijet
• S-duct

Notes and references