Empennage

Lore & Background

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.

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. In some aircraft the horizontal stabiliser and elevator are one unit, 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, and the rear section is the rudder, a movable aerofoil 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.

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.

Reader's Guide

The empennage is notable as the primary structure ensuring stability and control in flight, analogous to the feathers on an arrow. Its design encompasses a variety of configurations, each with specific advantages and trade-offs. The conventional tail, with vertical and horizontal stabilisers mounted to the rear fuselage, is the simplest and most common, used on around 60% of current aircraft and about 80% ever built, from general aviation types like the Cessna 172 to the largest airliners such as the Airbus A380. Cruciform tails place the horizontal stabilisers midway up the vertical stabiliser to avoid engine wake while avoiding some T-tail disadvantages. T-tails mount the horizontal stabiliser on top of the fin, offering better pitch control and glide ratio but being heavier and more prone to deep stalls. V-tail and X-tail designs combine pitch and yaw control into diagonal surfaces, potentially reducing drag and radar signature. Twin fins and twin booms provide alternative fin arrangements. Trim devices, such as trim tabs, adjustable stabilisers, bungee systems, anti-servo tabs, and servo tabs, are used to reduce the need for constant pilot pressure on controls. Multi-engined aircraft often have rudder trim tabs to manage asymmetrical thrust. The empennage's role in housing crash-survivable recorders underscores its structural significance.

Did You Know?

• Many early aircraft lacking a stabilising empennage were virtually unflyable.
• A T-tail is more likely to enter a deep stall and is more difficult to recover from a spin than a conventional tail.