A tailplane, also known as horizontal stabilizer (or horizontal stabiliser), is a small lifting surface located on the tail (empennage) behind the main lifting surfaces of a fixed-wing aircraft as well as other non-fixed wing aircraft such as helicopters and gyroplanes. However, not all fixed-wing aircraft have tailplanes, such as those configured with canards (where the "tail-plane" is located in front), flying-wing aircraft, where there is no tail, and v-tail aircraft where the fin/rudder and tail-plane are combined to form two diagonal surfaces in a V layout. The tailplane serves three purposes: equilibrium, stability and control.
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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:
Fuselage mounted |
Cruciform |
T-tail |
Flying tailplane |
An aeroplane must be in balance longitudinally in order to fly. This means that the net effect of all the forces acting on the aeroplane produces no overall pitching moment about the centre of gravity.
In a conventional aircraft, the center of gravity is ahead of the center of lift, which would cause the aircraft to pitch forward without the downward force of the tailplane to balance this. In cases where the two forces are close together, the control inputs required to fly the aircraft may be too difficult to apply precisely enough for many pilots to maintain control of the aircraft.
Examples of aircraft that had this setup include Charles Lindbergh's Spirit of St. Louis, the Sopwith Camel and the Gee Bee R Racer - all aircraft with a reputation for being difficult to fly. With computer controls this is no longer a problem and aircraft as different as the Airbus and the F-16 are flown in this condition. The advantage to this is a significant reduction in induced drag caused by the tailplane, and in the case of the F-16, improved maneuverability.
In addition the tailplane helps adjust for changes in the center of lift, and center of gravity caused by changes in speed and attitude, or when fuel is burned off, or when cargo or payload is dropped from the aircraft.
Not all aircraft have tailplanes - the downward force is instead provided by the trailing edge of the wing on straight wing flying wings such as the Fauvel AV.36 and by the wingtips on swept wing flying wings. In both cases, because the force being applied is acting on a short moment arm, the forces must be larger, which incurrs a larger induced drag penalty that may outweigh the reduction in weight the lack of a fuselage brings, particularly for larger aircraft where weight is less critical than drag.
On some airplanes built before World War I, such as the Bleriot XI, the center of gravity is between the center of lift from the wings, and the tailplane, which instead of providing a downward force, provided an upward one, however there are severe handling issues with this arrangement that were beyond the capabilities of designers at the time to fix, so the layout was abandoned.
An aeroplane with a wing not designed to work independently of a tailplane is normally unstable in pitch (longitudinal instability). This means that any disturbance (such as a gust) which raises the nose produces a nose-up pitching moment which tends to raise the nose further. With the same disturbance, the presence of a tailplane produces a restoring nose-down pitching moment which counteracts the natural instability of the wing and makes the aircraft longitudinally stable - much the same way a windvane always points into the wind. A stable aeroplane can be flown "hands-off" and will not depart significantly from its airspeed and pitch attitude. Aircraft which have the center of gravity and centre of lift close together are considered to be unstable or having relaxed stability. Using a computer to control the elevator allows these aircraft to be flown in the same manner.
In addition to giving a restoring force (which on its own would cause oscillatory motion) a tailplane gives damping. This is caused by the relative wind seen by the tail as the aircraft rotates around the center of mass. For example when the aircraft is oscillating, but is momentarily aligned with the overall vehicle's motion, the tailplane still sees a relative wind that is opposing the oscillation.
A tailplane has a hinged surface called an elevator, which allows the pilot to control the amount of lift produced by the tailplane. This in turn causes a nose-up or nose-down pitching moment on the aircraft, which is used to control the aircraft in pitch.
In transonic flight, however, shock waves generated by the tailplane render the elevator unusable. An all-moving tail was developed by the British for the Miles M.52, but first saw actual transonic flight on the Bell X-1; fortunately, although the tailplane was conventional in design, Bell Aircraft Corporation had included an elevator trim device that could alter the angle of attack of the entire tailplane. This saved the program from a costly and time-consuming rebuild of the aircraft.
Transonic and supersonic aircraft now have all-moving tailplanes to counteract the Mach tuck and maintain maneuverability when flying faster than the critical Mach number. While technically called a stabilator, this configuration is often referred to as an "all-moving" or "all-flying" tailplane.
Most aircraft have tailplanes, but some have instead a canard wing at the front. Being at the end of a long fuselage, a tailplane may have a greater leverage than a canard. However, whereas a tailplane provides negative lift (i.e. it forces the tail down), a canard provides positive lift. So, if an aircraft requires 10 units of lift to fly, and has a tailplane giving 0.5 units of downforce, wings giving 10.5 units of positive lift are needed. A comparable canard wing gives 0.5 units of lift, so the main wing need provide only 9.5 units of lift. In this scenario, the main wings of a canard could be smaller and lighter (9.5 vs 10.5 units of lift), which would seem on the surface to be an advantage. However, in practice, the ability to add high lift devices (e.g., flaps, slats) to wings having a tailplane (not practical with a canard) allow much higher wing loading for a given stall speed, and this potential for higher wing loading more than compensates for the small drag penalty of the tailplane in most applications. It also bears noting that virtually all current high performance gliders (where the design goal is the highest possible overall lift-to-drag ratio) employ a tailplane rather than a canard.
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