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Horizontal stabilizer

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Horizontal stabilizer


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. Not all fixed-wing aircraft have tailplanes. Canards, tailless and flying wing aircraft have no separate tailplane, while in v-tail aircraft the vertical stabilizer, rudder, and the tail-plane and elevator are combined to form two diagonal surfaces in a V layout. The tailplane provides stability and control.

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.

Tailplane types

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

Equilibrium


An aeroplane must be in balance longitudinally in order to fly. This means that the net sum of the pitching moments about the center of gravity generated by the forces acting on the aeroplane must be zero; i.e. the overall center of lift of the aircraft must be vertically aligned with it. The aircraft is then said to be in trim.

A tailplane typically has a long moment arm from the centre of gravity, so trim forces applied by it can be small. It is often used to set and maintain trim, by adjusting the tailplane attitude, the elevator angle, or using smaller trim tabs hinged to the rear of the elevator.

As a flight progresses, the centre of gravity may shift due to fuel being used up or the payload changing. The centre of lift may also move due to changes in the aircraft speed and angle of incidence. At intervals during the flight the trim may need adjusting to compensate and restore equilibrium.

Stability

(See main article, Stabilizer (aircraft), for more references)

A wing with a conventional aerofoil profile makes a negative contribution to longitudinal stability. 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 may counteract the natural instability of the wing and make the aircraft longitudinally stable (much the same way a windvane always points into the wind).

The longitudinal stability of an aircraft may change when it is flown "hands-off"; i.e. when the flight controls are subject to aerodynamic forces but not pilot input forces.

Using a computer to control the elevator allows aerodynamically unstable 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.

On some pioneer designs, such as the Bleriot XI, the center of gravity was between the center of lift from the wings and the tailplane, which also provided positive lift. However this arrangement can be unstable and these designs often had severe handling issues. The requirements for stability were not understood until shortly before World War I when it was realised that moving the centre of gravity further forwards allowed the use of a non-lifting tailplane in which the lift is nominally neither positive nor negative but zero, which leads to more stable behaviour.[2] Later examples of aircraft that had positive lift tailplanes include Charles Lindbergh's Spirit of St. Louis, the Sopwith Camel and the Gee Bee Model R Racer - all aircraft with a reputation for being difficult to fly. But with care a lifting tailplane can be made stable. An example is provided by the Bachem Ba 349 Natter VTOL rocket-powered interceptor, which had a lifting tail and was both stable and controllable in flight.[3]

In many modern conventional aircraft, the center of gravity is placed ahead of the center of lift of the main wing. The wing lift then exerts a pitch-down moment around the centre of gravity, which must be balanced by a pitch-up moment (implying negative lift) from the tailplane. A disadvantage is that it generates trim drag.

Aircraft such as the F-16 are flown with artificial stability. The advantage of this is a significant reduction drag caused by the tailplane, and improved maneuverability.

Mach tuck

Main article: Mach tuck

At transonic speeds, and aircraft can experience a shift rearwards in the centre of lift due to the buildup and movement of shockwaves. This causes a nose-down pitching moment called Mach tuck. Significant trim force may be needed to maintain equilibrium, and this is most often provided using the whole tailplane rather than a small trim tab, in the form of an all-flying tailplane or stabilator.

Control

A tailplane usually has some means allowing 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.

Elevator A conventional tailplane normally has a hinged aft surface called an elevator,

Stabilator or all-moving tail In transonic flight shock waves generated by the front of the tailplane render any 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 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 Mach tuck and maintain maneuverability when flying faster than the critical Mach number. Normally called a stabilator, this configuration is often referred to as an "all-moving" or "all-flying" tailplane.

See also

References

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