Tailplane

Tailplane

A tailplane, also known as horizontal stabilizer, is a small lifting surface located 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

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. Without a tailplane there would be only one combination of speed and center of gravity position for which this requirement was met. The tailplane provides a balancing force to maintain equilibrium for different speeds and center of gravity positions. Because the tailplane is located some distance from the center of gravity, even the small amount of lift it produces can generate a large pitching moment at the centre of gravity.

Stability

An aeroplane with a wing only is normally unstable in pitch (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 counteracts the natural instability of the wing and make the aircraft longitudinally stable. A stable aeroplane can be flown "hands-off" and will not depart significantly from its airspeed and pitch attitude.

Control

Elevator

A tailplane has a hinged flap 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.

All moving tail

In transonic flight, however, shockwaves generated by the tailplane render the elevator unusable. This was first discovered in 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.

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