Tailplane - Stability

Stability

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

An aeroplane with a conventional wing and no 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. 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.