LONGITUDINAL STABILITY

If an aircraft is stable about its lateral axis, it will not pitch up or down unless some force raises or lowers the nose of the aircraft. If the aircraft is statically stable about its lateral axis, it will resist any force which might cause it to pitch, and it will return to level flight when the force is removed.

This resistance to pitch-up and pitch-down motion is called longitudinal stability because the air- plane is stable along the longitudinal axis. Of the three types of stability of an aircraft in flight, this type is the most important.

To obtain longitudinal stability , the air- craft is designed so it is slightly nose- heavy. During normal flight, the aircraft has a continuous tendency to descend. However, this tendency is offset by the hori-zontal tail surfaces (stabilizer) being set at a negative angle of attack_ This position produces a downward, or negative, lift so the downward force on the tail exactly counteracts the nose hea@ness at a pre- determined aircraft speed. (See Fig. 1-47_)