when disturbed from a given pitch, yaw, or bank. This type  of stability also has three subtypes:  [Figure 5-22]  considered to be the most affected by certain variables in  various flight conditions.  Stability in an aircraft affects two areas significantly:  In designing an aircraft, a great deal of effort is spent in  developing the desired degree of stability around all three  axes. But longitudinal stability about the lateral axis is  The Center of Lift (CL) in most asymmetrical airfoils has a  tendency to change its fore and aft positions with a change in  the AOA. The CL tends to move forward with an increase in  AOA and to move aft with a decrease in AOA. This means  To obtain static longitudinal stability, the relation of the  wing and tail moments must be such that, if the moments  are initially balanced and the aircraft is suddenly nose up,  the wing moments and tail moments change so that the sum  of their forces provides an unbalanced but restoring moment  which, in turn, brings the nose down again. Similarly, if the  aircraft is nose down, the resulting change in moments brings  the nose back up.  In analyzing stability, it should be recalled that a body free  to rotate always turns about its CG.  Longitudinal stability is the quality that makes an aircraft  stable about its lateral axis. It involves the pitching motion  as the aircraft’s nose moves up and down in flight. A  longitudinally unstable aircraft has a tendency to dive or  climb progressively into a very steep dive or climb, or even  a stall. Thus, an aircraft with longitudinal instability becomes  difficult and sometimes dangerous to fly.  Static longitudinal stability, or instability in an aircraft, is  dependent upon three factors:  • Positive dynamic stability—over time, the motion  of the displaced object decreases in amplitude and,  because it is positive, the object displaced returns  toward the equilibrium state.  • Neutral dynamic stability—once displaced, the  displaced object neither decreases nor increases in  amplitude. A worn automobile shock absorber exhibits  this tendency.  • Negative dynamic stability—over time, the motion  of the displaced object increases and becomes more  divergent.  1. Location of the wing with respect to the CG  2. Location of the horizontal tail surfaces with respect  to the CG  3. Area or size of the tail surfaces  • Maneuverability—the quality of an aircraft that  permits it to be maneuvered easily and to withstand  the stresses imposed by maneuvers. It is governed by  the aircraft’s weight, inertia, size and location of flight  controls, structural strength, and powerplant. It too is  an aircraft design characteristic.  •  Controllability—the capability of an aircraft to  respond to the pilot’s control, especially with regard  to flight path and attitude. It is the quality of the  aircraft’s response to the pilot’s control application  when maneuvering the aircraft, regardless of its  stability characteristics.  Longitudinal Stability (Pitching)  Figure 5-22.  Damped versus undamped stability.  5-15  Displacement  Positive static (neutral dynamic) Positive static (positive dynamic) Positive Static (negative dynamic) Time Damped oscillation  Undamped oscillation  Divergent oscillation 