PHAK 5-15

Displacement Positive static (neutral dynamic) Positive static (positive dynamic) Positive Static (negative dynamic) Time Damped oscillation Undamped oscillation Divergent oscillation Figure 5-22. Damped versus undamped stability. when disturbed from a given pitch, yaw, or bank. This type of stability also has three subtypes: [Figure 5-22] • 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. Stability in an aircraft affects two areas significantly: • 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) 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 considered to be the most affected by certain variables in various flight conditions. 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: 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 In analyzing stability, it should be recalled that a body free to rotate always turns about its CG. 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. 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