Aerodynamics of Flight

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