Characteristic Length and Dynamic Time Scale Associated with Aircraft Pitching Motion

Abstract

The 5 % -static-margin requirement once used by the military has been removed in favor of dynamic requirements. However, static margin is still commonly used as a preliminary guideline for establishing adequate pitch stability in piloted airplanes. Here, a dynamic margin is defined to be the distance that the airplane's stick-fixed maneuver point lies aft of the center of gravity, expressed as a fraction of the pitch radius of gyration. A proposed minimum-dynamic-margin constraint is shown to agree with the short-period-frequency requirements used in recent military specifications to an accuracy of about 2%.For some airplanes, this stability constraint is shown to differ from the 5 % -minimum-static-margin guideline by more than 400%. Two worked examples are presented, which demonstrate the significance of the proposed dynamic-margin constraint.