An Analysis of Longitudinal Stability in Power-Off Flight With Charts for Use in Design

This report presents a discussion of longitudinal stability in gliding flight together with a series of charts with which the stability characteristics of any airplane may be readily estimated.

The first portion of this report is intended, primarily, for students of the subject. The relationships governing stability characteristics are derived from equations of equilibrium referred to moving axes that are tangent and perpendicular to the instantaneous flight path. It is shown that instability of the motion can arise only through an increase of linear and angular momentum in the system during one complete cycle. The interaction of events leading to increase or decrease of momentum during a cycle is explained in detail. The construction of charts showing the effects of the nondimensional parameters CL, CD, dCL/dα, dCD/dα, -mq, and -μmα upon the stability characteristics is explained and the effects of the more important of the aerodynamic and mass characteristics of the airplane, as revealed by the charts, are discussed.

The latter portion of the paper is devoted to a series of 40 related charts with which the dynamic stability of any airplane in power-off flight may be readily estimated. The use of the charts is explained in detail so that reference to the earlier discussion is unnecessary.