9 - Longitudinal dynamic stability

Abstract

The main aim of this chapter is to discuss the stability of aircraft in longitudinal motion. It first presents some general remarks on stability derivatives. Like all other aerodynamic coefficients, stability derivatives are functions of Reynolds and Mach numbers, the geometry of the body and its orientation to the airflow. These stability derivatives are restricted to small disturbances. It emphasizes that the result of all the experimentation is that in general the concept of stability derivatives is a valid one which can be relied on well away from incidences at which flow separations may occur. Also, in some cases, this can be validated at incidences at which flow separations are liable to occur. The chapter then deals with the estimation of the longitudinal derivatives. It provides solutions to the longitudinal equations. The longitudinal stability equations for the case of free motion are then solved. This is followed by a detailed discussion on the longitudinal modes thus obtained. Finally, it concludes with an appendix that describes the solution of longitudinal quartic using a spreadsheet.