Chapter 6 - Longitudinal Dynamics

Abstract

Publisher Summary This chapter explores the longitudinal dynamics of motion. The longitudinal dynamics of the aeroplane may be likened to a pair of loosely coupled mass-spring-damper systems. The interpretation of the motion of the aeroplane following a disturbance from equilibrium may be made by direct comparison with the behavior of the mechanical mass-spring-damper. However, the damping and frequency characteristics of the aeroplane are not mechanical in origin they derive entirely from the aerodynamic properties of the airframe. The chapter explains the dynamic stability modes. Both longitudinal dynamic stability modes are excited whenever the aeroplane is disturbed from its equilibrium trim state. The reduced order models are explored in detail. The longitudinal stability modes play a fundamental part in determining the longitudinal flying and handling qualities of an aircraft and it is essential that their characteristics must be correct if the aircraft is to be flown by a human pilot. The chapter explores the mode excitation. Since the longitudinal stability modes are usually well separated in frequency, it is possible to excite the modes more or less independently for the purposes of demonstration or measurement. The modes may be excited selectively by the application of a sympathetic elevator input to the trimmed aircraft. The methods developed for in-flight mode excitation reflect an intimate understanding of the dynamics involved and are generally easily adapted to the analytical environment.