THE SMALL-DISTURBANCE THEORY

A particular form of the system equations that has been used with enor­mous success ever since the beginnings of this subject is the linearized model for small disturbances about a reference condition of steady rectilinear flight over a flat Earth. This theory yields much valuable information and many important insights with relatively little effort. It gives correct enough results for engineering purposes over a surprisingly wide range of appli­cations, including stability and control response. There are, of course, limitations. It is not suitable for spinning, post-stall gyrations, nor any other application in which large variations occur in the state variables.

The reasons for the relative success of this approach are twofold: (i) in many cases the major aerodynamic effects are truly nearly linear functions of the state variables, and (ii) disturbed flight of considerable violence can correspond to relatively small values of the linear – and angular-velocity disturbances.