ABSTRACT
It is well established that an airplane exhibits five distinct modal behaviour when disturbed from a level flight trim. Short period and phugoid modes are motions confined to longitudinal plane, and other three modes, Roll, Dutch roll, and Spiral can be strictly defined as off-longitudinal or lateral-directional motions. These off-longitudinal modes are completely decoupled from the longitudinal modes, and so, are treated exclusively using remaining off-longitudinal motion variables. Different time scales specific to lateral-directional modes are introduced in this chapter. An aerodynamic model including flow curvature effect is presented and lateral-directional perturbation equations are derived. First and second order modal approximations for the three lateral-directional modes are finally extracted based on time scales and motion residues.
