pt.3: The dynamic coupling of rigid and elastic degrees of freedom of an airplane are described by two methods. In the first, coupling is described by the changes in airframe characteristic equation roots caused by the introduction of coupling terms to the equations of motion. The second method employs modal response coefficients to compare the relative amplitudes of rigid and elastic degrees of freedom at each coupled mode frequency. Simple literal expressions are obtained for each of these descriptors and physical interpretations given. Time vector diagrams are also used to show the major parameters affecting coupling. (Author).