Published online by Cambridge University Press: 04 July 2016
A new approach to deriving analytical approximations for the free modes of motion of an aircraft is presented. Based on an asymptotic analysis, it eliminates the need for a priori assumptions regarding the characteristic motion of each mode, and is thus particularly suitable for an introductory exposition. In its basic form, it yields expressions for natural frequencies, damping factors and time constants which agree, to leading order, with well established results. At a more advanced level, it allows us to assess the validity of these results. In particular, we find that the two degree-of-freedom approximations to the short period oscillation and Dutch roll are asymptotically incorrect. This is unlikely to have a significant effect on the accuracy of the former, but is the reason for the poor damping predictions of the latter.