Abstract
When a feedback system is devised to control a mechanical member that is structurally limber, unstable (“self-excited”) vibrations may be encountered at approximately a natural frequency of the structural member. Cures are generally easy to effect once the phenomena are understood. Two interesting cases are described: ground vibrations of an airplane control system due to a limber fuselage, and vibrations of a stable platform system due to limberness in the platform structure. The investigations are carried out using the root-locus technique, which provides a plot of system characteristics as explicit functions of control strength. In the case of the stable platform, the analysis is found to be more reliable than physical intuition.