INSTABILITY DUE TO SIMULTANEOUS TIME-VARIATION IN DAMPING AND SPRING CONSTANTS

Parametric instabilities due to simultaneous time variation in both th e viscous damping and spring constants of a single-degree-of-freedom s ystem have been investigated. Two types of variations, viz., stepped a nd harmonic, are considered. Numerical results are presented for prima ry and secondary unstable zones. It is observed that the widest and na rrowest unstable zones occur if the variation in the damping and sprin g constants of same frequency are in quadrature with each other. A lit tle negative damping during some part of the cycle more than offsets t he stabilizing effect of correspondingly higher positive damping durin g some other part of the same cycle.