I have a second order system with open-loop transfer function $G(s)={a\over(s+b)(s+c)}$, $a,b,c$ are already known. This system has unity feedback.
I am interested in the time consant of this system, but I need to know should I calculate based on the open-loop or closed-loop transfer function?
The second question is how to calculate the time consant of a second order system? On this webpage (Second Order Systems), it says a second order system may be the combination of two first order systems. If the time constant for the two first order system is $\tau_1$ and $\tau_2$, the time constant for the second order system is $\tau^2=\tau_1 \tau_2$. Can I compute according to this equation?