 Damped pendulum

In units where the small-amplitude natural frequency is one, linear velocity drag gives $\ddot{\theta }+c\dot{\theta }+\sin \theta =0$ with $c>0$. The even multiples of $\pi$ are attracting equilibria; the odd multiples are saddle equilibria. Small displacements obey a damped harmonic oscillator; the energy dissipation of a damped pendulum governs the global phase portrait.