The theory of the self-excited oscillation of the hinged blades of a helicopter rotor has been treated by Coleman and Feingold and their paper, although written several years ago, remains a standard reference on the subject. Since then, the scope of theoretical studies and calculations has been broadened to include hingeless rotors, more degrees of freedom and blade aerodynamic forces. All this has led inevitably to a great complexity in the equations of motion, with the result that, although the dynamicist can perform adequate calculations, he loses the insight and physical appreciation that he needs, for example, to identify the important parameters in the system. In order to maintain these qualities a different mathematical model is required, one that retains only the most basic parameters and yet still exhibits the fundamental ground resonance instability.