Reversible electric machine for aircraft

The machine (1) has electric devices (10, 20) e.g. motor and motor-generator, comprising rotors (12, 22) co-operating with stators (11, 21). A disengageable coupling unit (30) attaches or disconnects the rotors during rotation. Another disengageable coupling unit (40) mechanically connects one of the rotors to an output shaft (50) under predetermined effort. The motor transmits a mechanical power to the shaft. The motor-generator transmits a complementary mechanical power to the shaft in a driving mode and receives the mechanical power from the shaft in a generating mode. An independent claim is also included for a hybrid power plant of a rotary wing aircraft.