A method for controlling a thermal and electrical power plant for setting in motion at least one rotary member of a rotorcraft, the power plant comprising at least one heat engine and an electrical system provided with at least one electric machine. The method comprises: selecting, with a selector, an operating mode chosen from several operating modes; determining a density altitude and comparing, with a controller, the current density altitude and a threshold density altitude; and controlling, with the controller, the at least one electric machine depending on at least the chosen operating mode as well as the comparison and a necessary power to be supplied to the power transmission system.

A method of and system for operating a hybrid-electric propulsion system is provided. The system has a thermal engine and an electrical propulsion subsystem having a plurality of components. The method includes providing a set of original engine ratings for operating the hybrid-electric propulsion system, the original engine ratings are based on one or more first performance/capability parameters of an original electrical propulsion subsystem component; replacing the original component with an alternative component, the alternative component having one or more second performance/capability parameters, wherein the second performance/capability parameters are different from the first performance/capability parameters; producing a set of alternative engine ratings based on the second performance/capability parameters; and operating the hybrid-electric propulsion system using the set of alternative engine ratings.

A method of and system for operating a hybrid-electric propulsion system is provided. The system has a thermal engine and an electrical propulsion subsystem having a plurality of components. The method includes providing a set of original engine ratings for operating the hybrid-electric propulsion system, the original engine ratings are based on one or more first performance/capability parameters of an original electrical propulsion subsystem component; replacing the original component with an alternative component, the alternative component having one or more second performance/capability parameters, wherein the second performance/capability parameters are different from the first performance/capability parameters; producing a set of alternative engine ratings based on the second performance/capability parameters; and operating the hybrid-electric propulsion system using the set of alternative engine ratings.

A system is provided for an aircraft. This aircraft system includes a propulsor, a powerplant and a control system. The powerplant is configured to output mechanical power to drive rotation of the propulsor. The powerplant includes a heat engine and an electric machine. The heat engine is configured to provide a first portion of the mechanical power. The electric machine is configured to provide a second portion of the mechanical power. The control system is configured to operate the powerplant to control a rotational speed of the propulsor by adjusting the second portion of the mechanical power.

A system is provided for an aircraft. This aircraft system includes a propulsor, a powerplant and a control system. The powerplant is configured to output mechanical power to drive rotation of the propulsor. The powerplant includes a heat engine and an electric machine. The heat engine is configured to provide a first portion of the mechanical power. The electric machine is configured to provide a second portion of the mechanical power. The control system is configured to operate the powerplant to control a rotational speed of the propulsor by adjusting the second portion of the mechanical power.