An engine system for an aircraft comprising: a propulsor configured to drive an aircraft; a thermal combustion engine configured to drive the propulsor; an electric motor-generator connected to the thermal combustion engine and configured to drive the propulsor; a power converter configured to apply a torque to the electric motor-generator and generate electric energy from the torque; and an engine controller configured to: determine a current power output of the thermal combustion engine; determine a most fuel efficient power output of the thermal combustion engine based on current operating conditions; compare the current power output with the most fuel efficient power output; and increase or decrease the torque applied to the electric motor-generator so as to increase or decrease a load on the thermal combustion engine, wherein the torque is increased or decreased by an amount required to increase or decrease the power output of the thermal combustion engine to the determined most fuel efficient power output; wherein: the engine controller is configured to increase the torque applied to the electric motor-generator by the power converter if the current power output is below the most fuel efficient power output, so as to increase the load on the thermal combustion engine; and/or the engine controller is configured to decrease the torque applied to the electric motor-generator by the power converter if the current power output is above the most fuel efficient power output, so as to decrease the load on the thermal combustion engine. a propulsor configured to drive an aircraft; a thermal combustion engine configured to drive the propulsor; an electric motor-generator connected to the thermal combustion engine and configured to drive the propulsor; a power converter configured to apply a torque to the electric motor-generator and generate electric energy from the torque; and an engine controller configured to: determine a current power output of the thermal combustion engine; determine a most fuel efficient power output of the thermal combustion engine based on current operating conditions; compare the current power output with the most fuel efficient power output; and increase or decrease the torque applied to the electric motor-generator so as to increase or decrease a load on the thermal combustion engine, wherein the torque is increased or decreased by an amount required to increase or decrease the power output of the thermal combustion engine to the determined most fuel efficient power output; determine a current power output of the thermal combustion engine; determine a most fuel efficient power output of the thermal combustion engine based on current operating conditions; compare the current power output with the most fuel efficient power output; and increase or decrease the torque applied to the electric motor-generator so as to increase or decrease a load on the thermal combustion engine, wherein the torque is increased or decreased by an amount required to increase or decrease the power output of the thermal combustion engine to the determined most fuel efficient power output; wherein: the engine controller is configured to increase the torque applied to the electric motor-generator by the power converter if the current power output is below the most fuel efficient power output, so as to increase the load on the thermal combustion engine; and/or the engine controller is configured to decrease the torque applied to the electric motor-generator by the power converter if the current power output is above the most fuel efficient power output, so as to decrease the load on the thermal combustion engine. the engine controller is configured to increase the torque applied to the electric motor-generator by the power converter if the current power output is below the most fuel efficient power output, so as to increase the load on the thermal combustion engine; and/or the engine controller is configured to decrease the torque applied to the electric motor-generator by the power converter if the current power output is above the most fuel efficient power output, so as to decrease the load on the thermal combustion engine.