An aircraft propulsion system in which a combustion engine is arranged to drive an electrical generator. An electrical energy store is provided within the system. A propulsive rotor is arranged to be driven by an electric motor and a controller selectively varies the supply of power to the electric motor from the generator and/or energy store in dependence on one or more property of a vapor trail resulting from the engine exhaust flow. The controller may also control the supply of power to the energy store for charging.