Architecture of a multiple-engine helicopter propulsion system, and corresponding helicopter

An architecture of a propulsion system of a multi-engine helicopter, comprising:

• just one hybrid turboshaft engine configured to operate in a nominal operating mode, in which a shaft of a gas generator of the just one hybrid turboshaft engine rotates at a nominal speed and a combustion chamber of the said gas generator is ignited, and a standby mode during flight of the multi-engine helicopter, in which the shaft of said gas generator of the just one hybrid turboshaft engine rotates at a speed less than that of said nominal speed;
• at least one non-hybrid turboshaft engine that is not coupled to any restart packs, wherein during an economical flight phase, the at least one non-hybrid turboshaft engine operates in a stable flight mode and the hybrid turboshaft engine operates in said standby mode;
• just one restart pack configured for restarting the just one hybrid turboshaft engine in order to bring said just one hybrid turboshaft engine out of said standby mode and to reach said nominal operating mode and;
• just one auxiliary power unit that is connected to said just one restart pack and is capable of providing said just one restart pack, on demand, with power necessary for bringing said just one hybrid turboshaft engine out of said standby mode and to reach said nominal operating mode.

An architecture of a propulsion system of a multi-engine helicopter, comprising:

• a hybrid turboshaft engine configured to operate in a nominal operating mode, in which a shaft of a gas generator of the hybrid turboshaft engine rotates at a nominal speed and a combustion chamber of the said gas generator is ignited, and configured to operate in a standby mode during flight of the multi-engine helicopter, in which the shaft of said gas generator of the hybrid turboshaft engine rotates at a speed less than that of said nominal speed and the hybrid turboshaft engine is ignited, wherein during standby mode said shaft of the gas generator of the hybrid turboshaft engine rotates at a speed selected from a group consisting of between 60% and 80% of the nominal speed, between 20% and 60% of the nominal speed, and between 20% and 60% of the nominal speed with mechanical assistance;
• at least one non-hybrid turboshaft engine that is not coupled to any restart packs, wherein during an economical flight phase, the at least one non-hybrid turboshaft engine operates in a stable flight mode and the hybrid turboshaft engine operates in said standby mode;
• a restart pack configured for restarting the hybrid turboshaft engine during said flight in order to cause the hybrid turboshaft engine to leave said standby mode and to reach the nominal operating mode in less than ten seconds from receipt of a control signal indicative of an emergency condition;
• an auxiliary power unit connected to the restart pack and configured for providing said restart pack, on demand, with power required for causing the hybrid turboshaft engine to leave said standby mode and to reach the nominal operating mode, said auxiliary power unit arranged to provide power to non-propulsive equipment of the multi-engine helicopter.