What is the purpose of the free-wheeling unit in a helicopter drive system?

The purpose of the free-wheeling unit in a helicopter drive system is to disengage the engine from the main rotor when the engine RPM is less than the rotor RPM. This function is crucial because it allows the rotor blades to continue spinning freely even if the engine fails or its power output drops. During such a situation, the rotor can maintain its momentum, which is vital for safe autorotation—a condition where the helicopter can glide down to the ground without engine power.

By allowing the rotor to decouple from the engine, the free-wheeling unit prevents damage that could occur from an overloaded or stalling engine trying to drive a rotor that is spinning faster. This design ensures a smooth glide and gives pilots control over the descent, enhancing safety during emergencies.