Final Answer:
When a helicopter engine is running, a clutch mechanism called a freewheeling unit allows the rotor blades to rotate independently of the engine.
Step-by-step explanation:
In a helicopter, the rotor blades are essential for generating lift, and they need to continue spinning even if the engine fails. The freewheeling unit is a crucial component that allows the rotor blades to spin freely without being driven by the engine. When the engine is running, the freewheeling unit engages, connecting the engine to the rotor system and enabling it to drive the rotor blades.
However, if the engine fails or is turned off, the freewheeling unit disengages, allowing the rotor blades to spin freely. This feature ensures a controlled descent in case of engine failure, enhancing the safety of helicopter operations.