What is one purpose of the freewheeling unit required between the engine and the helicopter transmission?

The function of the freewheeling unit, which is essential in the connection between the engine and the helicopter transmission, is to provide a crucial safety mechanism during an engine failure. When the engine fails, the freewheeling unit automatically disengages the rotor from the engine. This allows the rotor to continue spinning due to the momentum generated by the helicopter's forward flight, which is critical for maintaining control and allowing for a safe autorotation descent.

This mechanism is vital because if the rotor were to remain directly connected to a non-functioning engine, it could lead to a catastrophic loss of control. By disengaging the rotor in such a scenario, the helicopter can use the remaining rotor momentum to gradually lose altitude safely.

The other options do not accurately describe the purpose of the freewheeling unit. For instance, while engaging the rotor in response to engine failure or connecting it during normal flight are both aspects of rotor system design, they do not reflect the specific function of the freewheeling unit, which focuses on disengagement during failure scenarios.