The main rotor system on a helicopter is classified into several groups. Which of those listed is NOT one of these groups?

The classification of helicopter rotor systems is essential for understanding how different designs affect the performance and handling of the aircraft. The correct answer, the flexible-beam rotor system, is not a standard classification of rotor systems.

Helicopter rotor systems primarily include fully articulated, rigid, and teetering rotor systems, each characterized by their unique design and mechanics. A fully articulated rotor system allows for blade movement in multiple directions, providing greater flexibility to accommodate aerodynamic forces during flight. A rigid rotor system, on the other hand, has blades that are fixed at the hub, allowing for minimal flexing, which can enhance responsiveness and stability. The teetering rotor system features blades that can pivot around a central hub, enabling a free movement that can adapt to changing forces and balance the aircraft.

The flexible-beam rotor system does not represent a recognized or established group in the context of rotor configurations as it does not conform to the typical operational designs found in modern helicopters. Therefore, identifying it as the option that does not belong aligns with established helicopter rotor system classifications.