Which of the following is NOT a component of the main rotor system?

The main rotor system of a helicopter consists of several crucial components that work together to enable controlled flight. The main rotor hub is the structure that connects the rotor blades to the helicopter, acting as the central component from which the rotor blades extend. The pitch control rods are linked to the swashplate assembly, which is responsible for altering the angle of the rotor blades as they rotate, allowing the pilot to control lift and maneuver the aircraft.

The swashplate assembly itself is integral to the main rotor system, facilitating the transmission of pilot input from the cyclic and collective controls to the rotor blades. These components are vital for the operation of the rotor system, ensuring that the helicopter can respond precisely to pilot commands.

On the other hand, a piston assembly is not part of the main rotor system. Instead, it is typically associated with the helicopter's engine or hydraulic systems, which may assist in operating other functions of the aircraft but do not directly contribute to the rotor dynamics. Therefore, the correct answer identifies the pistons as components external to the main rotor system, emphasizing the specific roles of the other three components within that critical structure.