From SKYbrary Wiki
Rigid rotor is a type of rotor system used on helicopters. The rigid rotor system resembles an ordinary propeller and is the least common type of rotor system. The helicopter blades are fixed to the rotor hub without hinges and the hub is fixed to the rotor shaft. The blades of rigid rotor systems are able to flex, which allows the blades freedom to feather but not to flap or lead. With a rigid rotor, the rotor head must absorb the forces generated during flight maneuvers. Control loads in this type of rotor head are high. However, this type of rotor system provides the most control power to the helicopter during flight.