The three basic designs that have traditionally been taught to students are semi-rigid, fully articulated, and rigid. Today there are versions that make extensive use of composite materials and are known as hinge less systems.
Rigid rotor systems do not use hinges and limited movement is absorbed through the hub and rotor blades. Many of the modern composite rotor systems also do not use traditional hinges, but have elastomeric and specially designed composites structures (flextures) that allow the blades to flap, feather, and hunt. Manufactures do not use the term rigid rotor system, opting instead to describe these systems as a fully articulated hinge less rotor system.
One helicopter that has a composite hinge less rotor system is the Eurocopter AS350 AStar. The basic component of the rotor system is the Starflex with 3 arms made from a glass resin laminate and flexible in the flapping direction. On the Starflex rotor hub, rigid sleeves join the blades to the star arms so that the hub allows blade flapping, lead/lag and pitch change without bearings or hinges. The sleeves also transmit the blade centrifugal loads to the rigid center part of the star. For this reason, flexible couplings are fitted between the sleeves and the star arm. The inboard ends of the sleeves are attached to the star by a laminated spherical stop (also called a thrust bearing) which is flexible in torsion, flapping and lead/lag but rigid in compression. The blade end contains elastomeric blocks called frequency adapters that provide stiffness and damping but can move in a shear direction. This rotor hub design allows for all the required blade movement through the bending and twisting of composite or elastomeric materials.
Some advantages of the Starflex hub are less maintenance, a fail-safe design through the use of composite materials (any damage evolves very slowly and is visible) and on-condition maintenance (no required overhauls). Eurocopter is one of the first manufactures to use composite materials in rotor hub design and the AS350 Starflex has proven extremely reliable for over 30 years.
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