The Bell AH-1 is an example of a semi-rigid rotor system.
A helicopter "rotor system" is the unit that is composed of
a hub and the blades attached to it. Each rotor is considered a separate
system, and a helicopter may have more than one main rotor.
There are three primary types of rotor systems: articulated, semi-rigid, and hingeless.
The articulated rotor system first appeared on the autogyros of the 1920s
and is the oldest and most widely used type of rotor system. The rotor blades
in this type of system can move in three ways as it turns around the rotor
hub and each blade can move independently of the others. They can move up
and down (flapping), back and forth in the horizontal plane, and can change
in the pitch angle (the tilt of the blade).
The Messerschmitt-Bölkow-Blohm BO-105 (Germany) of the late 1960s marked a significant transition from the Sikorsky-type articulated rotor to a high-performance hingeless rotor system.
The combination of a hingless rotor hub and advanced composite blades of the Westland Lynx (UK) allows a full range of aerobatic maneuvering and was critical in allowing the type to set the absolute speed record of helicopters (400km/hr).
In the semi-rigid rotor system, the blades are attached rigidly to the
hub but the hub itself can tilt in any direction about the top of the mast.
This system generally appears on helicopters with two rotor blades. These
systems resemble a seesaw—when one blade is pushed down, the opposite
one rises.
The hingeless rotor system functions much as the articulated system does, but uses elastomeric bearings and composite flextures to allow for flapping and lead-lag movements of the blades in place of conventional hinges.
Its advantages are improved control response with less lag and substantial improvements in vibration control. It does not have the risk of ground-resonance associated with the articulated type, but it is considerably more expensive.
A hingeless rotor system.
The Hughes TH-55A Osage has a fully articulated rotor system.
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