A rotor system refers to the bladed, propeller-like mechanism that is attached to the top of a helicopter. The system is made up of several blades connected to a hub, which is fastened to the aircraft’s mast. Rotor systems are responsible for providing a helicopter with vertical movement and propulsion. The three main variations of this system are articulated, semi-rigid, and rigid.
Helicopters rely on the main rotor, located on top of the craft, for both lift and thrust. Lift is responsible for vertical movement and for sustaining the weight of the helicopter once in the air, while thrust is responsible for propelling the craft once aloft. Thrust accounts for forward, lateral, and reverse movements. Similar to propeller driven airplanes, the movement of the blades through the air creates thrust, not the engine itself. Unlike propeller driven airplanes, the blades of the helicopter are also responsible for lift, which complicates movement once airborne.
While there are different rotor system designs, most contain the same key structural elements. Rotor systems are affixed to a central mast that rises from the helicopter’s engine and transmission. The mast is a single beam that is turned by the engine and causes the blades atop the mast to rotate. Below the blades is the hub, which consists of various mechanisms that allow the blades to be adjusted for flight in different environments.
In 2011, the majority of helicopters in service utilize an articulated rotor system. Articulated systems employ three or more blades, with hinges that allow each blade to move individually. The blades can move up and down relative to the hub, forwards and backward, and can be tilted relative to the central parallel axis running through the length of the blade. This is advantageous in most environments and less expensive than its hingeless counterpart.
Capable of the same function as an articulated system, a rigid rotor system forgoes the traditional hinges of the articulated type. Instead, the rigid system allows the same individual movement of each blade as the articulated system by utilizing composite flexures and elastomeric bearings. These materials allow the blades and portions of the rotor system to bend, negating any need for hinges. The bending blades reduce oscillation and improve the aircraft’s responsiveness. Rigid rotor systems are often used in specialized aircraft, such as those used by the military, where price is of no concern, but limited in other areas because of the high cost.
Semi-rigid rotor systems have only two blades, and neither is capable of independent movement. Instead, the entire hub moves atop the mast when in flight. This system is also called a seesaw rotor because of how the blades move. As the hub tilts and one blade moves down, the opposite blade is forced up. This system is not as frequently used as the other two.