Main Rotor System Fuselage Semirigid Rotor System A semirigid rotor system is usually composed of two blades  that are rigidly mounted to the main rotor hub. The main rotor  hub is free to tilt with respect to the main rotor shaft on what  is known as a teetering or flapping hinge. This allows the  blades to flap together as a unit. As one blade flaps up, the  other flaps down. Since there is no vertical drag hinge, lead/ lag forces are absorbed and mitigated by blade bending. The  semirigid rotor is also capable of feathering, which means that  the pitch angle of the blade changes. This is made possible  by the feathering hinge.  [Figure 4-2] The rotor system is the rotating part of a helicopter which  generates lift. The rotor consists of a mast, hub, and rotor  blades. The mast is a hollow cylindrical metal shaft which  extends upwards from and is driven and sometimes supported  by the transmission. At the top of the mast is the attachment  point for the rotor blades called the hub. The rotor blades are  then attached to the hub by any number of different methods.  Main rotor systems are classified according to how the main  rotor blades are attached and move relative to the main rotor  hub. There are three basic classifications: semirigid, rigid,  or fully articulated. Some modern rotor systems, such as the  bearingless rotor system, use an engineered combination of  these types. If the semirigid rotor system is an underslung rotor, the center  of gravity (CG) is below where it is attached to the mast. This  underslung mounting is designed to align the blade’s center  of mass with a common flapping hinge so that both blades’  centers of mass vary equally in distance from the center of  rotation during flapping. The rotational speed of the system tends to change, but this is restrained by the inertia of the  engine and flexibility of the drive system. Only a moderate  amount of stiffening at the blade root is necessary to handle  this restriction. Simply put, underslinging effectively  eliminates geometric imbalance.  [Figure 4-3] The fuselage, the outer core of the airframe, is an aircraft’s  main body section that houses the cabin that holds the crew,  passengers, and cargo. Helicopter cabins have a variety of  seating arrangements. Most have the pilot seated on the right  side, although there are some with the pilot seated on the  left side or center. The fuselage also houses the engine, the  transmission, avionics, flight controls, and the powerplant.  [Figure 4-1]  Main rotor system Landing gear Tail rotor system Airframe Fuselage Transmission Powerplant Figure 4-1.  The major components of a helicopter are the airframe, fuselage, landing gear, powerplant, transmission, main rotor  system,and tail rotor system.  4-2