Main rotor hub Blade pitch horns Pitch change links Main rotor blades Main rotor blades Main rotor mast Figure 1-95.  Four-blade hingeless (rigid) main rotor. The hub is a single piece of forged rigid titanium. up or flap. When blade rotation reaches the point where the  blade becomes the retreating blade, the extra lift is lost and  the blade flaps downward.  [Figure 1-97] rotor blades are attached and move relative to the main rotor  hub. There are three basic classifications: rigid, semirigid,  or fully articulated.  The simplest is the rigid rotor system. In this system, the  rotor blades are rigidly attached to the main rotor hub and  are not free to slide back and forth (drag) or move up and  down (flap).  [Figure 1-95]  The forces tending to make the  rotor blades do so are absorbed by the flexible properties of  the blade. The pitch of the blades, however, can be adjusted  by rotation about the spanwise axis via the feathering hinges.  Fully articulated rotor blade systems provide hinges that  allow the rotors to move fore and aft, as well as up and  The semirigid rotor system in  Figure 1-96  makes use of a  teetering hinge at the blade attach point. While held in check  from sliding back and forth, the teetering hinge does allow  the blades to flap up and down. With this hinge, when one  blade flaps up, the other flaps down. Flapping is caused by a phenomenon known as dissymmetry  of lift. As the plane of rotation of the rotor blades is tilted and  the helicopter begins to move forward, an advancing blade  and a retreating blade become established (on two-bladed  systems). The relative windspeed is greater on an advancing  blade than it is on a retreating blade. This causes greater lift  to be developed on the advancing blade, causing it to rise  1-44 Figure 1-96.  The semirigid rotor system of the Robinson R22. Semirigid Rotor System Rigid Rotor System Fully Articulated Rotor System Blade grip Blade grip Blade pitch change horn Pitch link Coning hinge Teetering hinge Swash plate Coning hinge