Static stops Pitch horn Main rotor mast Feathering hinge Teetering hinge Center of gravity Center of gravity Mast axis Figure 4-3.  With an underslung rotor, the center of gravity (CG) remains in the same approximate location relative to the mast before  and after rotor tilt. 4-3 Rigid Rotor System Figure 4-2.  The teetering hinge allows the main rotor hub to tilt, and  the feathering hinge enables the pitch angle of the blades to change.  semirigid rotor system dictates downward flapping of the  blades must have some physical limit. Mast bumping is the  result of excessive rotor flapping. Each rotor system design  has a maximum flapping angle. If flapping exceeds the design  value, the static stop will contact the mast. The static stop is  a component of the main rotor providing limited movement  of strap fittings and a contoured surface between the mast  and hub. It is the violent contact between the static stop and  the mast during flight that causes mast damage or separation.  This contact must be avoided at all costs. Mast bumping is directly related to how much the blade  system flaps. In straight and level flight, blade flapping is  minimal, perhaps 2° under usual flight conditions. Flapping  angles increase slightly with high forward speeds, at low  rotor rpm, at high-density altitudes, at high gross weights, and  when encountering turbulence. Maneuvering the aircraft in a  sideslip or during low-speed flight at extreme CG positions  can induce larger flapping angles. The underslung rotor system mitigates the lead/lag forces by  mounting the blades slightly lower than the usual plane of  rotation, so the lead/lag forces are minimized. As the blades  cone upward, the center of pressures of the blades are almost  in the same plane as the hub. Whatever stresses are remaining  bend the blades for compliance. Helicopters with semirigid rotors are vulnerable to a  condition known as mast bumping which can cause the rotor  flap stops to shear the mast. The mechanical design of the  The rigid rotor system shown in  Figure 4-4  is mechanically  simple, but structurally complex because operating loads  must be absorbed in bending rather than through hinges. In  this system, the blade roots are rigidly attached to the rotor  hub. Rigid rotor systems tend to behave like fully articulated  systems through aerodynamics, but lack flapping or lead/ lag hinges. Instead, the blades accommodate these motions  by bending. They cannot flap or lead/lag, but they can be  feathered. As advancements in helicopter aerodynamics 