Rotor blade Lagging position Leading position Rotor hub Center of rotation Lead/lag hinge (Vertical hinge) Pure Radial Position Lead/lag or drag hinge Lagging position Leading position Figure 4-6.  Lead/lag hinge allows the rotor blade to move back  and forth in plane.  Figure 4-7.  Fully articulated flapping hub. Figure 4-8.  Fully articulated rotor blade with flapping hinge. 4-5 Following a single blade through a single rotation beginning  at some neutral position, as load increases from increased  feathering, it flaps up and leads forward. As it continues  Either way, some motion must be accommodated. The  centrifugal force is nominally constant; however, the flapping  force is affected by the severity of the maneuver (rate of  climb, forward speed, aircraft gross weight). As the blade  flaps, its CG changes. This changes the local moment of  inertia of the blade with respect to the rotor system and it  speeds up or slows down with respect to the rest of the blades  and the whole rotor system. This is accommodated by the  lead/lag or drag hinge, shown in  Figure 4-9,  and is easier to  visualize with the classical ‘ice skater doing a spin’ image.  As the skater moves her arms in, she spins faster because her inertia changes but her total energy remains constant (neglect  friction for purposes of this explanation). Conversely, as  her arms extend, her spin slows. This is also known as the  conservation of angular momentum. An in-plane damper  typically moderates lead/lag motion.  As the lift on a given blade increases, it tends to flap upwards.  The flapping hinge for the blade permits this motion and is  balanced by the centrifugal force of the weight of the blade,  which tries to keep it in the horizontal plane.  [Figure 4-8] As the rotor spins, each blade responds to inputs from the  control system to enable aircraft control. The center of lift  on the whole rotor system moves in response to these inputs  to effect pitch, roll, and upward motion. The magnitude of  this lift force is based on the collective input, which changes  pitch on all blades in the same direction at the same time. The  location of this lift force is based on the pitch and roll inputs  from the pilot. Therefore, the feathering angle of each blade  (proportional to its own lifting force) changes as it rotates  with the rotor, hence the name “cyclic control.” to the others. Fully articulated rotor systems are found on  helicopters with more than two main rotor blades.