To: Vaqueroaero
On the Bell rotorheads placing the rotor mass below the teeter hinge minimizes the tendency to lead and lag. This is known as underslinging. When the Bell blade flaps the rotor mass tilts below the teeter hinge which places the rotating axis very close to the driving axis. On a multi blade helicopter that has blade offset from the driving axis when the blade flaps and the disc tilts there is a deviation between the two axes and this results in lead and lag. Some helicopters incorporate a lead lag hinge to permit movement of the blade on this axis. Other systems allow the blade to flex in plane accomplishing the lead lag in this manner or, the rotorhead had a flex beam to allow this movement. On the Bell system the two axes are not totally coincident with each other so there is a minor amount of lead/lag and the drag links on the larger bell models and the blade restraints on the smaller (206) types resist this. This results in a slight amount of in plane bending.
There is another phenomenon at work as well. All helicopters compensate for the tendency for in plane bending by moving the pitch axis slightly ahead of the rotor centerline. This compensates for the fact that it is nearly impossible to get the blade mass CG on the same line as the pitch axis and this causes an in plane bending of the blade. By offsetting the pitch change axis from the rotor centerline the tendency for bending is negated or minimized.
The Robinson helicopter is unique in the fact that the rotor head is not only free to teeter but also it is capable of flapping (on the cone hinges). When the blades teeter they are also free to flap to minimize the bending loads on the blades. The Robinson head is much like the Bell system in the fact that it is underslung for the same reasons as on the Bell. But since the blades are free to flap the disc center of drive deviates from the center of rotation. Here again, this induces leading and lagging. Since the blades are rigidized in the inplane rotation direction the blades will bend inplane at two times the rotor speed. This can cause problems such as flexing on the pitch axis, it can result in the fatigue of the blades and as stated above it can lead to the accelerated wear of the cone hinges and the teeter hinge as well as fatiguing the rotor mast.