HELICOPTER AERODYNAMICS WORKBOOK
The fully articulated rotor system incorporates more than two blades. Lead/lag is possible by
use of vertical hinge pins. Horizontal hinge pins allow for flapping. The movement of each
blade is independent of the other blades and independent in respect to the rotor head.
The term rigid as applied to rotor systems is generally misleading due to the considerable
flexibility in the systems. "Hingeless" may be a better description in most cases. The hub itself
bends and twists in order to provide for flapping, lead-lag, and pitch control.
The semi-rigid rotor system uses two rotor blades and incorporates a horizontal hinge pin
only for flapping. Pitch change movement is also allowed. We will spend most of our time
investigating this system since it is the type you will become most intimately familiar with first.
Semi-rigid rotor systems are attractive due to their simplicity. They are limited to two
blades, have fewer parts to maintain, and do not use lead-lag hinges. So how does the semi-rigid
system compensate for geometric imbalance? Remember, the semi-rigid system uses
underslinging. This underslung mounting is designed to align the blade's center of mass with a
common flapping hinge (figure 2-14) so that both blades' centers of mass vary equally in
distance from the center of rotation during flapping. The rotational speed of the system will tend
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.
ROTOR BLADE AERODYNAMICS 2-11