8 - Design

Summary

Rotor Configuration

The helicopter rotor type is largely determined by the construction of the blade root and its attachment to the hub. The blade root configuration has a fundamental influence on the blade flap and lag motion and hence on the helicopter handling qualities, vibration, loads, and aeroelastic stability. The basic distinction between rotor types is the presence or absence of flap and lag hinges, and thus whether the blade motion involves rigid-body rotation or bending at the blade root. A simple classification of rotor hubs has the categories articulated, teetering, hingeless, and bearingless, as sketched in Figures 8.1 to 8.4. With real designs (see Figure 1.2) the distinctions are not as clear as in these drawings.

An articulated rotor has its blades attached to the hub with both flap and lag hinges (Figure 8.1). The flap hinge is usually offset from the center of rotation because of mechanical constraints and to improve the helicopter handling qualities. The lag hinge must be offset for the shaft to transmit torque to the rotor. The purpose of the flap and lag hinges is to reduce the root blade loads (since the moments must be zero at the hinge) by allowing blade motion to relieve the bending moments that would otherwise arise at the blade root. With a lag hinge a mechanical lag damper is also needed to avoid a mechanical instability called ground resonance, involving the coupled motion of the rotor lag and hub in-plane displacement.