Purpose of Main Rotor Hub Horizontal Hinges

The horizontal hinge (HH) has its axis in the plane of rotation of the hub, perpendicular to the longitudinal axis of the. blade (Figure 35a). The blade thrust develops a moment which rotates the blade about this hinge. The thrust moment M.^, = Та causes rotation of the blade relative to this hinge, /50

and this means that the moment is not transmitted to the hub (the thrust overturning moment is eliminated) (Figure 35b).

When the horizontal hinge is used, the thrust-force bending moment at the root of the blade becomes zero, thus unloading the root section; the blade bending is reduced and therefore blade fatigue stresses are reduced and blade service life is increased. The vibrations caused by azimuthal variation of the blade thrust-force moment are also reduced. Summarizing, we can say that the horizontal hinges are intended to:

eliminate the main rotor thrust overturning moment in the forward flight regime;

relieve the blade root section of the thrust bending moment;

reduce fatigue stresses in the blade and vibrations caused by azimuthal variation of the blade thrust moment.

In addition, the horizontal hinges simplify control of the main rotor and helicopter, improve helicopter static stability, and reduce the magnitude of the azimuthal blade thrust variation.