Dave & Lu,
In my opinion the Robinson hub is a small work of genius! [Here we go....]
Consider a standard Bell type semi-rigid teetering rotor as used on the JetRanger. As the rotor is loaded (as the collective is raised) the blades will bend upwards elastically since there is no coning hinge to allow the CF and Aero forces to find a natural equilibrium.
Since the blade has now been bent above the pitch change bearing axis (radial from hub) the drag forces on the blade can actually create an additional moment about the pitch change axis which will be dependent on azimuth position and flight regime.
Which in English means.....very large and fatigue inducing oscillating control loads - STICK SHAKE.
By adding a coning hinge, the hub cannot resist a bending moment at the blade root because its free to move, therefore, providing the pitch change bearing is outboard of the coning hinge - as it is, then the blade fundamental bending mode (curving upwards) is largely eliminated as is stick shake. Of coarse the higher harmonics are still present but the advantages in stick shake reduction are significant.
If you would like a further explanation of this have a look at dear Mr. Robinson's patent application on the US Patent site (do a search for tri-hinge rotor system)
In summary, without the coning hinges, we would have a cyclic that jumped around so much that helicopter pilots the world over would be suffering with 'vibration white finger'! Not to mention that blades would probabley have to be strengthened, and wouldn't last as long.
Hope this helps
CRAN
