Originally Posted by
3top
Also, it is NOT a Robinson only "problem". ANY helicopter can experience it.
Actually, I think that you'll find that it is a problem that is somewhat unique to helicopters with teetering rotorheads, and other head designs with a small effective hinge offset.
The key issue is that in a heli with a teetering head or small hinge offset, the torque to change the orientation of the helicopter fuselage is created by tilting the lift vector. When there is little or no lift (low or zero g situation), you can tilt the disk, but it isn't going to cause the fuselage to tilt in response.
On helicopters with a larger hinge offset, you get a "hub moment". Basically, whenever the rotor disk is not perpendicular to the mast, the disk will exert a torque on the fuselage in addition to that caused by tilting the lift vector. If the hinge offset is sufficiently large, then the helicopter has no control issueas at low/zero G. An example would be the bo105.