The Robinson is not a semi rigid like Bell  

Unlike any other helicopter, Robinson has an extremely unorthodox underslung system that has an added flapping hinge along with the teetering hinge. This is because they refused to design a blade that would flex accordingly. Do not assume this has the same design and characteristics as the typical Bell semirigid. 

This added flapping hinge which otherwise offsets blade flex, has caused countless rotor “divergence”  from the normal plane of rotation fatal accidents. In other words chopping the tail boom off. Add to that an incredibly low inertia system with no energy. 

This was a horrible design that should have never been certified from day one under part 27.