Sikorsky,
"A
semi-rigid main rotor is always a 2 bladed rotor system. It gets its name from the fact that it does not have a lead-lag hinge." This is basically the same as what Nick said.
The following is an interesting story about a
semi-articulated rotor, which had three blades. I suppose that the hub was called "semi-articulated" because the blades did not have lead/lag hinges.
Semi-articulated rotor
Nick,
It appears that semi-rigid and semi-articulated may just be two ways of refering to the same thing.
The following two web pages may be of value.
Pictures of rotor hubs; including four of the Robinson hubs.
Technical information on the Robinson R-22 rotor.
Dave J.