The two blade teetering head design is not peculiar to Robinsons.
Among those heads I've maintained and overhauled are the Bell 206, and Hiller UH12-E and Soloy conversions.
Others are any of the Bells, from the Bell 47 (Souix in the RNZAF), UH1-D ( Iroquois) and civil derivatives up to the 212/214, and the AH-1 Cobra......
Mast bump is a possibility with that head design, but none of those machines mentioned fall out of the sky with the regularity of the Robinson.
Pretty easy for Robinson to point the finger at the pilot, when for the majority of the time the poor bugger isn't around to defend himself.
Myself and another member here had first hand experience of these POS's, we were interviewed by NZCAA as the last people to ride in a well known operator's R-66 prior to its fatal crash while returning to base after dropping us into our block. The thinking was he encountered turbulence/burble going through a high pass and the disc became unloaded, the mast bump scenario. RIP Mark.
"Death on a budget" is how I've heard a high hours fling winger refer to them.
I save my hard earned and pay for the 500...
B. ( Fixer of flying objects since Feb 7, 1977 )