It's the R44s inverted flight tanks which cause the ground resonance and because of its LTS tail construction it is more susceptible to tail wind conditions,this tail cone construction causes an accerated mass flow down through the advancing side of the rotor disc(similar to a convergent duct effect)creating a increase in velocity and decrease in pressure.this phenomenon would effect the surrounding air pressure substantially.The Astar does not suffer from these flight conditions because of its clockwise main rotor rotation.