Smooth EC130B4
The EC130B4 is very smooth up to about 125kts. At the recommended "fast cruise" of 130kts there is a low frequency oscillation, I guess you could call it a vibration, which develped when I fited emergency floats. Overall it is an extremely quiet and smooth helicopter, I am told the quietest light helicopter on the market, and the only one allowed to fly the Grand Canyon for that reason. I don't think there is anything magical about 90 kts, thats slow, and no advantage.
Cheers