A good presentation (unfortunately triggered by a fatality) of the path of turbulence from a helicopter…
YouTube > Lessons learned: helicopter wake turbulence / a dangerous phenomenon.
Main lesson. We often consider the downward turbulence when a helicopter is hovering (near stationary), but for helicopters with significant horizontally speed, the path of turbulence is at the same height, drifting with the wind and can last several minutes. We can be at risk when we think the helicopter is far away.
I experienced this at Torrey Pines on 2007/10/13, not fun.
Related discussion on PGF.
A video example: YouTube > Heli turbulence.