Thanks - so yesterday's scenario (Merlin hovering on a virtually windless morning) is about as bad as it gets...
I have so far treated it as one would wake vortex i.e. take off from a point before the helo's takeoff position and maintain a flight path above where it has just been. Does anyone know how far and how fast the vortices spread outwards? (I assume they do, like wake vortices?)
Tim