The rotor wake changes within three distinct speed ranges:
Up to 20 mph, the air moves primarily downwards, most of it descending from the outer edges of the blades, so you get a relatively calm area around the fuselage (in other words, you are in the middle of a ring, like a doughnut - you can see this by hovering over water)
Image 1
At 18-22 mph (on a Bell), the annular ring shortens in the direction of movement to become an ellipse, coinciding with translation. Above 20 mph, the annular ring disappears, and a large amount of separate, small airflows coalesce to provide an area of ill-defined downward airflow
Image 2
Above 35 mph, two distinct rotating vortices are formed from directly behind the machine (they are fully developed about 1 rotor diameter behind the mast, and can be sustained for up to 2500 feet). Each vortex starts from where the annular ring would be in the hover, and is relatively calm in the centre (in fact, the centre-to-centre distance between them is just under the rotor diameter, and slightly displaced from the centre towards the retreating blade). Regard them as large funnels extending rearward and downward, getting bigger as they go, looking rather like those from a fixed wing, but still with a downward flow.
This is what it looks like from the side:
Image 3
Phil
PS. The piccies (if you can see them) are my copyright!
