I thought the vibration was due to the uneven forces across the disc, when inflow roll was at its worst. That period in slow acceleration when only part of the disc has moved out the induced flow. What would be causing a vibration at ETL?
the rotor has to pass through the tip vortices which, in still air IGE, are spread outwards from the rotor - it is passing through those vortices that creates the vibration of ETL.
If you do an IGE transition over longish grass (an inch or two) you can see the pattern of the outflow of the rotor on the grass and as you catch up with the forward edge of that, you will experience the vibration and the onset of ETL.
AC - I'm with you on that one - heard all sort of theories about the aircraft being sucked towards the obstacle or requiring more forward or aft cyclic to prevent the aircraft moving forward or back - never felt a problem either on ops or in training moving in from a stable OGE hover to close proximity to cliffs/hangars/buildings. if there is an effect it is negligible.
Full recirculation is a different matter - had a colleague make a very firm landing in a wriggly tin fort in South Armagh - we 'fell out of the sky' as we got to the hover and used a lot of unanticipated power to cushion the landing.