One for the aerodynamicists, I think
Over the years I have read lots of documents about wake turbulence, most containing dire warnings of the risks associated with their presence. Almost every one talks about vortices being generated (on take-off) from the point of rotation, described often from what I recall as the point when the wing starts generating lift.
I really don't know much about how wings work but to my simple mind, as soon as the wing moves through the air around it has the potential to generate a vortex at the tip as the air going over and under the wing meets. Clearly, more significant vortices will be generated as speed increases, and they may not be a practical threat to other aircraft until rotation or thereabouts but it seems to me that it is incorrect to say that they are not generated until the wing produces lift (or something similar). Or am I wrong?
This is not completely an academic question - in the context of training a newly-minted air traffic controller I was asked a question about clearing a helicopter to cross a runway behind a departing aircraft. After dealing with jet blast and the like, there was a residual thought in my mind about the risk of the helicopter encountering wake vortex disturbances.
Any expert input welcome.