Thanks guys. I'm pretty comfortable with the idea that the downwash is blocked, and that as a result the relative airflow changes to more horizontal, and that the TR vector therefore moves more vertical etc. etc. From a teaching perspective that ought to pretty much cover it.
But suppose your student has done a bit more reading or listening in the flying club bar and comes up with the 'urban myth' of a ground cushion. My problem with simply dismissing it as an urban myth is that it seems to have a grain of truth in it.
My issue concerns the mechanism whereby the blocked downwash alters the airflow at the rotor ? Surely the dynamic pressure of the inflow hitting the ground is the mechanism. Now maybe to talk about as a 'ground cushion' is sloppy terminology, but surely it IS a pressure effect.
One of the counterarguments that is put foward against this 'myth' is the ground effect of a fixed wing which since it is travelling along cannot have a ground cushion. Well maybe not in the static sense, but you can easily envisage a dynamic ground cushion forming, a bow wave if you like, which is again a pressure effect causing the relative airflow changes.
I stress I'm not trying to make a strong theoretical point here, simply to try and separate the mythical from the factual. Maybe you can dismiss the ground cushion as sloppy terminology, but not dismiss pressure effects from being involved with Ground Effect ?