I agree with your reasoning for the lift-off and hover. NO WAY, the recirculation would be so severe as to produce an inflow into the top of the rotor that removed virtually all of the incidence of the blade sections. If you were to continue adding collective pitch eventually you would have the lift force for the aerofoil section pointing backwards (i.e. DRAG, resisting rotoation) and just consuming torque and power without producing any lift in the vertical direction.
Furthermore, the whole process would be horribley unsteady, and their would be lots of uncommanded pitching and rolling that at the high power settling we are discussing are likely to rollover the aircraft. However, the comment about ring vortex is not accurate. Ring vortex is when the vortex structures in the wake build up under the disc and in the plane of the disc due to a high Rate of descent. In the tunnel case the same vortex structures would be recirculated by the tunnel wall and injested from the top of the disc. Much of the unpleasantness would result in the same manner, i.e. high loads, high vibration, uncommanded pitching and rolling but it wouldn't be Ring Vortex it would be Blade Vortex Interaction due to Recirculation.
The interesting one is high speed foward flight. Fundementally as long as the tunnel was big enough, then I see no reason why a helicopter could not be flown into it at speed, through it and out the other end. As we know in high speed flight the wake is skewed almost horizontally behind the helicopter, and the streamtube in which the wake can be considered to exist in the regime is much flatter and diagonally orientated. So the flow into and out of the rotor would be affected only subtley in high speed flight. I guess with a lightly loaded rotor and the helicopter flying at say 5ft, you would need about 1.5 rotor diamaters of head room in a semi-circular tunnel. (The number is a finger in the air guess I haven't done the sums!)
However, I think that the wake of the helicopter would be very severe, again due to recirculation and so a second helicopter would not be able to follow......which brings me onto the final point. Even if you could fly a helicopter in a tunnel, you couldn't chase a train - the wake from the train would flip you over!
Hope this helps
Open the flood gates!
CRAN (..........not Cron, or Crum......C R A N!)
