Originally Posted by
bayou06
While I don't quite agree that the severity of the turbulence and unstable hovering flight would be tremendously severe (I believe the size and shape of the tunnel would be a significant factor), just much higher power required to hover.
Why exactly?
Imagine a helicopter hovering in a well just slightly wider than the rotor... no recirculation, because the only way for the air to get back out of the well is up through the disc.
Does it take less or more power than hovering in ground effect? In free air?
What happens if the rotor disc is near a ceiling?
What happens in vicinity of walls? Probably a question of stability...