Originally Posted by Chickenhawk1
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.
Countary to expectation this: The downwash flowing over the wall has a boundary layer, where the air is stagnant. This region causes the downwash to be pulled outwards, as a local rotation is set up (coanda effect - yup same as wing). This means that the conditions are right for the downwash to be returned, by squeezing past the blade end gap. Heli ends up as if in a weak vortex ring state. Net result is collective goes up so you need more power.
Originally Posted by Chickenhawk1
What happens if the rotor disc is near a ceiling?
The restriction to inflow means that to maintain the same disk pressure (which is what keeps the heli airborn), the velocity through the rotor is reduced. Net result is collective goes down, like ground effect, so you need less power.
Quite sad that i studied this stuff really...
Mart