Can a helicopter operate continuously in a small, enclosed, sealed environment?
A helicopter develops lift by accelerating air downwards through its rotors.  This relies on the air above the rotors being slow enough for the rotors to accelerate that air enough to provide sufficient lift. When hovering in still air, the incoming air has zero velocity relative to the helicopter. While of course the air is conserved, in a large volume of air, the downwash is able to diffuse much more, and much of it does not immediately circulate back up above the helicopter.

If the helicopter were enclosed in a sphere, the smaller the sphere, the less the air would be able to diffuse. The downwash would circulate back to the top of the sphere, where the rotors would take that air in again. The rotors would be less effective at accelerating the air that is coming in with significant velocity, just as a propeller develops less thrust with increasing forward speed. Successful hover would depend on the helicopter being able to develop lift at a rate of circulation where the air can slow down enough for the rotors to be effective.

This seems like a experiment manageable enough that someone could try it at home...