Pressure bubble
Two remarks
1. The fact that I claim that the static pressure under that rotor is higher in IGE than in OGE does NOT mean I am advocating an extrapolation of the metaphore to the point of stating that the heli sits on a (spherical) pressure bubble. May be a better metaphore could be : put a propeller on the top part of a tube that you put vertically on the ground. On the bottom side of the tube you have some air escape holes, that you can make smaller (IGE) or bigger (OGE)
2. Instability comes from a combination of all effects, among which the reaction of rotor efficiency with respect to height and cyclic angles as inputs to a comprehensive dynamic model.
Personally I never used pressure as any "driver" in the model nor in my "personal" thinking, I fully stand behind Nick's reasoning of induced flows, less induced drag etc as "a thinking" model".
But as I stated before impuls theory implies the above claim....
Two "ex aburdo" reasonings (without the need of going to the molecular level, which -as you state- is of course also possible, but only will make the model and convincing others more difficult).
1. If pressure does not increase how do axial pumps work, in particular all the turbines many of the PPruners use every day...
2. If impuls theory is wrong many schools will have to rewrite their books...
d3