Thought I’d wade in with Wagtendonk’s two cents worth. (From the W.J.Wagtendonk’s book Principles of Helicopter Flight, which I used as my bible when I was doing my exams.)
W.J.W.’s answer mirrors Nick’s (or vice versa)
He specifically says the bubble of high pressure is an oversimplification but if it helps you remember the practicalities then “so be it”.
Induced flow is the rate of bulk movement of air downwards through the disc.
When there is nothing below the disc (eg. HOGE) the downwash can dissipate into the surrounding air below the disc and induced flow remains constant.
When there is a flat surface (eg the ground) below the disc the air flow is impeded and it can’t dissipate as rapidly. The flow through the disc is thus slowed because it can only flow in as fast as the air below can dissipate.
Wagtendonk doesn’t approach whether there really IS a higher pressure under the disc or not. I wonder has anybody actually got under the blades during HIGE and actually measured the pressure? Might be an interesting point.
Wagtendonk then gives a series of those airfoil diagrams with arrows going everywhere which translates decreased induced flow into 1. Increased Angle of Attack
2. Same Drag
3. Greater Rotor thrust
(I wish I knew how to draw and post the diagrams but I can’t)
If you wanted to hover (IGE) you must maintain a constant but now lower Angle of Attack (and thus the same Lift). You are able to reduce the pitch on the blades, decrease the drag and suddenly be able to hover with less effort from the engine.