Nick: hang in there, we're not having a go at you personally, please believe this. may I also suggest that when someone of your calibre with your knowledge, tries to explain simple theories (in your mind) it may prove difficult for you to understand why mere mortals like us are having problems getting it
ALL of the UK military, and I would argue all civilian trained helo pilots, are taught that air under the disc, when close to the ground, is compressed thus exerting a back pressure down the line. Its effect on AofA/Induced Flow etc is another issue.
How, then are we to be asked to accept that this concept is incorrect?
Question: When a blade/wing/rotor, moves through the air, do the molecules of air in front of that leading edge (albeit for a millisecond) get compressed because they cannot move out of the way fast enough? Does this become a serious problem when that object moving through the air, reaches close to Mach 1?
Is compressibility not the same as pressure build up, and does it not have an effect on the air flow over/around that leading edge?
Please be gentle with me, as I think I represent 90%+ of all helo pilots...