I'm not trying to run a guessing game. This is supposed to be an
educational experience.
Remember the thought I attributed to Hans Bethe: I don't want to be
thought of as one of those humdrum magicians who mystifies people by
concealing how he does things. I don't want people to say "Gee, I guess
Denker's predictions all come true, but we can't figure out how he knew all
that."
I thought I was pretty clear earlier, but people seem to be barking up the
wrong tree, so here are some reminders for those of you who are trying to
figure out the induced drag and helicopter issues:
1) Upwash is important!
2) Upwash makes a helpful upward contribution to lift, and a helpful
negative contribution to induced drag. Downwash makes another helpful
upward contribution to lift, and a draggy positive contribution to induced
drag.
3) The infamous Anderson & Eberhardt paper got it diametrically wrong,
assigning upwash an unhelpful positive contribution to induced drag. Bill
B. and others have made similar errors on various occasions. The error is
quite large; induced drag is the small difference between large numbers;
if you flip one of the large numbers the effect is huge.
4) The machine-gun model and all similar models are doomed to gross failure
because they don't account for the upwash.
5) In ground effect, it is fairly obvious how the air can make multiple
"round trips" bouncing between the wing and the ground, contributing N
times the momentum for a given amount of energy.
But give a long wing, sections far from the wingtips perform about the same
whether they're in ground effect or not. Does this mean the air is
bouncing off of something? Well, yes, that would not be such a bad way to
think of it. The air is bouncing off of the lower air.
6) This is why I made a big fuss about the wing (however indirectly) being
able to push against the earth (or to be less indirect, to push against the
air that rests on the earth). I wasn't just saying that to be nitpicky,
and although I'm sorry that nobody understood it, I stand by the point I
was trying to make.
7) Induced drag is not very significant for airplanes in normal cruising
flight. It can be considered a minor complication, and ignored in an
introductory discussion. At the opposite extreme, a hovering helicopter
suffers enormously from induced-drag-like effects, which is why (I say
again) it would not be a good place to start a pedagogical discussion of lift.