Originally Posted by Mugi
Nice link. Thanks, HWD. Convincing but again (and I'm playing devil's advocate here) hints at more magic and mumbo jumbo.
Now I have to believe that because the air in a vortex I left behind in my wake meets the ground, I get a magic drag reduction, therefore lift boost
But I left that air way behind - it's 30m gone, history etc...
OK. Let us imagine it in this way:
Put a lid on a vessel so that the lid is slightly smaller than the vessel.
The lid could sink into the vessel, because the lid is not actually stuck to the walls - there is a gap all around. But the gap is narrow everywhere. Therefore the air in the vessel is compressed until it supports the weight of the lid and will only slowly escape through the gap. It does not take much power to pump the air back under the lid and keep the lid suspended.
Now, imagine that the wing is in a long and narrow ditch, the wingtips nearly touching the banks. The air can escape in front and behind of the wing - but not to the sides. So, you have to do quite some work to fly along the ditch.
But the moment you leave the ditch, air gets the possibility to escape around the wingtips, too. Which means that you have to do somewhat more work to replace the air cushion beneath you.
Is this a meaningful explanation of ground effect?