CV wrote in message ...

What makes the difference there is less headwind close to
the ground. The ground effect would of course not increase
on account of the wind gradient.

I would bet that if the wind is stong enough to have a strong
gradient, then there will be enough residual wind (and turbulence)
down low to affect the ability to stay low enough to use ground effect
- ever tried a high speed low pass on a windy day? It can be scary!

With bushes and fences it's not on. I was thinking more along the
lines of a big plowed field, perhaps with some ditches crossing
and maybe just a low fence between the field and the runway.

The obvious way to check this assumption is to test it with two
gliders - start off in formation, co-speed, on final; then one glider
dives into "ground effect" and the other stays at L/D max. Both land
at minimum speed. Repeat a few times, alternating who does what, and
examine the results.

If there is some ground-effect benefit at 2 ft though, it won't
be magically "eliminated" at 2,5 ft. The effect will decrease
gradually with height.

Apparently the drop off isn't gradual, but rapid - so it may be
significant at 2 feet, but insignificant at 5 feet. The Soaring
article gets into this, I think.

It doesn't have to be very turbulent there. We are assuming
flat ground. When the gradient is strong the surface wind is
much weaker than winds aloft. That is the meaning of "gradient".

Again, if the "gradient" is that strong (and we are talking about the
area up to say 200' above the surface, not 1000'), then there is
likely some surface wind also - especially over smooth terrain that
would favor ground effect?

The slowing down factor is a valid point. And ground effect may
help a little, or more than a little, if it is true that it can
doube the L/D as someone mentioned.

Russian work with Wing-In-Ground Effect (WIG) aircraft is fascinating
(also some German work in the area, and Boeing has a concept for a
huge WIG cargo plane, called the Pelican). Size seems to help, and
the ground effect was augmented by directing jet engine exhaust under
the wing. Double the L/D may be true, but likely only when inches
above the runway!

I suspect at altitude I wouldn't be able to appreciate the exact
distance covered and fail to be amazed.

Easy to do - just time how long it takes to slow down, then it is
pretty easy to approximate the distance. Or use a logger trace. I
tried it once in my LS6 at 8600', starting at 140 knots IAS and
slowing to 50 knots, trying to stay at the same altitude the whole
time (top of the start cylinder was 8600' - surprise!) Note that I
did this several miles outside the cylinder, flying towards it. I
know that it took at least 2 minutes to slow down (a very rough
number, since this was a real impromptu test, and 2 minutes was the
important number for me, for obvious reasons). So that is what? 2-3
miles? I'm sure there is a math whiz who can do the aero math to give
us the theoretical distance it should go.

IMHO, this idea (diving into ground effect) is like the idea that wet
pullups go higher - obvious, but incorrect.

Kirk