Ground effect effectiveness

Tony wrote:
I haven't found a decent reference for this -- can anyone help?

Consider a clean low speed airplane -- maybe one of the kit built ones.
Does anyone have some quantitative measure of how much drag is reduced
if the airplane is flown say half or quarter of a wingspan above the
ocean?

Would we be talking about a few percent less drag, or is it a big
number, like 30%? Sea gulls and other long winged birds tend to fly
just above the water, ducks and geese like to reduce drag by flying in
vees, but don't often cruise just above the water.

References would be helpful: I hate having stories I write wrong for
technical reasons.

There was an article in Soaring magazine years ago about some tests
done at Edwards AFB by USAF test pilot students on ground effects -
using a Blanik and a Grob-103, I think.

That might be available somewhere - there is a Soaring directory
somewhere, try SSA.org.

In gliding, especially with state of the art gliders (L/D in the 40 to
60 range), failure of your landing drag devices (dive brakes, 90 degree
flaps, even tail chutes) can be a real emergency - you can float in
ground effect for miles without slowing down, unable to land! And with
wingspans of 50 to 80+ feet, slipping at ground effect altitude is a
dangerous proposition! In the pattern, I would much rather have my
gear fail to extend than my spoilers fail!

A classic glider landing mishap is watching a pilot in a new-to-him
glider float the whole length of the runway raising and lowering the
gear, until he does a "tree-stop" off the far end - the result of
confusing the manual gear handle for the spoiler handle!

Kirk
Ls6-b "66"