At 18:53 16 December 2016, Jonathan St. Cloud wrote:
The reason I asked in the Russell Holtz book, he states that "The
closer
the wings are to the ground, the more the ground interferes with the
development of the wing tip vortices..."

That is because the flow becomes more and more 2 dimensional. The
ground effect goes up exponentially, the closer you get to the ground.
You can measure it in a wind tunnel, but it would be very hard to
measure in an actual glider because a difference of 6" or a foot in
height above the ground would have more of an effect than the
winglets probably do on their own. (presuming that one is already
flying really, really low) At least in a wind tunnel, you can set the
parameters and then make measurements. I don't have my references
handy, but I believe that the ground effect starts to become noticeable
when one is about 1/2 wingspan AGL. It then rapidly increases the
closer one gets to the ground. That is why I think that one won't be
able to see much of a difference with or without winglets. You are in
the area of 3'-4' off the ground, and in that area of the ground effect
curve, being off a foot in height (one way or the other) will make a
much bigger difference on that exponential curve than winglets (or
no winglets) ever will. In real life you just can't fly it that close
unless
you have a real sensitive radio (type) altimeter to measure exactly how
low you are.

RO