On Friday, December 16, 2016 at 1:53:05 PM UTC-5, 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..."

True. That said, how far the wing is from the ground would have a bigger affect on drag than whether the wing has winglets.
FWIW
UH