Eric Greenwell wrote:
Can you point out where these articles compare the pressures on the top
and bottom surface, or some reference that gives the relative
magnitudes? I was unable to find it.

For "classic" airfoils, try the book Theory of Wing Sections, by Abbott
& VonDoenhoff, THE reference for us old aero types.

You'll see section pressure distributions that clearly show far more
suction on the top (area under the curve of pressure coefficient vs.
chord), than high pressure on the bottom.

People are forgetting that there are other factors affecting the impact
of winglets, including the height of the winglet and more importantly,
the toe in -- which some have claimed generates "thrust."

I won't pretend to be a winglet expert (flying a non-wingletted 1-26),
but it has been pointed out to me that winglets tend to be point design
items (e.g., Global Flyer, Voyager), or compromises that provide
different benefit at different angle of attack (or C-L).

In some bathroom stall (at Boeing, Douglas or Northrop -- I've worked
for all 3), there was posted a truism:
"There is no substitute for span."

However, more span means more wing root bending (trying to pull the
wing tips up until they touch...), and some other problems (e.g., stall
characteristics, tail power required...). The complaint about hangar
space is in there, too, but look at the span of the 777 and the A380...
If you build it, they'll make room. Or maybe the Eta?

Winglets have competition from their more contemporary cousin, the
raked tip (fashionable on aircraft such as the 767-400). The raked
tips begin to lose thier effectiveness at higher angles of attack
(e.g., near stall), and thus mitigate some of the downsides of winglets
or added span.

You'll still have vortices and wake. One day I flew through the wake
of the late Mark Navarre (OD), and told him he "thumped" me. He
complained that his glider (ASW-20?) wasn't supposed to have a wake.
If you're heavier than air and you're flying, you ARE going to generate
a wake.