In article ,
Eric Greenwell wrote:

Winglets reduce induced drag by effectively making the wingspan longer.

Perhaps in the past, but maybe not anymo from page 104 of the
Fundamentals of Sailplane Design...

"Subsequently, it has been shown that good results may be obtained with
relatively small winglets. In contrast to early winglets, which were
essentially upward wing extensions, recent winglet designs are optimized
to diffuse the vortex rollup at the wingtip, thus reducing its strength.
There is also evidence that the velocity field induced by the winglets
can improve airfoil aerodynamics in the vicinity of the wing tip by
prolonging laminar flow and delaying separation."

But the question is, would similar-shaped horizontal extensions to the
wing have the same effect? What about multiple small span "winglets"
off the end of the wing? Birds do that. The modern winglets look a lot
like a single tip-feather.

-- Bruce