Why are downward pointing winglets more efficient? While most aircraft have upward-pointing winglets


Source

there are also some UAVs (I couldn't find any manned aircraft$^1$) with downward pointing winglets.


Source

I'm pretty sure that other considerations such as ground clearance, propeller/camera system protection and stability were the determining factors in the examples showed above.

However, according to a study (p.11) referenced in this question downward-pointing winglets are more efficient.


  This also means that winglets sticking up are not as efficient as
  folding them down.


I couldn't find the sources mentioned in the paper.

What makes downward pointing winglets more efficient? What are the efficiency-related differences between the two directions?



$^1$: No, neither the A-10's nor similar wingtips count.
For efficiency the orientation of winglets is not important. Best would be to stretch them out horizontally, but on a canard they do double duty as vertical tails.

Things look different when maneuvering is added: Now the downward-pointing winglet is superior especially if it carries a rudder. When roll is commanded by deflecting the ailerons, the lift change on the outer wing will carry over to the winglet. On upward-facing winglets this creates an opposite yawing moment while on downward-facing winglets this yawing moment supports the intended turning maneuver.

Also, they look cooler.