my two pence:
the spillage flow pattern is predominant from the high (underwing) pressure zone to the low pressure zone.. from my shadey past I know wing tip clearance is a significant factor in them not going downwards.. indeed me thinks downwards is actually a fair bit more effective than upwards.. (the high pressure can't spill laterally at all, instead of a half roll round to the upward winglet).. the C-17's (overwing) winglets are upwards presumably due to protection from military ground vehicles...
I have heard the winglet kits available for 737's (as on some BBJs) take 3-4 years to pay for (cost saving from reduced fuel consumption v investment) for the average 737 operator..
King Patience
