Negative drag forces of fanwing airfoil
 

Hi foamers,

I'm trying to extract drag and lift force on a fanwing airfoil. Everythings is ok except when i come to the drag and realize a negative value of the drag.
My cases is as below:
Case1:
freestream velocity = 10m/s oriented towards X direction
Angular velocity (clockwise) of the fan = 250rad/s
=> negative drag
Case2:
freestream velocity = 60m/s
Angular velocity of the fan = 250rad/s
=> positive drag

I used functionObject forces to extract the force.
Could anyone explain to me how the drag can be negative in case number 1? Is it because the thrust is bigger than the drag?
Please see the picture of case1 below for better understand my situation

Thanks in advance for any of your help.
https://dl.dropboxusercontent.com/co...TZsCGkJZM/file