I am simulating an airfoil to get lift and drag values. Using a What-if approach in a parametric study I step the foil's angle of attack in a constant wind velocity. The drag force values decrease with increase in angle of attack and become negative. I use surface goals to calculate the forces. How can drag force become negative? I use a local coordinate system to calculate the forces relative to. Any explanation for the negative drag values?

I have used a 3-layered mesh and well refined (2 million cells).