[StudentIB]

Hi,

I have a question regarding the evaluation of lift and drag forces acting on an airfoil. Assume inclined arifoil (non-zero angle of attack) placed in a virtual wind tunnel aligned with global coordinate system axes so that the flow is parallel to X axis. I've read various articles and I've noticed that some of them treat drag and lift as X and Y component of force acting on an airfoil (green vectors in the picture below). But others project them on the airfoil axis (blue vectors in the picture below).



Are both these approaches correct or only one of them ? Which one would you use when postprocessing CFD analysis ?

Thanks in advance for your help