[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

[agd]

The force on an airfoil can be represented by multiple sets of components. Lift and drag refer to components in a reference frame with axes normal and parallel to the freestream flow. These are not the same as the x and y components in general. The x-y components are simply components in that particular reference frame, with an orientation relative to the airfoil dependent on the choice of the analyst.

[StudentIB]

Thanks for reply. So in the case shown in the picture below drag and lift should be read as X and Y components of the force acting on an airfoil, right ?

[agd]

Yes, but the figure is confusing with labeling of the components. Fx and Fy would be components of the force in a body-fixed coordinate frame that rotates with the airfoil.

[sbaffini]

Also, your FL and FD are not really relevant (they are not aligned with body, cartesian axes or free stream) and having them in the picture only makes confusion