Originally Posted by Sparky Paul
THE force on the plate is resolvable into whatever frame of reference one desires.
This is certainly true, but as you point out, some choices are better than others. Why not define the lift force as the component of aerodynamic force acting perpendicular to the chord, and the drag as the component acting along the chord?
Seems like a pretty small and inconsequential shift from the traditional definitions.
The plot below shows CL and CD as a function of AOA for a rectangular wing with a Clark Y airfoil. The solid lines show the traditional resolution axes (perpendicular and parallel to the air flow). The dashed lines show the same force resolved perpendicular and parallel to the chord. The CL behavior doesn't change dramatically, but the CD behavior changes completely. In this coordinate system the "drag" has to become negative in order for the "lift" and "drag" components to sum up to the correct net aerodynamic force. I think we'd all agree that the traditional definition of drag far better captures the concept we are referring to when we say "drag".