Abstract: Drag reduction is one of the major tasks of aerodynamics. Traditional principles of flow diagnosis and drag reduction are based on concepts of linear-decomposition/superposition. In our recent work, for steady flows around a given configuration and flow condition, the induced drag as well as the profile drag has been found to depend on the streamwise location of the wake planes, and the widely-used existing theories can only be applied to simple attached flow. Based on these findings, all force components and their correlation are proved to have a common root at the Lamb-vector field produced by the body's steady motion. In order to diagnose complex flows, a new technical principle is proposed in this article, which defines aerodynamic components by using Lamb-vector field observed on a chosen set of planes in the flow field. As an example, the steady separated flow over a slender delta wing at large angles of attack is computed to determine the flow structures responsible for the aerodynamic force components. It is pointed out that the secondary vortex structure near the wingtip produces negative lift and profile drag. The root of the structure can be traced back to the specific area of the wall according to the boundary vorticity dynamics, which then guides a conceptual configuration optimization to increase the lift-to-drag ratio.