Prandtl's lifting-line theory models a three-dimensional finite wing by replacing it with a bound transverse vortex line whose circulation Γ(y) varies across the wingspan. Because vortex lines cannot terminate within a fluid by Helmholtz's theorems, trailing vortex sheets continuously shed from the span, coalescing into intense concentrated wingtip vortices. The downward velocity field (downwash w) induced across the wing tilts the effective lift vector backward, generating induced aerodynamic drag.