XX The two vortices tend to drag the surrounding air with them, and this secondary movment induces a small component is called downwash(下洗) X The downwash velocity combines with the freestream velocity to produce a loca/relative wind which is canted downward in the vicinity of each airfoil section of the wing X definition of induced angle of attack※ The two vortices tend to drag the surrounding air with them, and this secondary movment induces a small component is called downwash(下洗）. ※ The downwash velocity combines with the freestream velocity to produce a local relative wind which is canted downward in the vicinity of each airfoil section of the wing. ※ definition of induced angle of attack