Kutta–Joukowski theorem

For steady two-dimensional inviscid flow of density $\rho$, far-field velocity $\mathbf{U}$, and circulation $\kappa$, the force per unit span is perpendicular to the flow. With counterclockwise circulation taken positive and $\mathbf{U}=U{\mathbf{e}}_x$, it is $-\rho \kappa U{\mathbf{e}}_y$.

For uniform flow $\mathbf{U}$ plus a line vortex of counterclockwise circulation $\Gamma$, the outward advective momentum flux around a large circle is $\rho \mathbf{\Gamma }\times \mathbf{U}/2$. The Bernoulli equation supplies an equal pressure contribution. Their sum is the force on the fluid; the force on an obstacle per unit span is its negative, $\rho \mathbf{U}\times \mathbf{\Gamma }$. Retaining only the advective term loses half the lift and obscures its sign.