where cv and lv are the characteristic vortex velocity and length scales. Note that the work term is used here since it is proportional to rotor-induced circulation. A reasonable choice for the characteristic velocity is the average of the velocities at stations 2–4, and assuming an elliptical vortex shape, the average of the major and minor radii of the vortex provides a suitable characteristic length scale. With these assumptions the vortex power can be approximated as