Schematic representation of the actuator disc model in horizontal flight including the inherent assumptions. Where w is the downwash at the disc, V is the freestream velocity, V′ is the resultant velocity vector, the subscript, ∞, represents the far wake, and −∞ represents the volume of fluid before it has been affected by the disc. The disc exists in the x–y plane, but the far wake is measured in the y–z plane. Once rotated through 90° the plane in the far wake is analogous to the Trefftz plane of conventional wing theory.