DOWN was created to implement a flat wake theory, a simple analysis for the calculation of rotor inflow and wake velocities. The analysis essentially treats the rotor wake geometry as rigid without interaction between induced velocities and wake structure. The code will predict three orthogonal incremental components of flow velocity at any point in any plane parallel or perpendicular to the rotor disk. Input to the code is entered interactively through the computer keyboard. The coded version of flat wake theory as implemented in DOWN provides vertical inflow patterns similar to experimental patterns for helicopter flight speeds greater than 60 knots.