This article states a numerical method of finished differences, making it possible to calculate the variables describing an average flow, no viscous and no heavy, through a mobile wheel of a wind mill with horizontal axis, to deduce its performances from them, by using a grid with irregular steps. Centered space discretization’s diagrams inside the calculation domain and decentered towards the interior for the nodes placed on the borders are used. The temporal discretization uses an explicit diagram with two steps of time, with about two precisions. Calculations are made in two stages: a prediction stage and a correction stage. They preceded to calculations of the three-dimensional flow initial state, realized thereafter in its bypass sections. Boundary conditions are imposed on the borders of the calculation domain. Calculation convergence is ensured by numerical viscosity implicitly introduced by the time discretization’s diagrams. The internal stability is ensured by a constraint on the step of temporal discretization in accordance with CFL condition Initial conditions are calculated while realizing, on the flow bypass sections, characteristic quantities initially imposed in the calculation domain volume.