The nature of the differential equation for steady compressible flow is different for subsonic and supersonic flows. For subsonic flow the equation, the equation is of hyperbolic type. In this case the simple wave equation linearised theory. The exact differential equation for the steady flow of a compressible fluid as the same nature as that of linearised theory for steady subsonic flow, the differential equation of the elliptic type and for the steady supersonic flow, the differential equation is of the hyperbolic type. The general theory of the method of characteristics for the case of two independent variables is particularly easy to visualize and computation methods for this case need to be studied extensively. In this paper we discussed some basic properties of irrational flows and boundary conditions that occur in aeronautical engineering problems.