XFOIL is an analysis and design system for low Reynolds number airfoils. It provides calculation procedures for viscous/inviscid analysis and mixed-inverse design of subcritical airfoils. The system uses an inviscid linear-vorticity panel method with a Karman-Tsien compressibility correction for direct and mixed-inverse modes. Source distributions are superimposed on the airfoil and wake to model the influence of viscous layers on potential flow. A two-equation lagged dissipation integral method is used to represent viscous layers, with both laminar and turbulent layers treated. The boundary layer and transition equations are solved simultaneously with the inviscid flowfield by a global Newton method. The procedure is especially suitable for rapid analysis of low Reynolds number airfoil flows with transitional separation bubbles. Surface pressure distributions and entire polars are calculated and compared with experimental data. Design procedure examples are also presented. Effective airfoil design procedures require a fast and robust analysis method for on-design and off-design performance evaluation. Only the interacted viscous/inviscid zonal approaches have been fast and reliable enough for routine airfoil design work. For low Reynolds number airfoils (Re < 1/2 million), the demands on the analysis method become especially severe. Not only must the complex physics of transitional separation bubbles be captured, but the solution algorithm must be able to handle the very strong and nonlinear coupling between the viscous, transition, and inviscid formulations at a separation bubble. ISES has been successfully applied to the design of low Reynolds number airfoils for human-powered aircraft, analysis of established airfoils, and the design of high Reynolds number transonic transport airfoils. However, it can be demanding in terms of computer time. A major goal in the development of the present XFOIL code was to significantly reduce these computational requirements while retaining the ability to predict low Reynolds number flows. The analysis formulation was also embedded in an interactive driver which also allows the designer to exercise an inverse solver and a geometry-manipulation facility. The overall goal is to improve the productivity of the designer. The present paper will outline the basic inviscid and viscous formulations of XFOIL, and demonstrate its performance on a number of airfoil cases. The mixed-inverse formulation and associated user interface will also be described. Finally, the code's overall design/analysis environment will be discussed.XFOIL is an analysis and design system for low Reynolds number airfoils. It provides calculation procedures for viscous/inviscid analysis and mixed-inverse design of subcritical airfoils. The system uses an inviscid linear-vorticity panel method with a Karman-Tsien compressibility correction for direct and mixed-inverse modes. Source distributions are superimposed on the airfoil and wake to model the influence of viscous layers on potential flow. A two-equation lagged dissipation integral method is used to represent viscous layers, with both laminar and turbulent layers treated. The boundary layer and transition equations are solved simultaneously with the inviscid flowfield by a global Newton method. The procedure is especially suitable for rapid analysis of low Reynolds number airfoil flows with transitional separation bubbles. Surface pressure distributions and entire polars are calculated and compared with experimental data. Design procedure examples are also presented. Effective airfoil design procedures require a fast and robust analysis method for on-design and off-design performance evaluation. Only the interacted viscous/inviscid zonal approaches have been fast and reliable enough for routine airfoil design work. For low Reynolds number airfoils (Re < 1/2 million), the demands on the analysis method become especially severe. Not only must the complex physics of transitional separation bubbles be captured, but the solution algorithm must be able to handle the very strong and nonlinear coupling between the viscous, transition, and inviscid formulations at a separation bubble. ISES has been successfully applied to the design of low Reynolds number airfoils for human-powered aircraft, analysis of established airfoils, and the design of high Reynolds number transonic transport airfoils. However, it can be demanding in terms of computer time. A major goal in the development of the present XFOIL code was to significantly reduce these computational requirements while retaining the ability to predict low Reynolds number flows. The analysis formulation was also embedded in an interactive driver which also allows the designer to exercise an inverse solver and a geometry-manipulation facility. The overall goal is to improve the productivity of the designer. The present paper will outline the basic inviscid and viscous formulations of XFOIL, and demonstrate its performance on a number of airfoil cases. The mixed-inverse formulation and associated user interface will also be described. Finally, the code's overall design/analysis environment will be discussed.