The aim of the following text is to illustrate a method to predict the aerodynamic impact of a local wing geometry change. In the presented example a winglet was designed for a given configuration. To select the most promising design under consideration of given structural boundary conditions a lifting-line-method was used to reduce the amount of numerical computation time. As lifting-line-methods are only capable to predict the induced drag effect, initial numerical simulations of the basis reference geometry were used to consider some of the additional effects, like the one of changing the wetted surface area, by comparing those numerical results with the known equations of a quadratic drag polar. It will be shown that the resulting method is under limitations accurate enough to pre-predict the numerical results of certain winglet designs, and therefore is capable to help reducing the time needed for the overall design process.