Improvement of XFOIL lift and drag predictions

Abstract

XFOIL is a fast and relatively accurate panel method to analyse 2D airfoil performance. XFOIL has proven itself over many years to be a powerful airfoil design and analysis tool. From the literature and through the various study cases in this report, it is clear that XFOIL has a tendency to overpredict lift and underpredict drag in certain cases. This inaccuracy in predictions becomes increasingly apparent when analysing thick airfoils and increasing Reynolds numbers, where the effect of high and low angles of attack also need to be considered. In this research, improvements have been made to XFOIL by incorporating an interactive vortex panel method to predict lift and drag in better agreement with experimental results. It should be noted that experimental data does not represent the absolute truth, but is merely an indication of common fight predictions that might contribute to understanding actual ight performance. The reason for inaccurate predictions lay within assumptions made to allow for quicker converging to final answers. These assumptions were studied and are discussed in this research. Proposed improvements to XFOIL, identifed through the literature review, were then implemented and validated against experimental data obtained from literature. The improvements under scrutiny were developed by Ramanujam and Ozdemir (2017) and Ramanujam, Ozdemir, and Hoeijmakers (2016) for another panel method software based on XFOIL, called RFOIL. RFOIL is also a vortex panel method that was derived from XFOIL but was developed to better predict the performance of wind turbine airfoils. The proposed method focuses on the integral momentum quantities and updates the model assumptions in the lift calculation. Ramanujam, Ozdemir, and Hoeijmakers (2016) had studied the boundary layer quantities' momentum and displacement thickness, as predicted by XFOIL and stated that an underprediction of the quantities is the cause of the drag underprediction of XFOIL. Ramanujam and Ozdemir (2017) found that XFOIL utilises the inviscid solution to obtain the results for a viscous analysis. A new approach was developed to remedy this overprediction of lift by updating the vorticity strengths. These improvements showed good agreement with experimental data on RFOIL but they have not been tested on XFOIL. The goal of this report is to implement those improvements proposed for RFOIL, into XFOIL and to then validate them to study the improvements in both lift and drag predictions. As with RFOIL, the proposed improvements increased the accuracy of the predictions in accordance with experimental data, but an inaccuracy still exists with thick airfoils as XFOIL is not suited for such analyses. This will also be shown in this report.