Post-Stall Performance of Cambered Airfoils

Abstract

The design and optimisation of turbines, rotors, and propellers rely on airfoil pre- and post-stall aerodynamic data. Airfoil post-stall data is scarce compared to pre-stall data, especially for cambered airfoils. This scarcity has led to little insight into the effects of the airfoil shape on the post-stall performance and vortex-shedding behaviour. This thesis aims to partially fill this gap through wind-tunnel testing of four airfoils: the NACA 0018, the Eppler E387, the Wortmann FX 63-137, and the Martin Hepperle MH 113.

New post-stall data is presented for the airfoils, and the novel measurements demonstrate that the airfoil shape heavily influences its post-stall performance. The FX 63-137 and MH 113 airfoils have a maximum drag coefficient of 2.03 and 2.04, respectively. On the other hand, the E387 airfoil has a peak drag coefficient of 1.99. A significant change is observed for the maximum post-stall lift coefficient, increasing from 1.02 for the NACA 0018 to 1.31 for the FX 63-137.

Post-stall approximation methods are typically used when experimental data is absent. Therefore, their accuracy is tested against the new post-stall data. A novel maximum drag coefficient prediction method, based on an upwind ordinate and the airfoil's tail angle, is developed that is more straightforward to use and reduces the standard deviation by 35% in the positive angle regime compared to existing methods.

Vortex shedding heavily influences aerodynamics, aero-elastic behaviour, structural integrity, and noise generation of airfoils at angles of attack past stall. However, data on the vortex-shedding behaviour of airfoils is scarce. Developing a new vortex-shedding correction method led to a 60% improvement in the agreement between two different-sized E387 models compared to existing methods. In addition, it was found that the airfoil shape greatly influences the vortex-shedding behaviour, with significant changes in the vortex-shedding frequency between the four airfoils.