On the origin of the ground effect

Abstract

The ground effect is well known to pilots and aerodynamicists alike. However, the current explanations found in undergraduate (and pilot-focused) textbooks can be inconsistent, often attributing the phenomena to the interaction between tip vortices at the ground. Others invoke the method of images to show that, when the flow is forced to have a straight streamline on the ground, ground pressure must increase. These must prescriptively choose an airfoil circulation. Meanwhile, a simple panel code can be used to show both that the lift on an airfoil in ground effect is significantly two-dimensional, and that the circulation about an airfoil near the ground is not constant. In particular, circulation will be found to grow as altitude decreases, magnifying the ground effect. A simple graphical panel method solver is provided, such that this exercise is accessible to students without the longer task of writing a panel code for themselves. This exercise can provide students with greater insight into the Kutta condition, the method of images, and panel methods themselves. The resulting streamline pattern can also be used to explain the phenomenon to more general audiences, by observing the relationship between lift and streamline curvature.