Designing Foils with XFLR5

For any object moving through a fluid, forces are applied to the object as the fluid moves around it. A fluid can be something like water, or even something like the air around us. When the object is specifically designed to maximize the forces that the fluid can apply, you can designate these designs as airfoils. A more common name that most people would use is a wing. The shape of a wing, or airfoil, determines the forces that are applied to it when it moves through a fluid or the air. These forces also depend on the speed of motion through the fluid and the direction of flow around the airfoil.

With all of these parameters, how can you design airfoils? How do you optimize airfoils for a particular use? You need some way of analyzing all of this information—specifically, you need software that can run the numbers and do the calculations. There are very complex pieces of software that can analyze hydrodynamic problems in the abstract. But, with airfoils, you can limit the problem to such a degree that the equations are greatly simplified.

One of the software packages available to do these kinds of calculations is XFLR5. XFLR5 started as a fork of the much older xfoils program, but it has been extended with extra functionality.

Installation on Debian-based distributions can be done with the command:

sudo apt-get install xflr5

That command should install the XFLR5 documentation package as well.

When you start XFLR5 the first time, it is not very flashy. In fact, on my system, I end up with a plain black window.

Although you can design your own airfoil from scratch, doing so can be fairly tedious. It is much easier to take a previously designed airfoil as a starting point and make alterations. A good database of airfoil designs is located at the UIUC Airfoil Coordinates Database, containing nearly 1,600 airfoils. The database contains DAT files, which contain the information you need to use in XFLR5. They also have GIF files, allowing you to see what the airfoil looks like before downloading the DAT file. Once you choose one, download the related DAT file and open it in XFLR5 by clicking on the menu item File→Open.

Figure 1. Opening a DAT file loads the data and switches to the polar view.

You can change the view to the OpPoint View by clicking the menu item View→OpPoint View or by pressing the F5 key.

Figure 2. The OpPoint View gives you a traditional cross-section view of an airfoil.

At the bottom of the window, you can see airfoil characteristics, such as the thickness. Let's say that the first design change you need to make is to generate a thinner airfoil. You can do this by clicking the menu item Design→Scale camber and thickness or pressing the F9 key. This pops up a new window where you can change those characteristics.

Figure 3. A new window lets you change the thickness and camber of your airfoil.

When you make your changes and click OK, XFLR5 will ask you if you want to overwrite the current airfoil or if you want to create a new one. If you choose to create a new one, you will be able to switch between the various loaded airfoils using the drop-down at the top of the window.