In this tutorial, you will learn how to create custom airfoil for your aircraft in X-Plane by using Airfoil-Maker.
Conditions and Assumptions
This tutorial was made based on X-Plane 9’s Airfoil-Maker with Windows Vista 32 operating system. In this tutorial, it is assumed that you are using the same version of X-Plane’s Airfoil-Maker and you are familiar with basic computer file operation. It is also assumed that you already have your own airfoil shape and its aerodynamic characteristics (lift polar, drag polar, and moment polar, etc). If you don’t have it, you need to search the internet for your airfoil’s aerodynamic characteristics or obtain it yourself by using CFD software. If you don’t have aerospace engineering background. it is also recommended that you understand the basic concept of airfoil aerodynamics first to make this tutorial easier for you.
Before taking further steps in this tutorial, you should prepare the items/data listed below. It is important that you have them all completely, so, you can go through all part of this tutorial without any obstacle.
Reynolds number at which the airfoil performance is evaluated.
Airfoil’s thickness in fraction of chord length.
Airfoil’s Drag Divergence Mach number (required for high subsonic aircraft, transonic aircraft, and supersonic aircraft)
Reference Graph of Cl (airfoil’s lift coefficient) vs AoA (angle of attack) at Reynolds number given in previously.
Reference Graph of Cd (airfoil’s drag coefficient) vs AoA (angle of attack) at Reynolds number given in previously.
Reference Graph of Cm (airfoil pitching moment coefficient) vs AoA (angle of attack) at Reynolds number given in previously.
Drawing of the airfoil (optional).
1. Introduction to Airfoil-Maker
Airfoil-Maker is one of the three X-Plane’s supplemental programs. As the name suggest, this program is used to create custom airfoils for further use in Plane-Maker. In Airfoil-Maker, creating custom airfoils is done by inputing your airfoil’s aerodynamic characteristic together with its shape and then save it to afl files, which can be read by Plane-Maker to be used as aircraft’s lifting surface’s cross section.
2. Airfoil-Maker user interface
Airfoil-Maker’s user interface consist of toolbar, four selection window tab, and the main window itself. The toolbar consist of two main buttons: ‘about’ button where you can check the version of your Airfoil-Maker and ‘file’ button where you can create, open or save your airfoils (afl files). The main window itself consists of a big figure box on the rightside, a smaller box on the lower-leftside, and some white spaces on the upper-leftside. Just right below the toolbar, there are four selection tab, each labelled ‘coeff-20’, ‘coeff-180’, ‘finite L/D’, and ‘camber’.
When you start your Airfoil-Maker, ‘coeff-20’ tab is automatically selected. In this tab, the figure box shows a graph of Cl, Cd, and Cm versus AoA (each presented by the green, red, and yellow line respectively) with AoA ranging from -20 degrees to 20 degrees, while there are a lot of input boxes on the leftside, as shown in figure 1 below. In this tab, you can input your airfoil’s aerodynamic characteristics from those input boxes and see if the graph on the rightside matches your airfoil’s aerodynamic characteristics reference graph.
Figure 1 : Airfoil-Maker coeff-20 tab
In ‘coeff-180’ tab, you’ll see similar window with the one when the ‘coeff-20’ tab is selected (see figure 2 below). The only difference is that the figure box on the rightside is showing a graph of Cl, Cd, and Cm versus AoA with AoA ranging from -180 degree to 180 degree instead of from -20 degree to 20 degree. You can also input your airfoil’s aerodynamic characteristics from the input boxes in this tab. It is the same as inputting it on ‘coeff-20’ tab.
Figure 2 : Airfoil-Maker coeff-180 tab
In ‘finite ‘L/D’ tab, you’ll notice that there are only two input boxes on the leftside and the figure box shows a graph of Cl, Cd, and L/D versus AoA (each represented with the green, red, and blue line respectively), as shown in figure 3 below. Note that these graph is showing wing’s aerodynamic characteristic, instead of airfoil’s. It is because the purpose of this tab is to show you the aerodynamic characteristics of a wing with planform parameters defined from the input boxes and with the custom airfoil as its cross section. This tab serves no purpose for airfoil customization and will not effect the airfoil data.
Figure 3 : Airfoil-Maker finite L/D tab
In ‘camber’ tab, there is no input box at the leftside and the figure box shows the shape of your airfoil. Figure 4 below shows the appearance of ‘camber’ tab. In this tab, you can modify your airfoil’s shape for visual purposes.
Figure 4: Airfoil-Maker camber tab
Note that if your hover your mouse cursor for a while on an input box, a dialog box will appear explaining what value should be inputted into the box. If you place your mouse over the graph, the small box on the lower leftside of the main window will show you the AoA where your mouse is pointing at and airfoil’s aerodynamic characteristic at that AoA.
3. Creating afl file
When you start Airfoil-Maker, it will automatically open your last opened airfoil. To create a new custom airfoil, click file>new. You may notice that the input boxes and the graph changes to default but the lower leftside box on your windows still showing that you are currently opening the last opened airfoil. In order to avoid confusion, save and rename your airfoil by clicking file>saveas. Airfoil-Maker will create an afl file to save your custom airfoil data. Note that you can only save your afl file anywhere inside X-plane directory. If you are creating custom airfoil for a specific custom aircraft, it is recommended that you save your afl file inside airfoils folder under the root of your custom aircraft folder (see How to Create Aircraft with Plane-Maker section 3 for more detail on this topic).
4. Setting up airfoil basic parameter
After you have created your afl file, the first thing to do is to input your airfoil’s basic parameter. They are Reynolds number where the airfoil is evaluated (Re), airfoil thickness ratio (thickness), and drag divergence Mach number at zero lift (drag div Mach). You can find the input boxes for these parameters on the upper region. Here is a brief explanation about what value should you put in each box.
Reynolds number (Re) is one of flow parameters which affects airfoil’s aerodynamic characteristics. Typically, when the aerodynamic characteristics of an airfoil is evaluated, it is evaluated for a certain Reynolds number (because different Reynolds number leads to different characteristic even if the shape is the same). So, in the Reynolds number input boxes, you should put the value of Reynolds number where your custom airfoil is evaluated at in unit of mega. X-Plane need this information to predict the aerodynamic characteristics of the airfoil at various different conditions.
Airfoil thickness (thickness) means the maximum thickness of your airfoil. In this input box, insert your airfoil’s maximum thickness in terms of fraction of chord. Note that although in reality, airfoil’s maximum thickness affects its characteristics, the number of maximum airfoil thickness you are inputting here won’t affect your airfoil characteristics. X-Plane needs this information for visualization purposes only (for rendering your lifting surface).
Drag divergence Mach number (drag div Mach) means the Mach number when your airfoil starts to experience transonic drag rise. Typically, airfoil’s Cd does not change much as Mach number increases. However, at Mach number close to 1 (around 0.8 or 0.9), airfoil experiences a sudden significant increase of Cd because of the occurance of shockwave (or usually called transonic drag rise). So, drag divergence Mach number is the maximum Mach number an airfoil can reach before it experiences transonic drag rise. Insert this box with your airfoil’s drag divergence Mach number at zero lift. Note that this input is important for high subsonic and supersonic airfoil. If your airfoil is used only for low subsonic flight (such as for RC model), you can leave this box’s value at 0.75.
5. Inputing Cl vs AoA graph
After setting up your airfoil basic parameter, the next step is to input your Cl vs AoA graph into Airfoil-Maker. In this step, you will be working with eight input boxes below airfoil basic parameter input boxes and two input boxes at the most lower region. Remember that you can always see what value should be inputted into a box by hovering your mouse cursor on that box. Here is a brief explanation about what value should you put in each box. A typical graph of Cl vs AoA (figure5) is also provided below to provide more clear explanation about each input box.
Figure 5: Typical airfoil’s Cl vs AoA graph with Airfoil-Maker’s parameters
Lift intercept (intercept) is the value of airfoil’s Cl at zero AoA. A symmetric airfoil always has lift intercept of 0, while a positive cambered airfoil always has positive value of lift intercept.
Lift slope (slope) is the increase of airfoil’s Cl value for AoA increase of 1 degree inside the linear range.
Linear range (lin range) means the range of AoA (from zero degree to both positive or negative direction) where the airfoil’s lift slope inside this range is linear.
Near-stall curve power (power) means the airfoil’s lift slope characteristic in AoA range from the outermost linear range to maximum AoA. A value of 1 means that the line is linear, a value of 2 means that the line is quadratic, and so on. Usually the value of this parameter cannot be found from your Cl vs AoA reference graph, so you need to tune this value until you found a profile that similar to the profile at your reference graph.
Maximum Cl (maximum) is the maximum value of Cl that an airfoil can achieve before it stalls and loses lift.
Stalling Cl drop (drop) is the amount of Cl value that an airfoil loses suddenly when it stalls or exceeds maximum AoA. Usually, an airfoil with good stall characteristic has Cl drop of 0.
Post-stall curve power (power) means the airfoil’s lift slope characteristic after it stalls and experiences sudden Cl drop. Similar to the near-stall curve power, a value of 1 means that the line is linear, a value of 2 means that the line is quadratic, and so on. The value of this parameter also cannot be found from your Cl vs AoA reference graph, so you need to tune this value until you found a profile that similar to the profile at your reference graph.
Post-stall Cl drop (drop) is the difference between airfoil’s Cl value right after it stalls (and experience sudden Cl drop) with airfoil’s Cl value at 20 degree AoA. The value of this parameter may be unavailable because typical airfoil’s Cl vs AoA graph do not cover AoA up to 20 degree. However, you can still obtain this value by doing simple extrapolation on your reference graph.
Minimum AoA (alpha min) is the value of AoA at which an airfoil produces maximum negative lift.
Maximum AoA (alpha max) is the maximum value of AoA that an airfoil can achieve before it stalls and loses lift. It is also means the value of AoA at which the airfoil produces maximum Cl.
6. Inserting Cd vs AoA graph
After done with Cl vs AoA graph, input your Cd vs AoA graph into Airfoil-Maker. This step is similar with the previous step but you will be working with eight input box below the input boxes for step 4. Notice that when you modify one of these eight boxes, the y-scale of graph at your rightside will changes to 0.001. Here is a brief explanation about what value should you put in each box. A typical graph of Cd vs AoA (figure 6) is also provided below to provide more clear explanation about each input box.
Figure 6: Typical airfoil’s Cd vs AoA graph with Airfoil-Maker’s parameters
Minimum Cd (d-min) is the minimum value of airfoil’s Cd that can occur. Usually, minimum Cd occured at zero AoA or close to it.
Cl for minimum Cd (d-min cl) is the value of airfoil’s Cl at which minimum Cd occurs. Note that from your Cd vs AoA reference graph, what you can get is the value of AoA for minimum Cd, not the value of Cl. You will your Cl vs AoA reference graph to obtain the value of Cl from the value of AoA that you get.
Cd at 10 degree AoA (d-alpha=10) is the value of airfoil’s Cd at 10 degree AoA.
Drag curve power (power) means the airfoil’s drag slope characteristic of the entire Cd vs AoA graph. A value of 1 means the line is linear, a value of 2 means that the line is quadratic, and so on. The value of this parameter also cannot be found from your Cd vs AoA reference graph, so you need to tune this value until you found a profile that similar to the profile at your reference graph.
Drag bucket Cl (cl location) is the center value of the airfoil’s Cl range where laminar flow drag bucket phenomenom occurs. This parameter only applies for special airfoils that was designed to maintain laminar flow throughtout entire airfoil surface. Note that from your Cd vs AoA reference graph, what you can get is the center value of AoA range of the drag bucket, not the value of Cl. You will your Cl vs AoA reference graph to obtain the value of Cl from the value of AoA that you get.
Drag bucket Cl range (width) is the range of airfoil’s Cl (from center value to both direction) where laminar flow drag bucket phenomenom occurs. This parameter only applies for special airfoils that was designed to maintain laminar flow throughtout entire airfoil surface. Note that from your Cd vs AoA reference graph, what you can get is the AoA range of the drag bucket, not the Cl range. You will your Cl vs AoA reference graph to obtain the value of Cl from the value of AoA that you get.
Drag bucket depth (depth) is the maximum amount of airfoil’s Cd value reduction that applied inside the drag bucket region.
Drag bucket curve power (power) means the airfoil’s drag slope characteristic inside drag bucket region (if any). A value of 1 means that the line is linear, a value of 2 means that the line is quadratic, and so on. Usually, a value of 2 is used for this parameter.
7. Inserting Cm vs AoA graph
After inputting your Cd vs AoA graph, the last step in creating your custom airfoil is to input your Cm vs AoA graph. You will be working with six input boxes at the bottom. This step is relatively easy than the two previous steps. You just have to input four values of your airfoil’s Cm, each in (cm1), (cm2), (cm3), and (cm4) boxes, where they respectively are the values of airfoil’s Cd at AoA of -20 degrees, (alpha1) input value, (alpha2) input value, and 20 degrees (see figure 7 below). Note that the y-scale of graph at your rightside will changes to 0.05 when you modify one of these input boxes.
Figure 7: Typical airfoil’s Cm vs AoA graph with Airfoil-Maker parameter
8. Drawing your airfoil (optional)
After finished inputing your airfoil aerodynamic characteristics, you can optionally modify the shape of your airfoil in the ‘camber’ tab. However, remember that modifying your airfoil shape in ‘camber’ tab will not affect your airfoil’s aerodynamics characteristic. It is only for visualization purposes.
To modify the airfoil shape, simply click and drag the white vertexes that forms up the airfoil shape with white lines inside the rightside figure box. Note that you can only move 4 vertexes on upper surface and 4 vertexes on lower surface. You cannot move airfoils leading-edge vertex and trailing-edge vertex (two dots connected by green line).
Notice that there are two horizontal yellow lines surrounding your vertexes. These two line marks your airfoil thickness as you defined in the (thickness) input boxes. The purpose of these yellow lines is to ensure that your airfoil shape has the thickness that you defined previously. So, you should only move your white vertexes around between these yellow lines.
9. Saving your airfoil and reopen it
After you finished inputting your airfoil’s aerodynamic characteristics and modifying its shape, don’t forget save your airfoil. Click file>save to save your airfoil into your previously named afl files. If you want to reopen and edit your airfoil, simply open it by clicking file>open. However, remember that you can only open afl files located inside X-Plane directory. So be sure not to move your afl files outside X-Plane directory.
This tutorial is not perfect! So, if you still have any question regarding Airfoil-Maker, critics, or suggestions, please feel free to leave a comment or email me at firstname.lastname@example.org.