Q. How do I use an inverted airfoil for my horizontal stabilizer in AirplanePDQ?

Some aircraft use a cambered airfoil for their horizontal stabilizer that is inverted so that it generates negative lift when at zero angle of attack.  AirplanePDQ does not currently automatically handle inverting an airfoil shape for use in this manner.  Consequently, to use an airfoil in the AirplanePDQ airfoil library in an inverted manner, you must modify the data supplied in the airfoil library to create a new inverted airfoil.  For this tutorial we will create an inverted version of the NACA 2412 airfoil.  

In order to create our new inverted NACA 2412 airfoil we will need to modify or create three different files.  The first is the airfoil shape file.  This is a file that contains the profile of the airfoil shape in and drawing (DWG) file format.  The second is the table of aerodynamic coefficients for the airfoil in PEF file format.  The third is the airfoils file which contains a list of all of the airfoils in the AirplanePDQ library.

Before beginning this tutorial, you should read the section in the AirplanePDQ User's Manual entitled "Airfoil Database" which explains the database used to store the airfoil information.

Airfoil Shape

The first step in creating our new inverted NACA 2412 airfoil is to create the airfoil shape.  We will do this by inverting the existing NACA 2412 airfoil shape using the following steps.

1. Open the file C:\Program Files\AirplanePDQ\airfoils\2412.dwg in AirplanePDQ.
2. Once the file is open, you will see just the top of the airfoil in the lower left corner of your window.  
   
   
   
3. Use the Zoom Extents command (View->Zoom->Extents on the menu bar) to center the airfoil on the screen and zoom in.
   
   
   
4. Click with the mouse on the line representing the airfoil to select the airfoil shape.
   
   
   
5. Click on the Mirror command on the Modify menu to invert the airfoil shape.  After you click on the command you will be prompted for the first point about which to mirror the shape.  Type in "0,0" (without the quotes and with no spaces) then the Enter key.  Next you will be prompted for the second point about which to mirror the shape.  Type in "1,0" (again without the quotes and with no spaces) then the Enter key.  Finally, you will be prompted whether you would like to delete the original/source shape.  Type in "Y" (no quotes) and then the Enter key.  The airfoil shape should then be inverted as shown below.
   
   
   
6. Click on the Save As command on the File menu to save the inverted shape under a new name.  A dialog box entitled "Save Drawing File" will appear.  Please ensure that you are saving into the folder C:\Program Files\AirplanePDQ\Airfoils, then save the file as 2412inverted.dwg.

Airfoil Aerodynamic Data

The second step is to create a new aerodynamic data file for the inverted airfoil.  

1. Start the Notepad application by clicking on the Start menu button, then on Programs, then on Accessories, and then on Notepad.
   
2. Open the file C:\Program Files\AirplanePDQ\Airfoils\2412.pef.  Note that on the File Open dialog box, you will need to select the "All Files" option for "File of Type" at the bottom of the window in order to see the *.pef files listed.  Once the file is opened it should look like the following.
   
   
   

3. If you didn't read the section in the AirplanePDQ User's Manual entitled "Airfoil Database" which explains the database used to store the airfoil information before, please read it now so that you understand the format of the PEF file.
   
   

4. You now need to reverse the order of the lines shown above that start with Value01 ... Value17.  After you've done that it should look like the following.
   
   
   
5. Now you need to edit the numbers on each of those lines.  The two digits after the "Value" label at the beginning of each line are now in reverse order and need to be changed so they are in ascending order.  After you've done that it should look like the following.
   
   
   
6. The numbers on each of those lines after the equals sign (=) represent the angle of attack, the lift generated by the airfoil, the drag on the airfoil, and the pitching moment generated by the airfoil respectively.  You now need to reverse the sign on the angle of attack, the lift, and the pitching moment.  Those are the first and second numbers, and the fourth number.  On each line change the sign so that if the number has a negative sign (-) in front of it, delete the negative sign.  If it does not have a negative sign in front of the number, then place one there.  Remember to do that for each of the numbers except those in the third column after the equals sign.   After you've done that it should look like the following.
   
   
   
7. If you now scroll down through the remainder of the file, you'll notice that there are two additional sections of the file that look similar to the ones we just modified.  You need to follow the same steps 4 though 6 on the remaining two sections.
   
8. Change the description of the airfoil at the top of the file to indicate it has been inverted.  On the second line, change the description from "NACA 2412" to "Inverted NACA2412".
   
9. Save the file to a new name.  Click on the Save As command on the File menu.  A dialog box entitled "Save As" will appear.  Please ensure that you are saving into the folder C:\Program Files\AirplanePDQ\Airfoils, then save the file as 2412inverted.pef.  Please also select "All Files" for the "Save as type:" option which is second from the bottom.

Airfoil List

The final step is to add a line to the file C:\Program Files\Airfoils\Airfoils so that AirplanePDQ will recognize the new airfoil.  

1. Using the Notepad application you used above, open the file C:\Program Files\Airfoils\Airfoils.  Note that on the File Open dialog box, you will need to select the "All Files" option for "File of Type" at the bottom of the window in order to see the file listed.  Once the file is opened it should look like the following.
   
   
   
2. Look for the entry for the 2412 airfoil.  Immediately after that line add the line
   
           2412inverted  2412inverted.pef  2412inverted.dwg
   
   
3. Save the file.

If you now close AirplanePDQ and reopen it, you will be able to use the new inverted 2412 airfoil for your horizontal stabilizer.  The same steps can be used to generate an inverted version of any of the airfoils in the database.  Here are the DWG and PEF files for the inverted 2412 if you would like to use them.

2412inverted.pef

2412inverted.dwg