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How To Plot an Airfoil

September 17, 2011



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Airfield Models (http://www.airfieldmodels.com/)How to Plot a Model Aircraft Airfoil

I have received several e-mails asking how to plot an airfoil.  This is a task that is not difficult, but many people do not understand how it works.  The process is simple multiplication, drawing a few lines, a circle and some curves.

Note that plotting and designing an airfoil are two entirely different things.  If you draw a random curve that resembles an airfoil, you have designed an airfoil.  However, unless you have a way to gather empirical data about the airfoil you have just drawn, then you have no way to tell in advance how it will behave.

This article will not discuss airfoil design or aerodynamics.

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About Airfoil Plotting

How you choose the airfoil you want to plot is up to you.  I have enough experience building flying models that I can usually pick a good airfoil by looking at it without knowing much about it.  Unless an airfoil is radical, some things will clue you in to its performance, such as the maximum thickness of the airfoil, where the maximum thickness falls, reflex, camber and leading edge shape.

If the application is critical, then I go to one of the popular online forums, such as RC Universe, and present my design specifications and questions.  I always get good feedback - sometimes more than I can digest or understand.  But I end up with some good choices of airfoils.

Once you know what airfoil you want to plot, it is time to get busy.  One thing to note is that there are several computer programs that will do all this for you - many of them are shareware or freeware.  Plotting an airfoil is much faster using a computer if you understand the program.  Some programs are designed to create wing ribs and will print a ready-to-use pattern.

I still manually plot my airfoils because it only takes me about 10 minutes to do it and my printer never works.  A computer program can print an airfoil more accurately than I can draw it.

Which brings up the next point - accuracy.  Is it important?  Yes and no.  Yes, you should try to draw as accurately as possible, but in the end, you probably will not have the exact airfoil you plotted anyway.

If you manually plot the airfoil, then your drawing will be slightly inaccurate to begin with.  Even a printer is not 100% accurate.  Cutting and sanding the ribs introduces more inaccuracies.  The sheeting may or may not be the exact thickness you accounted for - especially after sanding it.  Finally, you apply a finish.

When everything is said and done, you will have a close approximation of the airfoil you chose, but it will not be exact.  Just be as accurate as you can in each step and do not sweat it too much.  Your plane will fly fine.

 
 

Obtaining Airfoil Ordinates

There are a variety of places you can find airfoil ordinate listings.  I have Theory of Wing Sections by Ira H. Abbott and Albert E. Von Doenhoff.  This book not only covers the theory, but also has a good list of airfoil ordinates to choose from.

Martin Simons' Book, Model Aircraft Aerodynamics, also has a large listing of ordinates that you can use and presumably, these are airfoils that Mr. Simons feels are appropriate for model aircraft.

Theory of Wing Sections

Model Aircraft Aerodynamics

More reference materials

There are also various airfoil databases available for free on the internet.  A web search will provide you with an extensive list of foils to choose from.

David Gell sent information providing links to additional airfoil information.  Thanks David!

"I was reading your article on plotting airfoils and thought you might consider adding a reference to UIUC Airfoil Data Site, Michael Selig Department of Aeronautical and Astronautical Engineering University of Illinois at Urbana-Champaign.

This site has a data base of 1500 + airfoils that can be downloaded and plotted.  It includes many sections that are more modern than those in "Theory of Wing Sections."

I found the site after reviewing the text "Model Aircraft Aerodynamics" by Martin Simons.  He had a reference to a Macintosh airfoil plotting program, MacFoil, which in turn lead me to the Selig site."

9-17-2011:  I can't find a link to MacFoil and don't know if it exists any more.  Please let me know if you have a link to a safe download site and I'll add it here.

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Styles of Model Aircraft Wing Construction
Using Shear Webs in Model Aircraft Wings

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Copyright 2004 Paul K. Johnson