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I am familiar with the NACA 4-digit and 5-digit airfoil design system where airfoils, but I was wondering if anyone had some good reference material or software for designing a completely custom airfoil.

From what I understand, many new aircraft use completely custom non-NACA airfoils, sometimes several different airfoils over the wingspan. I am curious about how the design process goes from specifications of the airflow (incidence angle, perhaps the sweep of the wing, air speed, density, reynolds number etc) to a new custom airfoil?

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Wing sweep, incidence, and airfoil geometry will all have to be adjusted to achieve the best performance at a certain air speed, density, and Reynolds number.

The software that egid mentioned is a good place to start. While studying aerospace in college, XFOIL was used for basic airfoil analysis in our classes.

Another program used was AVL. This analyzes the aircraft as a whole and can model other things such as dynamic stability.

I'm glad that XPlane was also mentioned, even though I wouldn't consider it the most scientific tool for testing wing geometry. It is an interesting way to test configurations since it does use the airplane geometry to simulate the performance characteristics.

In order to consider more variables such as sweep, dihedral, and different airfoils over the wingspan, computational fluid dynamics (CFD) software is used. ANSYS Fluent is one example. There are many other packages out there, and anyone doing serious commercial airfoil design will have proprietary software and methods for this.

Of course, even in the days of supercomputers and CFD, wind tunnel testing is still very important. Scale models can be tested to validate the computer models and demonstrate performance.

As far as requirements, they can get very complicated. For aircraft, of course, the primary goal is to provide the lowest drag at cruise conditions. However, many other factors include:

  • Preventing flutter
  • Provide room for sufficient supporting structure
  • Provide room for fuel
  • Methods to manufacture the shape
  • Good stall characteristics
  • No bad aerodynamics with fuselage and engines
  • Enough lift with flaps for low landing speed
  • Able to accommodate flaps, slats, spoilers, and ailerons
  • Tip treatment like winglets/raked tips
  • Wingspan must fit into available space (airport, hangar, aircraft carrier, etc.)

And probably many more. Of course, the air speed and conditions will also affect the fuselage, empennage (tail) and its control surfaces, and engines, so this will also be taken into account.

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XFOIL is a piece of free software that appears to be under at least semi-active development (latest update this year, nothing for two years prior). I've never used it and know nothing about it; this was merely the product of a google search for "airfoil design".

XPlane has a feature called Airfoil Maker that I messed with once. It hurt my brain, but not necessarily because it was bad - it's just complicated stuff. The fun part about XPlane for design is that you also get to design or modify aircraft and fly them to see how the changes affect performance, handling, or other characteristics.

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XFLR5 is fantastic sofware for airfoil design. It is basically a graphical user interface for xfoil @egid mentioned.

It is very good for low Reynolds number flows, i.e. quite slow flying larger aircraft or model airplanes. Here is a link to a quick tutorial on the software, and there are many other tutorials you will find on Google.

In the latest versions you can even design whole planes.

Larger companies will make use of computational fluid dynamics (CFD) software, which actually solve the governing equations of fluid dynamics for a selected model (Xfoil and XFLR5 solve other very much simplified equations) Comercial CFD packages are however generally VERY expensive, although OpenFOAM is an open source package that is gaining popularity.

CFD is however a science of it's own and if you don't know what you are doing you are probably going to get useless results.

Regards

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For a recent design task involving reflex aerofoils we tried to get some data out of this program from www.mh-aerotools.de. There's both an applet and download, I wasn't able to run the applet but the download is decently extensive.

Your problem however is probably that any decently-sized aircraft manufacturer will have their own proprietary CFD software for the purpose and skilled programmers and aeronautical engineers to go with it.

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