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How can a NACA2312 airfoil or similar have a camber location from 30 to 39% without a change in airfoil name?

Was fooling around in javafoil, and noticed I could go from 30% camber to 39% camber, without the NACA number of the airfoil changing!! What the heck?

What name is used to differentiate NACA2312 with 30% camber location and NACA2312 with 39% camber location?

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The NACA airfoil numbers are coded as follows:

$$ \mathrm{NACA} \; M P XX $$

  • $M$ is the maximum camber divided by 100. In the example M=2 so the camber is 0.02 or 2% of the chord
  • $P$ is the position of the maximum camber divided by 10. In the example P=4 so the maximum camber is at 0.4 or 40% of the chord.
  • $XX$ is the thickness divided by 100. In the example XX=12 so the thiickness is 0.12 or 12% of the chord.

source: airfoiltools.com


In your case you changed the position of the maximum camber ($P$) from 30% to 39%. Both result in $ P = 30 / 10 = 3 $ and $ P = 39 / 10 = 3 $ (when using integers, no rounding is applied). The airfoil name therefore only tells you the position of maximum camber with a 10% accuracy.

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