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Can NACA 4-series airfoil have decimal representation??

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Old   February 16, 2020, 05:27
Post Can NACA 4-series airfoil have decimal representation??
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Dear All,

In NACA 4-series airfoil, for example NACA-2412 has maximum camber of 2% located 40% (0.4 chords) from the leading edge with a maximum thickness of 12% of the chord.

In case, a NACA airfoil has a maximum camber of 2.5% located 45% (0.45 chords) from the leading edge with a maximum thickness of 12.5% of the chord, how to represent this condition? Can I represent as NACA 2.5 4.5 12.5 with decimals instead of integer ? I'm facing this issue while doing airfoil optimization & I'm not clear how to represent this.

Thanks a lot for your clarification in advance.!!!
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Old   February 18, 2020, 07:49
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Well, nobody is getting you to jail if you do, if this what you want to know.

Jokes apart, there are two points here:

1) Does the 4 digits formula keep working if they become decimal? That is, will they still provide a set of points that can be used to define an airfoil or, at least, a sort of? I never tried, but you can very easily.

2) If 1 applies, would be a decimal formula still useful if, say, it turns out that the relative profile is the optimum you were searching for? Unless your chaintool is somehow weird enough to be bounded to the 4 digit series, I think this is another yes.

In conclusion, I think that as long as that formula keeps working in producing a realistic profile for your intended use, you can keep using it even with decimal digits.
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Old   February 24, 2020, 14:30
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Go for it.

The NACA airfoil designation would be akin to "nomenclature" if it were in a paper today. It works for the specific paper. It clearly doesn't work for everyone.

You can't take this designation too seriously. Keep in mind the last two digits (the maximum thickness) aren't even exact when you punch them into the formula, but only an approximate thickness. E.g. a NACA 0320 has a max thickness of 20.006. Similar (small) inaccuracies you can find in the 1st and 2nd digits.

It is not uncommon for folks to modify the NACA family.
The Lockheed S-3 Viking has published airfoil sections: NACA 0016.3-1.03 32.7/100 at the root and NACA 0012-1.10 40/1.00 at the tip. The 0016.3 is for a 16.3% max thickness. These are severe modifications from the original NACA family nowhere mentioned in the original publication. No one us sueing Lockheed for deviating from the NACA formula.

Modifying the 1st and 2nd digits is not that aesthetically pleasing but I wouldn't worry about it. It is bound to happen in any optimization work. You might want to find creative ways of presenting the parameters, that's all.
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