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I have come across this NACA profile

NACA 0005.4 mod

and it is not clear how to interpret this airfoil. Is this considered a 4 digit NACA profile? What does the number after dot mean?

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Yes, it is a valid airfoil identifier. First two zeros indicate it is a symmetrical airfoil with no camber. The next two digits mean it has a five percent thickness to chord length ratio. The number after the decimal indicates a slight adjustment to the maximum thickness and location thereof. Instead of being 5% thick, this airfoil is 5.4% thick. There is quite a bit of very good info on NACA airfoils here. This airfoil is mostly used on helicopter blades, far as I can see.

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    $\begingroup$ And the "mod" addition means that is has some arbitrary changes applied to it which make the name almost meaningless. Remember that the Lockheed C-141 also uses a NACA 0013 mod. Heavily modded, with camber and all. The name just meant that Lockheed did not want anybody to know what really has been used. $\endgroup$ – Peter Kämpf Jun 1 at 19:56

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