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I am interested in identifying this prop airfoil due to its uncommon high .92% efficiency (the best metal props are closer to 83-87%). I have done an exhaustive search for it. It is labeled "NACA RML 9 G 06a". I do not know where I originally got this plot and therefore have no way to know what "ref 3" is.

It does not appear to be a airfoil number and a google search doe not return any "NACA RML" however I have a library of about 2000 "NACA RM" which refers to NACA Research Memos; perhaps the additional "L" means "Letter(s)".

I can find NACA reports starting with "RM G-10" but no G-9. I have also searched for derivatives of "06a" in an airfoil number. I would appreciate any assistance in identifying this airfoil.

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The label you seek is not an airfoil, it is a research memorandum produced at Langley. (RM Lxxxx) The subject airfoil is NACA 4-(5)(08)-03 and the maximum efficiency from the test as reported in the conclusion was 88% at m0.71, partway down the curve where compressibility effects become adverse. enter image description here enter image description here

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  • $\begingroup$ Thanks... wonderful! I see now the chart label was wrong and that explains why I could not find it. $\endgroup$
    – jwzumwalt
    Jun 8, 2018 at 2:19
  • $\begingroup$ @jwzumwalt What, if anything, did you wind up doing with the airfoil? It has interesting performance. $\endgroup$
    – Pilothead
    Aug 6, 2018 at 20:26
  • $\begingroup$ I am installing a VW engine in a home-built aircraft of my own design and was intrigued by the high speed efficiency. A VW engine is rated at 5000rpm but 3800-4000 (other builders typically run them at ~3400rpm) is more realistic and this puts a 55-60" prop near the speed of sound. This airfoil is about 5% better than similar designs at that speed. $\endgroup$
    – jwzumwalt
    Aug 6, 2018 at 23:10
  • $\begingroup$ @jwzumwalt How will you get the airfoil coordinates? It is not numbered like any NACA series with which I am familiar. Hope it works out for you. $\endgroup$
    – Pilothead
    Aug 7, 2018 at 0:25
  • $\begingroup$ I have pictures of it, and it appears to be similar in design and characteristics to a NACA 10-(5)(08)-03, NACA 4508 or 5508. dtic.mil/dtic/tr/fulltext/u2/a801420.pdf <br> airfoiltools.com/airfoil/… $\endgroup$
    – jwzumwalt
    Aug 7, 2018 at 2:18

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