# How reliable are the available airfoil data sources?

I am making calculations based on Blade Element (Momentum) theory (BEM) for propellers and I am trying to find the angle of attack at which the propeller performs most efficiently. If I have understood the theory correctly, the efficiency greatly depends on blade angle the instream velocity (and consequently phi) and the lift drag ratio. A higher L/D results in higher efficiency.

But I have a hard time determine which of the Cl and Cd plots from different sources is reliable enough for my purpose. As an example in below image I am showing three different plots for the same airfoil (RAF6) (Red, green and blue). The problem as you can see is that they are all very very different. I am comparing the green curve from the original plot (thinkness ratio 0.1) with airfoil data from airfoiltools (thinkness ratio 0.1 as well) and I also sketched a blue line that I sketched from an older NACA 650 document (pg. 28). The vertical red line indicates the angle of attack the max Cl/Cd corresponding to the one from airfoiltools.

Questions:

• Which of the three airfoil data sourced do you deem the most reliable?
• Does anyone know where the one from wiki (reference below) is coming from (It would be nice to know which Re number applies to them)?
• Are there any other (more reliable) sources for airfoil date (in this case for the puropse of BEM theory)?
• Is data from airfoiltools actually reliable?
• What is the highest L/D ratio ever measured (or what is the most efficient propeller ever)? (And how does this compare to L/D ratios from propellers from the early days of aviation)

Notes: Cl/Cd against airfoil plot from airfoiltools: I concluded that I should be calculating with the result velocity (Vr) when calculating the Reynolds number for a propeller so I got Re numbers of approx. 2,000,000 and consulted the raf6-il from airfoiltools with the highest available Re of 1,000,000 with Ncrit=9 and 5.

Attribution: Black and while Cl, Cd-plot coming from: https://en.wikipedia.org/wiki/Blade_element_theory Marino108LFS, CC BY-SA 4.0 https://creativecommons.org/licenses/by-sa/4.0, via Wikimedia Commons