Okay Stoney if its 'pretty close' then I suppose 'substantially more drag' cant be verified.
The point is Brooke is trying to substantiate his argument with data that is not relevant to the P-51D. That leads me to believe that he either doesnt have verifiable evidence (which admittedly would be difficult to find) and/or he is incapable of working it out logically. Thats why he has turned to the betting approach.
My point is that even the evidence that I have seen from history (wind tunnel tests of the later 40s) are not quantifiable because of the tunnels having inherent turbulence. Some of the tunnels of the day had a turbulence factor of as much as 2.64 which requires not only a quantitative but statistical adjustment rendering any result useless.
Now if you want to look at data that is 'pretty close' then I suggest you do some research on the work of Eppler and Wortmann (pre-Raspet) where Eppler and Wortmann independently tested the NACA 65 415 with the Eppler 266 profile. The double bucket result is very interesting but again... 'pretty close' doesnt cut it.
You are exasperating... I've never read any of Eppler and Wortmann's work, admittedly. I know Eppler was part of the NLF series airfoil designs. But I have read Theory of Wing Sections from stem to stern. You look at enough drag polars and you can see trends. I've plotted and compared the 45-100 root airfoil plot with the 64218 plot and they're nearly identical. At the tip, the 64212 is almost identical. At the P-51D MAC, the 64215 plot is almost identical. This is assuming that the plot that you got from the UIUC airfoils database for the 45-100 is dead-on accurate. They are close enough for any
reasonable person to use for comparisons. The differences? The 45-100 point of maximum thickness is at about 37%, same as the 64000 series. There is a bit of a variation in the mean lines used for the two, as NACA used the 1.0 mean line versus a shallower mean line used by North American. I don't know which because its impossible to tell visually. What does a shallower mean line mean? That the P-51D pitching moment would be a bit more benign, and add just a hair more profile drag. I'm talking variations of fractions of percent here. If I had time (probably take 6-8 hours of work), I could plot a 64418 a=.5, 64415 a=.5, 64412 a=.5 and see how much closer they would be, but I'm not.
I love these discussions when they're educational and enjoy the back and forth. This one could be, but isn't anymore.