When I am back for a couple days and caught up on more important stuff, I will run NACA and Riblett profiles, take apart the mean lines and thickness distributions and combination methods, and see just how big (how small?) the differences are.Riblett simply adds the thickness and camber together. NACA takes the camber line and then add the thickness to that curve. Riblett's book has a good explanation of the two methods.
I found that site once and then lost it. Thanks. I plugged in a 64-A415. The TE looks better, but the cd still seems high in Javafoil, but not in XFLR5...hmmm.
What would you recommend as an alternative? XFoil direct or pay for something?
xflr5 uses the xfoil code. I don't know of anything significantly better without a significant price tag...What would you recommend as an alternative? XFoil direct or pay for something?
I have noticed some inconsistency that Javafoil seems to give some good results, but drag can be predicted much higher and I thought the xflr5/xfoil results were too choppy to interpret.
Right, I'm stealing that!I plugged in the numbers for the NACA642A215 and Riblett 37A215 foils, and plotted them in the attached Excel file. The big differences are that Harry's foil has a tad more comber and with that a somewhat steeper midline apparent at the leading edge. The former is a bit of a surprise, the latter is Harry's big point... I even tried to use NACA's combination method for assembling the NACA foil. Lines up pretty darn close to the NACA coordinates, so I guess I did it right.
This points up the weakness and strength of these empirical methods. They only do a fair job of predicting actual numbers. Their strength is comparing similar airfoils. So if you have a good set of data on an airfoil similar to the one you want some numbers for, you can estimate the numbers of the comparison airfoil.
NACA Standard Roughness for their 24" chord models was 0.011" tall grit over the leading 8%, 5-10% coverage per area. That grit height/density is very severe when scaled up to a vehicle in normal service. (well ok maybe if you never clean the bugs off) Again suggest finding another foil with good wind tunnel data available to calibrate your computational techniques against, then applying that technique to the foils under consideration.In Javafoil there can be a large difference when using "smooth" vs. "NACA Standard" for roughness depending on the airfoil. The 93 series seems to give more reasonable numbers with standard roughness while being optimistic with smooth...or not. Who knows? I'm mechanical so anything over 2 to 3 significant digits and my eyes glaze over.