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Coordinates for NACA 23112

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Topaz

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Legally, yes. But all Riblett is doing is arithmetically adding the mean line and thickness distribution for each coordinate. And generally he's using the mean lines and thickness distributions right out of the NACA catalog. Nothing special there, and nothing you or I couldn't duplicate easily with a simple spreadsheet. Where his book is a good value is that he's already done the computer runs that show how the "new" versions of the airfoils perform. If you're good with XFoil or Profilii you could do that part yourself, too. So it becomes a measure of how much your time is worth to learn and use those tools. I think his book is a steal, in that context.

The over-complicated, innaccurate "rolling circle" method that the NACA came up with to do the same job is absurd when you see it written out. Only a research scientist could over-complicate simple arithmetic that much...
 

Norman

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all Riblett is doing is arithmetically adding the mean line and thickness distribution for each coordinate. And generally he's using the mean lines and thickness distributions right out of the NACA catalog. Nothing special there, and nothing you or I couldn't duplicate easily with a simple spreadsheet.
Yep, I have that book and have typed a few of his airfoils into Profili. After that it's just a mater of combining the mean line with a thickness form (a symmetrical airfoil section) with the back-end Xfoil airfoil blending functions to generate any of the other airfoils in his matrix or variations thereof. So far I've only used it to make a thicker section with an NACA 6 series thickness distribution but in fact it might not be a bad idea to explore other thickness forms because Riblett's mean lines only address the nose camber problem not the transition bubble so there's still room for improvement. In this screen capture I've circled the transition bubble for those who may not be familiar with this type of graph.

Where his book is a good value is that he's already done the computer runs that show how the "new" versions of the airfoils perform. If you're good with XFoil or Profilii you could do that part yourself, too. So it becomes a measure of how much your time is worth to learn and use those tools. I think his book is a steal, in that context.
I apologize to those who have heard this too many times before but it's especially relevant in this thread because the NACA 5 digit airfoils are notorious for their unusual stall. Software is no substitute for wind tunnel testing. It can be very accurate within the linear range and it can give you good data on when and where separation starts but after that things get more complex and the free or cheep programs don't always give accurate post stall results. Personally I always assume that the results after the lift curve in the cl/alpha graph begins to round off are 10% optimistic. That should be a very conservative guess for most airfoils.

The over-complicated, inaccurate "rolling circle" method that the NACA came up with to do the same job is absurd when you see it written out. Only a research scientist could over-complicate simple arithmetic that much...
**** math geeks:roll: I think they would have preferred "elegant" rather than "over-complicated". And it's certainly not inaccurate. Think of it this way: take a symmetrical airfoil made out of some elastic material with the mean line and several vertical station lines drawn on it. Now bend it to add camber. The top skin stretches and the bottom skin compresses and the station lines still cross the mean line at 90 degrees. The rolling circle method accurately reproduces this mathematically but the the expediency of drawing the leading edge circle introduced a little discrepancy in the very end of the line (it's a drafting error not a math error). I've noticed that this leading edge error creeps in to the last 1/2% of the chord when you change thickness or derotate an airfoil so I guess Xfoil does implement the rolling circle method (I suppose I should ask Dr Drella about that)
 

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Topaz

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Nice post, Norman. AFAIK, it's that leading edge error that's part of the stall issue with many of the NACA 'foils generated the "NACA way", although obviously there's more to it that just that. Fixing that leading edge camber issue is the crux of Riblett's work, and everything I've seen and read shows that his analysis and corrections are valid.

And a good point you make about computer analysis of airfoils. At least in the stuff we can get our hands upon, the results are only valid up until separation begins somewhere on the 'foil surface, which can be well below the stall in some cases. After that, they start to break down.
 

Norman

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The over-complicated, innaccurate "rolling circle" method that the NACA came up with to do the same job is absurd when you see it written out. Only a research scientist could over-complicate simple arithmetic that much...
Looks like Xfoil does NOT use the rolling circle method and with good reason. It's explained in this thread from the Xfoil mailing list. Read the whole thread. There's more to it than Riblett's rant
 
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