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A new airfoil mean line.

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bifft

Well-Known Member
Joined
Apr 17, 2011
Messages
429
Location
Utah
Been studying up on airfoils, reading the books, etc. Thinking about simple airfoils for low performance aircraft. Most of my designs (none of which have progressed beyond the X-plane model) just use the old NACA 4 digit series, but I have been thinking about moving a little beyond.

The other day, had an idea on changing the mean line. Because the NACA 4-digit mean line is two parabolic sections, the slope is increasing all the way to the trailing edge. This means you end up with fairly high moments. The NACA 5-digit uses a straight line from the max camber point to get low moments, but at the cost of having a sharp stall due to the kink (discontinous 1st derivative) in the mean line. So, my idea is to use a hyperbolic mean line after the max camber point. For the front, use the same parabola as the standard 4-digit, then for the back use a curve of the form a/x+bx+c where a,b,c are chosen to have the peak at the max camber point, then back to 0 at the x=1 position.

Worked this up in the attached spreadsheet. Just using the y+-thickness for now, instead of perpendicular to the mean line as the real NACA method, so that is also a little diffrerent, but using the NACA 4-digit thickness profile. Testing with javafoil (as it is free and easy to use) shows my foils as having slightly lower drag, the same Clmax, and about 25% lower moment than the same parameters for the NACA 4 digit.

I know that what I'm doing is basically 1930s tech, and there are many better airfoils available now. But, I want something that is less critical to exact shape (working with wood or Al instead of composite) at low Reynolds numbers (1-3x10^6). Just thought I'd throw this out here for the many more knowledgeable posters to take a look at. Thanks for your comments.
 

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  • myfoil.xls
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