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# Calculate inner surface Naca airfoil

## Calculate inner surface Naca airfoil

(OP)
Hello everyone, I know the equations from which I can calculate a Naca airfoil, if I have a thick, How is the inner surface?

I´m sorry for my english level.

### RE: Calculate inner surface Naca airfoil

google the thickness you want, i googled "NACA 0012" and found a data file defining the upper and lower surface.  there might be an equation defining the data, it'd be in an early NACA report, or you can curve fit the data.

### RE: Calculate inner surface Naca airfoil

you can use the point data from NACA to determine the perimeter of the airfoil.  if you're trying to determine the inner face, i'd suggest putting into autoCAD (or similar) and offsetting the external surface; or thinking about it for a second, if you determine the external perimeter, P, from the NACA data, then an equivalent circle id D = P/pi and the inner surface length would be pi*(D-2t) = P-2*pi*t

### RE: Calculate inner surface Naca airfoil

how did you generate the inner curve ?  it doesn't look like a constant offset from the outer curve ...

### RE: Calculate inner surface Naca airfoil

(OP)
The points outside the NACA multiply by a correction factor (eg 0.8), I understand that autocad can do the offset but I really wonder what he does.

### RE: Calculate inner surface Naca airfoil

looking at the result, that sounds about right.  I think autoCAD has drawn an 80% airfoil for you, and maybe centered both at the origin; but clearly this isn't what you want ... look at the nose area.

i would have all the lower surface points and copy them some +ve amount, and then all the upper surface points and copy them the same -ve amount, and then "fuss" the nose area; the inner point at the nose is the offset distance aft of the outer profile.

### RE: Calculate inner surface Naca airfoil

holyo,
One method that will give you a close approximation is to take your data points for the air foil curve, then insert a circle at each point, the radius of which is your desired offset.
Then draw a spline curve tangent to each circle.
You may have to do some adjusting at the trailing edge and leading edge especially if the leading edge is defined by a radius. If that is the case draw a smaller one off set by your thickness.
B.E.

The good engineer does not need to memorize every formula; he just needs to know where he can find them when he needs them.  Old professor

### RE: Calculate inner surface Naca airfoil

Another method would be to create a new point offset the thickenss that you want in a direction perpendictular to the curve of the airfoil.  A close approximation to this would be to calculate the slope of the line connecting the two points on either side of the current point, then move the current point along the negative reciprocal. Here's an example given the points p1 = (2,0) p2 = (1,1) and p3 = (0,3) to find the offset of point (1,1).  The slope is given as --> if p4 = p1-p3 = (2,-3) then the slope = b/a = -3/2.  For a 1/2 inch offset you would add 1/2 * 3/(sqrt(2^2 + 3^2)) to the x value and 1/2 * 2/(sqrt(2^2 + 3^2)) to the y value.  here 2/3 is the negative reciprocal of the slope of the line connecting the two adjacent points which is perpendictulaor to the surface.  So you would end up with (1.416, 1.277) as offset 1/2 inch from point (1,1).  As a check sqrt(.416^2 + .277^2) = .5 so our point is half an inch away from (1,1).  Do this for every point except the end points and you will have your curve except for the end points which if you use the trailing edge as the beginning and end point should be easy to come up with.  This should be pretty easy to do in a spreadsheet such as excel, but using a CAD program to create an offset edge would be much simpler.

-Kirby

Kirby Wilkerson

Remember, first define the problem, then solve it.

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