How fair is fair?

I'm currently lofting the lines of the Petrel to full scale using a CAD program and then printing.  Using the programs default 'spline' tool (connects points with a smooth curve) i am noticing that the curve is slightly wavy when trying to run a smooth curve directly through every point.  Is it more important/appropriate to hit every point exactly, or to have a very smooth curve.  Or should i worry about a curve at all and just connect the dots with a straight line?

Thanks for any advice!

6 replies:

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RE: How fair is fair?

preferably a smooth curve, I've never lofted anything and hit every point dead on. Traditionally lofting numbers are drawn to the nearest 1/8" so there's usually a small error involved. Modern designs are better but not perfect.



RE: How fair is fair?

I used to do a bunch of cad splining years a go and did it for a redesign of my strip canu's. There was two methods of splining lines in cad One caused what you are talking about the other did not but may not go thru all the points perfectly good luck 

RE: How fair is fair?

It may be more beneficial to use the curve tool with your object snap.  I have tried to trace out scanned imaged in AutoCAD and find that the curve tool is more accurate. I have a hard time explaining this but if you have questions I can try to help.  You could send me your CAD file and I could see what I can do for you. How experienced are you with AutoCAD? 

RE: How fair is fair?


 I'm an architect so i work in cad everyday, for better or worse.  By curve, do you mean the arc tool?  I've been using spline, trying varying degrees to see what gives me the smoothest curve/closest curve to the points but i'm not to confindent in the results.  I've also tried in Rhino (a 3d cad'esk program) because it gives me more options for adjusting the spline, but still not great results.  Maybe i'm just being a bit too picky, i just don't want to start the year long project to find out i messed it up in the first hour.  I suppose using the arc tool would give me the greatest control, even if it is a more manual approach to getting what i need out of the points.  I would post one of the forms but i cannot figure out how to insert an image that isn't on the web already.

RE: How fair is fair?

You absolutely want a fair curve (no waves), even if you don't hit every point.

If the offsets are precisely and accurately taken from a fair curve, then your CAD program's spline function isn't very good.  Does the documentation tell you what math they are using?  Haven't used CAD for a while but some programs used to use single "Bezier" curves, with "high polynomial order" needed to be able to get through all the points.  These caused wavy curves--the higher the polynomial order, the wavier. Some others used "parabolic" curves, which also caused waves and bumps.

Good programs have good math base ("rational B-Splines" or better yet non-uniform rational B-Splines or "NURBS"), plus powerful algorithms for high-quality curve-fitting and for allowing the designer to control bend radii, tolerance, tangencies, and radii.

So to make a long story short, you may need to find a better program, or tolerate some bigger deviations from the offsets.

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