Spline Curves

Spline curves are an integral element to MAD designs.  If your design calls for all straight edges, then you don’t need to worry about these, but for a small bit of work, you can really liven up your design with careful use of curves.  The Spitfire is one of WW-II’s most beautiful fighter airplanes and it didn’t get this recognition for it’s straight lines.  Typically curves are hard to design and build into model airplanes, but MAD makes it easy from start to finish.

What is a “spline curve”?

Spline curves were often used by ship builders to create curved hull designs.  Imagine you place some pins at key points along your curve outline and the fit a flexible ruler through the pins.  Conceptually that’s all there is to it.

How do I make a “spline curve” in MAD?

It’s really simple actually.  Let’s go back to a simple example of a wing with a 30″ half span.  The root station is station “0.0” and the tip station is station “30.0”.

We can define a very simple two point curve (which is really a straight line) by giving coordinate pairs in the form of (station pos, chord)

So to create a tapered wing with a 10″ root chord and a 6″ tip chord we could create the following spline curve “(0, 10) (30, 6)”.  Notice these are just pairs of numbers in parentheses.

Two points form a straight line, so that’s not very interesting.  Let’s make a 3-point curve.  “(0,10) (9.5,12) (30,6)”  If we look at these points carefully, you will see that at position 0 (the root rib) we ask for a 10″ chord.  9.5″ out from the root we ask for a 12″ rib.  Finally at position 30 (the tip) we ask for a 6″ chord.  Fit a flexibly ruler between those 3 points and that is exactly what MAD produces.  You can do a lot with only 3 points and may never need more than that.  Now you have a curvy sexy wing that stands out at the field from everyone else’s simple straight wing!

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