I was working on a turbocharger housing and needed some more information on 3D equation curves. I found a really sweet write-up on the discussion boards posted by Glenn Chun, Autodesk employee. If you are working with helixes, check this one out.

Variable pitch, variable radius helixes, differing variations and more. Examples in Cartesian and cylindrical coordinates.

Variable Helix Equation Curves in Inventor

Here’s the link: http://forums.autodesk.com/t5/Autodesk-Inventor/Variable-Pitch-Helix-by-Equation-Curve/td-p/3507770