Where can I find plans for building a helix?

I would like to utilize some of the space below my layout as a staging area, and I am considering a helix. I am looking for plans, recommendations and suggestions.

I drew my own. My helix will be 30" radius, with 3 turns (almost) with 4" of rise per turn. That will give me 2.1% of grade. I’m looking at 11" of elevation change. Keep in mind, a helix eats up alot of space, 30" radius is 60" in diameter, track center to track center, not including roadbed and any other overhang of the structure you put it on. I am building it on an octaginal base and each segment will be made identical to the base pieces and vertical support will be 1/4-20 threaded rod. So far I have only made the base, so I don’t have any good pics of it yet.

Glenn

here is a web site to down load helix cal it will help you find the sise and the height you want
http://www.fortunecity.com/westwood/beautiful/819/HeliCal.htm