Sorry I wasn't clearer about what I was meaning. I did make a drawing and tried to upload, but evidently didn't do it correctly. My intent was using two bars twisting in a helical pattern around each other.
I did do a trial using two pieces of 18" long, 1/2" round and measured the overall axial length with each rotation, then plotted it on excel and was suprised with results that were pretty linear (my r squared value was .98 for all you math types), and my formula generated showed a .41" decrease with each complete turn. I stopped at 6 turns because it still looked pretty good to the eye, but I know from twisting rope that you reach a point where the room left between twists won't allow the diameter to fit and the rope gets all wankled up.
So, my little experiment provided good enough results for me to plan my larger project which was to make a helical twist with two pieces that was a set length with a specific number of revolutions.
ps. I'll try to add my original drawing again. - didn't work with .png, .bmp, jpg, or gif, so I conclude I don't know how to add a drawing to the post.