@Tyglik
Curious that you obtain a perfect helix with an arc!
(see my previous posts and my little disappoinment with the "arc":D
I will study your method :)
But how do you draw the first "spiral" ?
With this method?
http://moi3d.com/forum/index.php?webtag=MOI&msg=277.3

With your method (very fine for automatic big spiral) (277.3) I obtain that !
Seems not so bad, but I have not the exact Rhino spiral :)
Maybe I have not found the good parameters?
I have same height, radius, start, end ...
not exactly the same numbers of spires!!! That must be the default of my try, I return to see that :)