What is the equation for a spiral path in 3D?
I have just found this correct answer on the web (by someone else):

A helix is a circle that moves up with time.  Hence, the equations for one helix are:

x(t) = sin t

y(t) = cos t

z(t) = t

A double helix include two helices that are offset by half a turn.  So, the equations for the second helix would then be :

x(t) = sin t

y(t) = cos t

z(t) = t + \pi