The Collaborative International Dictionary of English v.0.48:

Helical \Hel"i*cal\, a. [From Helix.]
   Of or pertaining to, or in the form of, a helix; spiral; as,
   a helical staircase; a helical spring. -- Hel"i*cal*ly,
   adv.
   [1913 Webster]

WordNet (r) 3.0 (2006):

helical
    adj 1: in the shape of a coil [syn: coiling, helical,
           spiral, spiraling, volute, voluted, whorled,
           turbinate]

Moby Thesaurus II by Grady Ward, 1.0:

35 Moby Thesaurus words for "helical":
   O-shaped, ambagious, anfractuous, backhanded, circuitous, circular,
   cochlear, cochleate, corkscrew, corkscrewy, deviating, deviative,
   devious, digressive, discursive, excursive, helicoid, indirect,
   meandering, oblique, orbital, out-of-the-way, rotary, round,
   roundabout, screw-shaped, scrolled, spiral, spiroid, turbinal,
   turbinate, verticillate, volute, voluted, whorled