The spar has an I beam or C beam cross section in {HEAD ~ the region} not bonded to {TAIL ~ the blades} so that the spar is twistable for pitch control 
not bonded to
The spar has an I beam or C beam cross section in { HEAD ~ the region } not bonded to { TAIL ~ the blades } so that the spar is twistable for pitch control
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD REL REL REL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH