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