A helicoid sheet with a generating line at, say, 85 degree to {TAIL ~ the axis}, and a second one with {HEAD ~ a generating line} at, say, 75 degree to the axis, can be attached to the same central cylinder at a small distance from one another, a distance less than the pitch distance 
A helicoid sheet with a generating line at , say , 85 degree to { TAIL ~ the axis } , and a second one with { HEAD ~ a generating line } at , say , 75 degree to the axis , can be attached to the same central cylinder at a small distance from one another , a distance less than the pitch distance
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH