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