The axis of {HEAD ~ the structure} about which the helicoid turns is making an angle with the flow vector in {TAIL ~ the order} of or slightly larger that the pitch angle 
The axis of { HEAD ~ the structure } about which the helicoid turns is making an angle with the flow vector in { TAIL ~ the order } of or slightly larger that the pitch angle
OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD 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 OTH OTH