{HEAD ~ The trough} has width substantially corresponding to that of the take off location and has a bottom and {TAIL ~ sidewalls} which preferably converge in the shape of a funnel 
has
{ HEAD ~ The trough } has width substantially corresponding to that of the take off location and has a bottom and { TAIL ~ sidewalls } which preferably converge in the shape of a funnel
HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH REL OTH OTH OTH TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH