Counter rotating fan design and variable blade row spacing {HEAD ~ optimization} for {TAIL ~ low environmental impact} 
for
Counter rotating fan design and variable blade row spacing { HEAD ~ optimization } for { TAIL ~ low environmental impact }
OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL