I still suggest: 
 # Create an Airplane article on top of the current redirect, heavily basing content on what is currently at Fixed-wing aircraft, but taking out unpowered content, etc. 
 # Redirect Aeroplane to Airplane 
 # Edit down Fixed-wing aircraft to removed duplicate information that is now in Airplane. 

 Since the history of Airplane will show in the edit summary that the content came from Fixed-wing aircraft, nothing will be lost.