Listen to tashley - he got it right. What ever it's called it is not necessary on all WWI airplanes with the possible exception of the Eindeckers. Even then it was a decoration and was often painted over. Perhaps It was used to impress the people who were reponsible for letting the construction contracts. Tony Fokker was good at that. Seldom used on allied aircraft and not too often on the German planes. Paiting was cheaper, easier in production and the first item grabbed by the piilot who was issued the airplane ususally grabbed the first canof paint available and decorated the plane his way. It's still called engine turning today.