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Since the advent of the turbojet engine, there has been much research by aircraft and engine manufacturers into the source of aircraft noise and its reduction. A review of this research is presented delineating the transition from turbojet engines to turbofan engines to the high by-pass ratio engines being introduced today, and the progress that has been made. Application of the current state-of-the-art to existing airplanes through engine replacement, nacelle retrofit, and flight procedures are also discussed.

This paper discusses the inter-relationship between turbojet and turbofan engines used in military and commercial service. This relationship has evolved from the modification of military engines for application to the initial commercial transport aircraft to the present where some engines have been designed from inception to meet commercial transport requirements. This trend is the result of the commercial transport engine market growing enough to justify new engine developments to meet its specific needs in some cases. There are future military and commercial propulsion requirements, however, where the anticipated market may not be sufficiently large or firm to provide the basis for a new development, for example, a medium size commercial or military STOL aircraft.

One of the problems associated with long part life in aircraft jet engines is wear in nonlubricated mating parts resulting from vibration, fretting, impact, or sliding during engine operation. A significant improvement in the life of engine parts subject to this type of wear can be achieved by coating mating surfaces with protective, wear resistant materials. Numerous plasma sprayed and detonation flame plated coatings have been developed for this purpose. Successful use of the plasma sprayed coatings in these applications is dependent on development of the necessary coating properties and strict control of the coating process. Engine experience with both types of protective coatings has been most successful when coatings are tailored to the environment and type of wear encountered.