Figure 3-36.  The air that passes through the inner part of the fan  blades becomes the primary airstream.  Figure 3-34.  A typical turbofan intake section.  Figure 3-37.  Air from the fan exhaust can be discharged overboard  through short ducts directly behind the fan. Figure 3-35.  Rubber stripping inside a turbofan engine inlet allows  for friction for short periods of time during changes in the flightpath. 3-22 F a n d i s c h a r g e The reciprocating engine exhaust system is fundamentally  a scavenging system that collects and disposes of the high  temperature  noxious gases being discharged by the engine.  Its main function is to dispose of the gases with complete  safety to the airframe and the occupants of the aircraft. The  exhaust system can perform many useful functions  but its  first duty is to provide protection against the potentially  destructive action of the exhaust gases. Modern exhaust  The air from the fan exhaust  which is ducted overboard  may  be discharged in either of two ways: The fan on high-bypass engines consists of one stage  of rotating blades and stationary vanes that can range in  diameter from less than 84 inches to more than 112 inches.  [Figure 3-36]  The fan blades are either hollow titanium or  composite materials. The air accelerated by the outer part of  the fan blades forms a secondary airstream  which is ducted  overboard without passing through the main engine. This  secondary air (fan flow) produces 80 percent of the thrust in  high-bypass engines. The air that passes through the inner  part of the fan blades becomes the primary airstream (core  flow) through the engine itself.  [Figure 3-36] as mentioned earlier. Inside the inlet by the fan blade tips is  an abraidable rub strip that allows the fan blades to rub for  short times due to flightpath changes.  [Figure 3-35]  Also   inside the inlet are sound-reducing materials to lower the  noise generated by the fan.  1.  To the outside air through short ducts (dual exhaust  nozzles) directly behind the fan.  [Figure 3-37] 2.  Ducted fan  which uses closed ducts all the way to the  rear of the engine  where it is exhausted to the outside  air through a mixed exhaust nozzle. This type engine  is called a ducted fan and the core airflow and fan  airflow mix in a common exhaust nozzle. Reciprocating Engine Exhaust Systems