Fan airflow Fan airflow Core airflow Core airflow Figure 3-44.  Path of both core exhaust flow and fan flow from the engine to separate nozzles.  the fan blades and exit vanes with little turbulence as possible.  [Figure 3-45]  The core airflow needs to be straightened as  it comes from the turbine. Through the use of a converging  nozzle, the exhaust gases increase in velocity before they are  discharged from the exhaust nozzle. Increasing the velocity of  the gases increases their momentum and increases the thrust  produced (20–15 percent total thrust). Most of the energy of  the gases have been absorbed to drive the fan through the  low-pressure turbine stages.  Turboprop exhaust nozzles provide small amounts of thrust  (10–15 percent) but are mainly used to discharge the exhaust  gases from the aircraft. Most of the energy has been transferred  to the propeller. On some turboprop aircraft, an exhaust duct  is often referred to as a tailpipe, although the duct itself is  essentially a simple, stainless steel, conical or cylindrical pipe.  The assembly also includes an engine tail cone and the struts  inside the duct. The tail cone and the struts add strength to  the duct, impart an axial direction to the gas flow, and smooth  the gas flow. In a typical installation, the tailpipe assembly is  mounted in the nacelle and attached at its forward end to the  firewall. The forward section of the tailpipe is funnel shaped  and surrounds but does not contact the turbine exhaust section.  This arrangement forms an annular gap that serves as an air  ejector for the air surrounding the engine hot section. As the  high-velocity exhaust gases enter the tailpipe, a low-pressure  effect is produced which causes the air around the engine hot  section to flow through the annular gap into the tailpipe. The  rear section of the tailpipe is secured to the airframe by two  support arms, one on each side of the tailpipe. The support  arms are attached to the upper surface of the wing in such  a way that allow movement fore and aft to compensate for  expansion. The tailpipe assembly is wrapped in an insulating  blanket to shield the surrounding area from the high heat  produced by the exhaust gases. Such blankets may be made of  a stainless steel laminated sheet on the outside and fiberglass  on the inside. This is used when the engine exhaust is located  some distance from the edge of the wing or aircraft structure. Immediately aft of the turbine outlet, and usually just forward  of the flange to which the exhaust duct is attached, the engine  is instrumented for turbine discharge pressure. One or more  pressure probes are inserted into the exhaust duct to provide  adequate sampling of the exhaust gases. In large engines,  it is not practical to measure the internal temperature at the  turbine inlet, so the engine is often also instrumented for  exhaust gas temperature at the turbine outlet. 3-27 Figure 3-45.  Fan air is directed through the fan blades and exit  vanes.