NAVWEPS 00-801-80  AIRPLANE PERFORMANCE  is subjected to the bending and torsion of  the tangential impulse-reaction forces. The  blade must wirhstand these stresses  which are  generally of a vibratory and cyclic nature  while  at high temperatures. The elevated  temperatures at which the turbine must func-  tion produce extreme conditions for struc-  tural creep and fatigue considerations.  Conse-  quently, the engine speed and temperature op-  erating limits demand very careful considera-  tion.  Excessive  engine temperatures  or speeds  may produce damage which is immediately  apparent.  However, creep and fatigue damage  is cumulative and even though damage may  not be immediately apparent by visual inspec-  tion, proper inspection methods (other than  visual) must be utilized and proper records  kept regarding the occurrence.  area is too large, incomplete expansion will  take place; if the exit area is too small, an over  expansion tendency results. The exit area can  affect the upstream conditions and must be  properly proportioned for overall performance.  When the ratio of exhaust gas pressure to  ambient pressure is greater than some critical  due,  sonic flow can exist and the nozzle will  be choked or limited to some maximum flow.  When supersonic exhaust gas velocities are re-  quired to produce the necessary momentum  change, the expansion process  will require the  convergent-divergent nozzle illustrated in fig-  ure 2.9. With sui?icient pressure  available the  initial expansion in the converging portion is  subsonic increasing to sonic velocity at the  throat.  Subsequent  expansion in the divergent  portion of the nozzle is supersonic and the re-  sult is the highest exit velocity for a given  pressure  ratio and mass flow.  When the pres-  sure ratio is very high the final exit diameter  required to expand to ambient pressure  may be  very large but is practically. limited to the  fuselage or nacelle afterbody diameter. If the  exhaust gases  exceed  sonic velocity, as is porsi-  ble in a ramjet combustion chamber or after-  burner section, only the divergent portion of  the nozzle may be necessary.  Actually, the development of high tempera-  ture alloys for turbines is a critical factor in the  develop,mcnt of high ei%ciciicy, high output  aircraft gas turbines. The higher the tem-  peratute of gases entering the turbine, the  higher can be the temperature and pressure  of  the gases at discharge from the turbine with  greater exhaust jet velocity and thrust.  The function of the t&pipe or exhaust no?&  is to discharge the exhaust gases  to the atmos-  phere at the highest possible velocity to pro-  duce the greatest  momentum change and thrust.  If a majority of the expansion occurs through  the turbine section, there remains only to con-  duct the exhaust gases rearward with a mini-  mum energy loss. However, if the turbine  operates  against a noticeable back pressure,  the  nozzle must convert the remaining pressure  energy into exhaust gas velocity.  Under ideal  conditions, the nozzle would expand the flow  to the ambient static pressure at the exhaust  and the area distribution in the nozzle must  provide these conditions.  When the ratio af  exhaust gas pressure to ambient pressure is  relatively low and incapable of producing sonic  flow, a converging nozzle provides the expan-  sion. The exit area must be of proper size to  bring about proper exit conditions.  If the exit  Figure 2.9 provides illustration of the func-  tion of the various engine components and the  changes in static pressure, temperature, and  velocity through the engine. The conditions  at the inlet provide the initial properties of the  engine airflow.  The compressor section fur-  nishes the compression pressure rise with a  certain unavoidable but undesirable increase  in  temperature.  High pressure air delivered to  combustion chamber receives heat from the  combustion of fuel and experiences  a rise in  temperature. The fuel flow is limited so that  the turbine inlet temperature is within limits  which can be tolerated by the turbine structure.  The combustion takes place at relatively con-  stant pressure  and initially low velocity. Heat  addition then causes  large increases  in gas vol-  ume and flow velocity.  114 