NAVWEPS 00-8OT-30  AIRPLANE PERFORMANCE  During deceleration conditions, the mini-  mum allowable fuel flow is defined by the lean  limit to support combustion.  If the fuel flow  is reduced below some critical value at each  RPM, lean blowout or flameout will occur.  This condition is illustrated by curve 3 of  figure 2.12 which forms the deceleration fuel  flow boundary.  The governing apparatus  must  regulate the deceleration fuel flow within this  boundary.  2 of figure 2.12. Curve 2 of this illustration  defines an upper limit of fuel flow which can  be tolerated within stall-surge and tempera-  ture limits.  The governing apparatus of the  engine must limit the acceleration fuel flow  within this boundary.  To appreciate the governing requirements  during the acceleration process, assume the  engine described  in figure 2.12 is in steady state  stabilized operation at point A and it is desired  to acceler&the engine to maximum RPM and  stabilize:at point C.  As the throttle is placed  at the position for maximum RPM, the fuel  control will increase the fuel flow to point B  to provide acceleration fuel flow.  As the  engine accelerates  and increases  RPM, the fuel  control will continue to increase the fuel flow  within  the acceleration boundary until  the  engine speed approaches the controlled maxi-  mum RPM at point C. As the engine speed  nears the maximum at point C, the fuel contrcl’  will reduce  fuel flow to produce stabilized oper-  ation at this point and prevent the engine  overspeeding  the commanded  RPM.  Of course,  if the throttle is opened very gradually, the  acceleration  fuel flow is barely above the steady  state condition and the engine does not ap-  proach the acceleration fuel flow boundary.  While this technique is recommended for  ordinary conditions to achieve trouble free  operation and good service  life, the engine must  be capable of good acceleration to produce  rapid thrust changes for satisfactory flight  control.  To appreciate the governing requirements  during the deceleration process, assume the  engine described in figure 2.12 is in stabilized,  steady state operation at point C and it is  desired to decelerate to idle conditions and  stabilize at point E.  As the throttle is placed  at the position for idle RPM, the fuel control  will decrease  the fuel flow to point D to provide  the deceleration fuel flow.  As the engine  decelerates  and decreases  RPM, the fuel gov-  erning will continue to decrease  the fuel flow  within the deceleration boundary until the idle  fuel flow is reached and RPM is established at  point E. Of course, if the throttle is closed  very slowly, the deceleration fuel flow is barely  below the steady state condition and the engine  does not approach the deceleration fuel flow  boundary. The fuel control must provide a  deceleration flow close to the boundary to  provide rapid decrease  in thrust and satisfactory  flight control.  In most cases, the deceleration fuel flow  boundary is considerably below the steady  state fuel flow and no great problem exists in  obtaining satisfactory deceleration character-  istics. In fact, the greater problem is con-  cerned with  obtaining  proper acceleration  characteristics.  For the majority of centrifu-  gal flow engines, the acceleration boundary is  set usually by temperature limiting conditions  rather than compressor  surge conditions.  Peak  operating efficiency of the centrifugal com-  pressor is obtained at flow conditions which  are below the surge limit, hence acceleration  fuel flow boundary is determined by turbine  temperature limits.  The usual result is that  In order for the powerplant to achieve mini-  mum acceleration times, the fuel control must  provide acceleration fuel flow as close as  practical to the acceleration boundary.  Thus,  a maximum controlled acceleration may pro-  duce limiting  turbine inlet temperatures or  slight incipient stall-surge of the compressor.  Proper maintenance and adjustment of the  engine governing apparatus is essential to  produce minimum acceleration times without  incurring excessive  temperatures  or heavy stall-  surge conditions.  123 