NAVWEPS 00-801-80  AIRMANE Pl?RFORMANCE  (c) Very high altitude flight produces  low  compressor  Reynolds numbers and an effect  similar to that of airfoil sections. As a  decrease  to low Reynolds numbers reduces  the section c&,  very high altitudes reduce  the maximum pressure ratio of the com-  pressor. The reduced stall margins increase  the likelihood of compressor  stall.  attack for the rotating blade with a subsequent  increase in pressure rise.  Of course, if the  change in angle of attack or pressure rise is  beyond some critical value, stall will occur.  While the stall phenomenon of a series of  rotating compressor blades differs from that  of a single airfoil section in a free airstream,  the cause and effect are essentially the same.  If an excessive pressure rise is required  through the compressor,  stall may occur with  the attendant breakdown of stable, steady flow  through the compressor.  As stall occurs, the  pressure  rise drops and the compressor  does not  furnish discharge at a pressure equal to the  combustion chamber pressure. As a result, a  flow reversal or backfire takes place. If the  stall is transient and intermittent, the indica-  tion will be the intermittent “bang”  as back-  fire and flow reversal take place. If the stall  develops and becomes  steady, strong vibration  and a loud (and possibly expensive) roar  develops from the continuous flow reversal.  The increase in compressor power required  tends to reduce RPM and the reduced airflow  and increased  fuel flow cause rapid, immediate  rise in exhaust gas temperature. The pos-  sibility of damage is immediate with the steady  stall and recovery must be accomplished  quickly by reducing throttle setting, lowering  the airplane angle of attack, and increasing  airspeed. Generally, the compressor stall is  caused by one or a combination of the fol-  lowing items:  Thus, the recovery from a compressor stall  must entail reduction of throttle setting to  reduce fuel flow, lowering angle of attack and  sideslip and increasing airspeed to improve  inlet condition, and reducing altitude if high  altitude is a contributing factor.  (3) While the j7ameout  is a rare occurrence  with  modern engines, various malfunctions  and operating conditions allow the flameout to  remain a possibility.  A uniform mixture of  fuel and air will sustain combustion within a  relatively wide range of fuel-air ratios. Com-  bustion can be sustained with a fuel-air ratio  as rich as one to five or as lean as one to twenty-  five. Fuel air ratios outside these limits will  not support combustion due to the deficiency  of air or deficiency  of fuel.  The characteristics  of the fuel nozzle and spray pattern as well as  the governing apoaratus must insure that the  nucleus of combt .,on is maintained through-  out the range of engine operation.  If the rich limit of fuel-air ratio is exceeded  in the combustion chamber, the flame will  blow out.  While this condition is a pos-  sibility the more usual cause of a flameout is  exceeding the lean blowout limit.  Any con-  dition  which produces some fuel-air ratio  leaner than the lean limit of combustion will  produce a flameout. Any interruption of the  fuel supply could bring on this condition.  Fuel system failure, fuel system icing, or pro-  longed unusual attitudes could starve the flows  of fuel to the engine. It should be noted the  majority  of aviation fuels are capable of  holding in solution a certain small amount of  water.  If the aircraft is refueled with rela-  tively w&m fuel then flown to high altitude,  (ti) A malfunctioning fuel control or gov-  erning apparatus is a common cause. Proper  maintenance  and adjustment is a necessity  for  stall-free operation. The malfunctioning is  most  usually  apparent  during  engine  acceleration.  (6) Poor inlet conditions are typical at  high angles of attack and sideslip. These  conditions reduce inlet airflow and create  nonuniform flow conditions at the com-  pressor  face. Of course, these conditions are  at the immediate control of the pilot.  127 