text: engine-propeller combination is operated at a 
  constant RPM throughout the major range of 
  output power and the principal variables ofcon- 
  trol are fuel flow and propeller blade angle. 
  In 
  the 
  the major range of power output, 
  throttle commands a certain fuel flow and the 
  propeller blade angle adjusts to increase the 
  propeller 
  load and remain at the governed 
  speed. 
  
  The operating limitations of the turboprop 
  powerplant are quite similar in nature to the 
  operating limitations of the turbojet engine. 
  Generally, the turbine temperature limnations 
  are the most critical items. 
  In addition, over- 
  speed conditions can produce overstress of the 
  gearing and propeller as well as overstress of 
  the turbine section. 
  
  The gas turbine engine is capable of processing 
  large quantities of air and can produce high 
  output power for a given engine size. Thus, 
  the principal advantage of 
  turboprop 
  powerplant is the high specific power output, 
  high power per engine weight and high power 
  per engine size. 
  
  The gas turbine engine must operate at quite 
  high rotative speed to process large airflows 
  and produce high power. However, high 
  rotative speeds are not conducive to high 
  propeller efficiency because of compressibility 
  effects. A large reduction of shaft speed must 
  be provided in order to match the powerplant 
  and the propeller. The reduction gearing must 
  provide a propeller shaft speed which can be 
  utilized effectively by the propeller and, be- 
  cause of the high rotative speeds of the turbine, 
  gearing ratios of 6 to 15 may be typical. The 
  transmission of large shaft horsepower with 
  such high gearing involves considerable desi,gn 
  problems to provide good service life. The 
  problems of such gearing were one of 
  the 
  greatest difficulties 
  the development of 
  in 
  turboprop powerplants. 