NAVWEPS Oo-ROT-30  AIRPLANE PERFORMANCE  the turboprop powerplant is rated by an  “equivalent shaft horsepower.”  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 major range of power output, the  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.  ESHP=equivalent shaft horsepower  EHP= brake horsepower, or shaft horse-  power applied to the propeller  T,= jet thrust, lbs.  V=flight velocity, knots, TAS  ‘ 1s = propeller efficiency  T,y  ESHP= BHP+325vp  where  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 the turboprop  powerplant is the high specific power output,  high power per engine weight and high power  per engine size.  The performance  of the turboprop illustrates  the typical advantages of the propeller-engine  combination.  Higher  propulsive efficiency  and high thrust and low speeds provide the  characteristic of range, endurance, and takeoff  performance superior to the turbojet.  As is  typical of all propeller equipped powerplants,  the power available is nearly constant with  speed.  Because  the power from the jet thrust  depends on velocity, the power available in-  creases slightly  with  speed. However, the  thrust available decreases  with  speed. The  equivalent shaft horsepower, ESHP, of the  turboprop is affected by mass ,flow and inlet  temperature in fashion similar to that of the  turbojet.  Thus, the ESHP will  vary with  altitude much like the thrust output of the  turbojet because the higher altitude produces  much lower density and engine mass flow.  The gas turbine-propeller combination utilizes  a number of turbine stages to extract shaft  power from the exhaust gases and, as high  compressor inlet temperatures reduce the fuel  flow allowable within  turbine temperature  limits, hot days will cause a noticeable loss of  output power.  Generally, the turboprop is  just as sensitive, if not more sensitive, to com-  pressor inlet air temperature as the turbojet  engine.  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 in the development of  turboprop powerplants.  The governing apparatus for the turboprop  powerplant must account for one additional  variable, the propeller blade angle. If the  propeller is governed separately from the tur-  bine, an interaction can exist between the  engine and propeller governers and various  “hunting,”  overspeed, and overtemperature  conditions are possible. For this reason, the  133 