The turboprop engine offers several advantages over other  types of engines, such as: • Lightweight • Mechanical reliability due to relatively few moving  parts • Simplicity of operation • Minimum vibration • High power per unit of weight • Use of propeller for takeoff and landing Turboprop engines are most efficient at speeds between 250  and 400 mph and altitudes between 18,000 and 30,000 feet.  They also perform well at the slow speeds required for takeoff  and landing and are fuel efficient. The minimum specific fuel  consumption of the turboprop engine is normally available in  the altitude range of 25,000 feet up to the tropopause. The power output of a piston engine is measured in  horsepower and is determined primarily by rpm and manifold  pressure. The power of a turboprop engine, however, is  measured in shaft horsepower (shp). Shaft horsepower is  determined by the rpm and the torque (twisting moment)  applied to the propeller shaft. Since turboprop engines  are gas turbine engines, some jet thrust is produced by  exhaust leaving the engine. This thrust is added to the shaft  horsepower to determine the total engine power or equivalent  shaft horsepower (eshp). Jet thrust usually accounts for less  than 10 percent of the total engine power. Although the turboprop engine is more complicated and  heavier than a turbojet engine of equivalent size and power,  it delivers more thrust at low subsonic airspeeds. However,  the advantages decrease as flight speed increases. In normal  cruising speed ranges, the propulsive efficiency (output  divided by input) of a turboprop decreases as speed increases. Although power decreases as the airplane climbs to  higher altitudes, engine efficiency in terms of specific  fuel consumption (expressed as pounds of fuel consumed  The gas mixture is initially ignited by high-energy igniter  plugs, and the expanding combustion gases flow to the  turbine. The energy of the hot, high-velocity gases is  converted to torque on the main shaft by the turbine rotors.  The reduction gear converts the high rpm—low torque of the  main shaft to low rpm—high torque to drive the accessories  and the propeller. The spent gases leaving the turbine are  directed to the atmosphere by the exhaust pipe. Fixed Shaft One type of turboprop engine is the fixed shaft constant speed  type, such as the Garrett TPE331.  [Figure 14-2]  In this type  engine, ambient air is directed to the compressor section  through the engine inlet. An acceleration/diffusion process  in the two stage compressor increases air pressure and directs  it rearward to a combustor. The combustor is made up of a  combustion chamber, a transition liner, and a turbine plenum.  Atomized fuel is added to the air in the combustion chamber.  Air also surrounds the combustion chamber to provide for  cooling and insulation of the combustor. Turboprop Engine Types  All turbine engines, turboprop or turbojet, are defined by  limiting temperatures, rotational speeds, and (in the case  of turboprops) torque. Depending on the installation, the  primary parameter for power setting might be temperature,  torque, fuel flow, or rpm (either propeller rpm, gas generator  (compressor) rpm, or both). In cold weather conditions,  torque limits can be exceeded while temperature limits are  still within acceptable range. While in hot weather conditions,  temperature limits may be exceeded without exceeding  torque limits. In any weather, the maximum power setting  of a turbine engine is usually obtained with the throttles  positioned somewhat aft of the full forward position. The  transitioning pilot must understand the importance of  knowing and observing limits on turbine engines. An over  temperature or over torque condition that lasts for more than a  few seconds can literally destroy internal engine components. per horsepower per hour) is increased. Decreased specific  fuel consumption plus the increased true airspeed at higher  altitudes is a definite advantage of a turboprop engine. 14-3