Turboprop Engines Turboshaft Engines The turbopropeller (turboprop) engine is a combination of  a gas turbine engine, reduction gear box, and a propeller.  [Figure 1-78]  Turboprops are basically gas turbine engines  that have a compressor, combustion chamber(s), turbine,  and an exhaust nozzle (gas generator), all of which operate  in the same manner as any other gas engine. However, the  difference is that the turbine in the turboprop engine usually  has extra stages to extract energy to drive the propeller. In  addition to operating the compressor and accessories, the  turboprop turbine transmits increased power forward through  a shaft and a gear train to drive the propeller. The increased  power is generated by the exhaust gases passing through  additional stages of the turbine. Some engines use a multirotor turbine with coaxial shafts  for independent driving of the compressor and propeller.  Although there are three turbines utilized in this illustration,  as many as five turbine stages have been used for driving the  two rotor elements, propeller, and accessories. The exhaust gases also contribute to engine power output  through thrust production, although the amount of energy  available for thrust is considerably reduced. Two basic types  of turboprop engine are in use: fixed turbine and free turbine.  The fixed turbine has a mechanical connection from the gas  generator (gas-turbine engine) to the reduction gear box  and propeller. The free turbine has only an air link from gas  generator to the power turbines. There is no mechanical link  from the propeller to the gas turbine engine (gas generator).  There are advantages and disadvantages of each system, with  the airframe generally dictating the system used.  Since the basic components of normal gas-turbine and  turboprop engines differ slightly only in design features, it  should be fairly simple to apply acquired knowledge of the  basic gas turbine to the turboprop. The typical turboprop engine can be broken down into  assemblies as follows: A gas-turbine engine that delivers power through a shaft to  operate something other than a propeller is referred to as  a turboshaft engine.  [Figure 1-79]  The output shaft may  be coupled directly to the engine turbine, or the shaft may  be driven by a turbine of its own (free turbine) located in  the exhaust stream. As mentioned with the turboprop, the  1. The power section assembly—contains the usual major  components of a gas turbine engine (i.e., compressor,  combustion chamber, turbine, and exhaust sections). 2.  The reduction gear or gearbox assembly—contains  those sections unique to turboprop configurations. 3.  The torquemeter assembly—transmits the torque from  the engine to the gearbox of the reduction section. 4. The accessory drive housing assembly—mounted  on the bottom of the compressor air inlet housing.  It includes the necessary gear trains for driving all  power section driven accessories at their proper rpm  in relation to engine rpm.  Figure 1-78. PT6 turboprop engine.  1-56