A characteristic of the turboprop is that changes in power are  not related to engine speed, but to turbine inlet temperature.  During flight, the propeller maintains a constant engine  speed. This speed is known as the 100 percent rated speed  of the engine, and it is the design speed at which most power  and best overall efficiency can be obtained. Power changes  are affected by changing the fuel flow. An increase in fuel  flow causes an increase in turbine inlet temperature and a  corresponding increase in energy available at the turbine. The  turbine absorbs more energy and transmits it to the propeller  in the form of torque. The propeller, in order to absorb the  increased torque, increases blade angle, thus maintaining  constant engine rpm with added thrust. The function of the reduction gear assembly is to reduce  the high rpm from the engine to a propeller rpm that can be  maintained without exceeding the maximum propeller tip  speed (speed of sound). Most reduction gear assemblies use  a planetary gear reduction.  [Figure 7-46]  Additional power  takeoffs are available for propeller governor, oil pump, and  other accessories. A propeller brake is often incorporated into  the gearbox. The propeller brake is designed to prevent the  propeller from windmilling when it is feathered in flight, and  to decrease the time for the propeller to come to a complete  stop after engine shutdown. The turbo-propeller provides an efficient and flexible  means of using the power of the engine at any condition  in flight (alpha range).  [Figure 7-47]  For ground handling  and reversing (beta range), the propeller can be operated  to provide either zero or negative thrust. The major  subassemblies of the propeller assembly are the barrel,  dome, low-pitch stop assembly, overspeed governor, pitch  control unit, auxiliary pump, feather and unfeather valves,  torque motor, spinner, deice timer, beta feedback assembly,  The power lever directs a signal from the cockpit to the fuel  control for a specific amount of power from the engine. The  fuel control and the propeller governor together establish  the correct combination of rpm, fuel flow, and propeller  blade angle to create sufficient propeller thrust to provide  the desired power. The turbofan engine produces thrust directly; the turboprop  engine produces thrust indirectly because the compressor and  turbine assembly furnishes torque to a propeller, producing  the major portion of the propulsive force that drives the  aircraft. The turboprop fuel control and the propeller governor  are connected and operate in coordination with each other.  The propeller control system is divided into two types of  control: one for flight and one for ground operation. For  flight, the propeller blade angle and fuel flow for any given  power lever setting are governed automatically according to a  predetermined schedule. Below the “flight idle” power lever  position, the coordinated rpm blade angle schedule becomes  incapable of handling the engine efficiently. Here, the ground  handling range, referred to as the beta range, is encountered. In  the beta range of the throttle quadrant, the propeller blade angle  is not governed by the propeller governor but is controlled by  the power lever position. When the power lever is moved below  the start position, the propeller pitch is reversed to provide  reverse thrust for rapid deceleration of the aircraft after landing. 7-31 Figure 7-46.  Reduction gearbox.  Figure 7-45.  Pratt & Whitney 150 turboprop engine.  Figure 7-44.  Pratt & Whitney PT6 engine.  Reduction Gear Assembly Turbo-Propeller Assembly