5-5 Turboprop and Turboshaft Engines

• Couples a gas generator with a reduction gear box and propeller, which is driven by the turbine section.
• Propeller provides thrust by imparting a small amount of acceleration to a large mass of air

• Propeller Assembly
• Consists of:
• blades - installed into the hub (barrel assembly
• hub - attached to the propeller shaft
• pitch change/dome assembly - changes the blade angle of the propeller
• Attached to the gas generator by either:
• Front of the compressor drive shaft
• free/power turbines

10% of thrust created by exhaust

• Reduction Gear Box (RGB)
• located between propeller assembly and gas generator.
• Prevents propeller from reaching supersonic speeds.
• Converts high RPM and low torque of the gas generator to low RPM and High torque
• T-6B - 30,000 RPM gas generator to 2000 RPM propeller

• 
• TP are most efficient at lower airspeeds.
• At higher airspeed, Turboprop has a reduction in efficiency, and will be less than both Turbofan and Turbojet at very high speeds.
• Turbofans are more efficient than Turbojets.
• Order of TSFC (Lowest to Highest)
• Turboprop
• Turbofan
• Turbojet

Turboprop has a lower TSFC because its thrust is produced by accelerating a large mass of air at low velocity for a relatively small amount of fuel.

• 
• The torquemeter assembly is a set of shafts located between the gas generator and RGB.
• Used to transmit and measure power output from gas generator to the RGB.
• Accurately measures the twisting movement that occurs in the torque shaft by magnetic pickups.

• Torque shaft (inner shaft) connects the compressor and the RGB. It carries the load from the propeller and produces torsional deflection.
• Reference shaft (outer shaft) is rigidly connected to RGB but free at the other end. It doesn't twist and provides reference

• Turboprop accelerates a very large mass of air to a moderate speed.
• Propeller assembly maintains the propeller at a constant 100% RPM.
• Increasing fuel flow causes an increase in energy available at the turbine.
• The propeller increases blade angle to absorb the increased torque and maintain constant RPM.
• Changes occur through coordination between propeller governor and turboprop engine fuel control unit.

• Two ranges of operation:
• Alpha Range:
• aka flight range
• Power control level (PCL) can be positioned from flight idle to full power.
• PCL sends signals to Fuel Control Unit (FCU) for fuel flow.
• FCU works with prop governor to ensure constant propeller RPM by adjusting blade angle.

• Beta Range:
• Used only during ground operations.
• PCL can be positioned from flight idle to max reverse.
• PCL mechanically connected to pitch change assembly and FCU to allow pilot direct control of blade angle.
• Reversing airflow by reversing blade angle decreases landing distance and increases maneuverability during ground operations.
• T-6B DOES NOT have beta range

Gas generator with a free/power turbine that drives something other than an aircraft propeller.

• Free/Power Turbine (PT)
• Gas generator produces the gases required for compressor, accessories, and power turbine.
• 75% of power produced is used to drive the compressor.
• Exhaust is negligible on thrust.
• Remaining energy is converted into shaft horsepower, used to power the rotor.