🔵✈️ 204. Most Important Characteristics of Turboprop Engines 🚁🎞️

Source: FAA-H-8083-32A, Aviation Maintenance Technician Handbook Powerplant, Volume 1, Pag: 1-56

Aviation: Turboprop Engines

The turbopropeller (turboprop) engine is a combination of a gas turbine engine, reduction gear box, and a propeller. 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:

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.  

Engine Type/Component	Description	Function/Mechanism	Operational Advantages	Mechanical Connection Type
Turboprop Engine	A propulsion system combining a gas turbine engine, reduction gear box, and a propeller.	The gas generator produces energy while additional turbine stages extract power to drive the propeller via a shaft and gear train.	Efficiently converts gas energy into shaft power for propulsion; exhaust gases provide supplemental thrust.	Shaft and gear train
Fixed Turbine	A turboprop configuration where the power section is physically linked to the drive components.	Utilizes a direct mechanical connection from the gas generator to the reduction gear box and propeller.	Provides direct and immediate power transfer from the gas generator.	Mechanical connection
Free Turbine	A turboprop configuration where the propeller is driven by a dedicated power turbine.	Uses an air link (gas coupling) to transfer energy from the gas generator to the power turbines.	Eliminates the mechanical link between the propeller and gas generator; selection is dictated by airframe requirements.	Air link (no mechanical link)
Reduction Gearbox Assembly	A specialized section unique to turboprop engine configurations.	Reduces high engine RPM to suitable propeller RPM and transmits power to the propeller.	Allows both the turbine and the propeller to operate at their respective optimal and efficient speed ranges.	Gear train
Power Section Assembly	The core gas turbine portion of the turboprop engine.	Contains major assemblies including the compressor, combustion chamber, turbine, and exhaust sections.	Generates the primary power and high-velocity exhaust gases for the propulsion system.
Torquemeter Assembly	Specific assembly within the turboprop engine structure.	Transmits torque from the engine power section to the reduction gearbox assembly.	Provides a dedicated means to measure and transmit engine torque levels.	Mechanical transmission
Accessory Drive Housing Assembly	An assembly mounted on the bottom of the compressor air inlet housing.	Uses gear trains to drive all power section accessories at the correct RPM relative to the engine speed.	Ensures that all engine accessories operate in synchronization with engine RPM.	Gear trains