The Parts of a Turboprop Engine and How they Work Together to Propel Flight

Chelsea Walker

English 202c

10/18/2012

Audience and Purpose: New pilots learn how to fly airplanes, but few learn how the airplane’s propulsion system works. This document provides an explanation of how a turboprop engine works and why it makes the airplane fly. This technical definition and description is intended for new pilots learning to fly aircrafts with turboprop engines. After reading this article, the reader should be able to identify the parts of a turboprop engine and how they are connected to propel flight.

The audience is young pilots that have been through college level math and science classes. Pilot’s licenses are not available until at least the age of 16. I am intending this document to be for students that have taken college freshman level physics and chemistry, whether through AP classes or college classes.

Parts of a Turboprop Engine


A turboprop is an aircraft engine, used primarily in regional flights, which utilizes a propeller to generate thrust for the aircraft. Regional jets use turboprop engines because they are more efficient for smaller planes with short flights. Regional jets perform short flights of less than 3 hours and have about 50 seats or less. A turboprop engine has six main parts; the propeller/gearbox/shaft, inlet, compressor, combustion chamber, turbine, and exhaust nozzle. Figure 2 displays the parts of a turboprop engine in order from front to back.The propeller uses a gas turbine engine to produce the energy to turn the propeller. Turboprops allow for a short duration flight aircraft to have high thrust and low fuel consumption.

Figure 1: Parts of a Turboprop Engine

This description will provide the parts of the engine, starting at the front and progressing towards the back, and how the parts work together to produce thrust. Each step has a picture of the engine displayed in Figure 2 with how the step works in that part of the engine.

Figure 2: Parts of the Engine used to Describe Engine Process

  1. Propeller, Gearbox, and Shaft

In a turboprop engine, the majority of the thrust comes from the propeller and not the jet. Two types of propeller configurations typically exist on a turboprop engine, three bladed propeller or four bladed propeller. A four propeller engine creates the same amount of thrust as a three propeller engine but at a lower RPM. The number of propellers directly relates to how hard the engine needs to work. The more propellers present, the slower the propellers need to rotate, allowing the engine to produce less energy.

By operating at a lower RPM, because of the extra blade, the engine produces less noise than at a higher RPM. For commercial travel, noise annoys not only the passengers on the plane but also the communities that the aircraft flies over. So why not just put five propellers on? Most of the airplanes with propellers have already been built and changing the propellers costs more than the benefits worth.

Figure 3: A four bladed propeller on the left; three bladed propeller on the right

The turbine towards the back of the engine drives the propeller through the use of the gearbox and the shaft. The reduction gearbox powers the propeller by converting the high RPM, low torque output from the turbine to low RPM, high torque for the propeller. The turbine and the propeller operate at their most efficient speeds through use of the gearbox. The propeller becomes inefficient at high speeds so the gearbox is an important aspect of the engine.The propeller usually rotates at a rate between 1000 and 1200 RPM, approximately 15 times slower than the turbine.

Figure 4: The blue lines show the propellers circulating the air over the engine.

Propellers require close calculations and tests when designing an engine and airplane. Propellers decrease in efficiency as the aircraft increases in speed; therefore, propellers work best for short missions with a lenient time requirement. Short missions comprise any flight less than three hours in length. The flight from JFK airport in New York to Ft. Lauderdale Airport in Florida is approximately three hours, to give some perspective. Cargo planes or regional aircraft use propellers because of the short flights. Cargo planes specifically do not have strict time requirements, so the engine can operate at its most efficient speed.

  1. Inlet


Air flows into the engine through the inlet at the front of the engine. This initiates the engine process. Figure 5shows the air flowing into the inlet, below the propeller and through the rest of the engine.

Figure 5: The air enters through the inlet below the propeller and travels through the engine

The air ingested by the inlet either enters the engine or completely bypasses the engine and leaves with the exhaust. Next, the air in the engine moves to the compressor to start the compression process.

  1. Compressor


The compressor prepares the air for the combustion chamber. When the air enters the compressor, the compressor constricts the air, which increases the pressure of the air. The compressor raises the temperature and pressure of the air so it can enter the combustion chamber. The air that leaves the compressor is typically on the order of 1000°F hotter and 30 times higher in pressure. The blue lines, in Figure 6, show that the air is still not hot enough to combust.

Figure 6: The air travels to the blue “gears” and is compressed

  1. Combustion Chamber

The air then moves from the compressor into the combustion chamber. With the air heated and pressurized, the air prepares for the injection of the fuel. In the combustion chamber, spark plugs ignite the fuel and air mixture. This ignition causes the mixture to rise in temperature once again, reaching over 3000°F. The red ring in Figure 7 shows the air changing drastically in temperature.


Figure 7: The air reaches the red ring where it is mixed with fuel and ignited

Combustion increases the internal energy of a gas, which can result in either an increase in temperature, pressure, or volume. For this specific process, the combustion of air results in an increase in temperature. A change in volume also occurs because the compressed air mixed with the fuel expands to the turbine.

  1. Turbine

The air/fuel mixture from the compressor goes to the turbine and expands.

The turbines, the yellow wheels in Figure 8, act like windmills. The hot gas acts like the wind and causes the turbines to rotate. The coupled turbines cause the shaft to rotate the compressors and the propeller. The act of capturing the air and accelerating it in the engine is called the Brayton Cycle.


Figure 8: The air travels through the yellow “gears” and causes them to rotate the shaft

  1. Exhaust

The last process for the turboprop engine disposes of the excess air that was not used to turn the shaft. The exhaustexits the aircraft at a high speedproducing additional thrust but not enough to completely propel the aircraft.


The cloud at the end of the engine in Figure 9 shows the exhaust exiting the engine. The red arrows represent the temperature of the exhaust.

Figure 9: The excess hot air leaves the engine through the exhaust nozzle

Conclusion

In conclusion, all parts of a turboprop engine work together to keep an aircraft flying and the engine running. If one part of the engine does not operate properly, the entire engine becomes useless. A short recap of the steps involved for a turboprop engine to operate:

  1. The air enters the inlet which then travels to the compressor. Some of the air may completely bypass the engine.
  2. The compressor increases the temperature and pressure of the air so that it is most effectively used in the combustion chamber.
  3. The combustion chamber adds fuel to the heated air, ignites the fuel which further increases the temperature.
  4. The turbine expands the heated air mixture and uses the change in internal energy to power the gearbox and propeller.
  5. The exhaust takes the leftover air from the turbine and allows it to exit the engine and create additional thrust.

The turboprop engine helps propel many small aircraft today. A turboprop engine cannot fly at the same speed as a turbojet but saves money in fuel consumption. With the cost of fuel steadily increasing, the emissions from airplanes need to be cut down any way possible. Turboprop engines give an eco-friendly option to airlines at a time when the environment is a concern.

Works Cited

"Engine Education."Pratt & Whitney:.Pratt & Whitney, n.d. Web. 15 Oct. 2012. <

Goodpaster, Ken. "Advanced Lesson on Turboprops and Propellers."Advanced Lesson on Turboprops and Propellers. West Wind Airlines, 14 Mar. 2006. Web. 17 Oct. 2012. <

"Turboprop Engine."Turboprop Engine.Ed. Tom Benson. NASA, 13 Sept. 2010. Web. 16 Oct. 2012. <

"Turboprop."Wikipedia. Wikimedia Foundation, 16 Oct. 2012. Web. 16 Oct. 2012. <