Editing Turboprop (section)

==Technological aspects==
[[File:Turboprop operation-en.svg|thumb|Schematic diagram showing the operation of a turboprop engine]]
[[File:Propulsive efficiency for different engine types and Mach numbers.png|thumb|Propulsive efficiency for different engine types and Mach numbers]]

Exhaust thrust in a turboprop is sacrificed in favor of shaft power, which is obtained by extracting additional power (beyond that necessary to drive the compressor) from turbine expansion. Owing to the additional expansion in the turbine system, the residual energy in the exhaust jet is low.<ref name=glennTur>{{Cite web
| last = Hall | first = Nancy
| year = 2021
|title=Turboprop Engine|url=https://www.grc.nasa.gov/WWW/K-12/airplane/aturbp.html|access-date=2023-03-14|website=Glenn Research Center
| publisher= NASA}}</ref><ref name=glennThr>{{Cite web|title=Turboprop Thrust
| last = Hall | first = Nancy
| year = 2021
|url=https://www.grc.nasa.gov/WWW/K-12/airplane/turbprp.html|access-date=2023-03-14|website=Glenn Research Center
| publisher= NASA}}</ref><ref name=lyle>{{cite web|url=http://lyle.smu.edu/propulsion/Pages/variations.htm|title=Variations of Jet Engines|work=smu.edu|access-date=31 August 2016}}</ref> Consequently, the exhaust jet produces about 10% of the total thrust.<ref name="srm">"[http://www.srmuniv.ac.in/downloads/turbofan-2012.pdf "The turbofan engine] {{Webarchive|url=https://web.archive.org/web/20150418181832/http://www.srmuniv.ac.in/downloads/turbofan-2012.pdf |date=18 April 2015 }}", page 7. [[SRM Institute of Science and Technology]], Department of aerospace engineering.</ref> A higher proportion of the thrust comes from the propeller at low speeds and less at higher speeds.<ref>{{cite book |title= Performance and Stability of Aircraft |author= J. Russell |publisher= Butterworth-Heinemann |date= 2 August 1996 |isbn= 0080538649 |page= 16 |url= https://books.google.com/books?id=IuGILwrEMBYC&q=0080538649&pg=PA16}}</ref>

Turboprops have [[bypass ratio]]s of 50–100,<ref name=kroo>Ilan Kroo and Juan Alonso. "[http://adg.stanford.edu/aa241/propulsion/propulsionintro.html Aircraft Design: Synthesis and Analysis, Propulsion Systems: Basic Concepts] {{webarchive|url=https://web.archive.org/web/20150418150746/http://adg.stanford.edu/aa241/propulsion/propulsionintro.html |date=18 April 2015 }}" Stanford University School of Engineering, Department of Aeronautics and Astronautics [http://adg.stanford.edu/aa241/AircraftDesign.html Main page] {{webarchive|url=https://web.archive.org/web/20010223232617/http://adg.stanford.edu/aa241/AircraftDesign.html |date=23 February 2001 }}</ref><ref name=Spak>[http://web.mit.edu/aeroastro/people/spakovszky.html Prof. Z. S. Spakovszky]. "[http://web.mit.edu/16.unified/www/FALL/thermodynamics/notes/node84.html 11.5 Trends in thermal and propulsive efficiency]" ''[[School of Engineering, Massachusetts Institute of Technology#Aeronautics and Astronautics|MIT turbines]]'', 2002. [http://web.mit.edu/16.unified/www/FALL/thermodynamics/notes/notes.html Thermodynamics and Propulsion]</ref> although the propulsion airflow is less clearly defined for propellers than for fans.<ref name=glennProp>{{Cite web|title=Propeller Thrust
| last = Hall | first = Nancy
| year = 2021
|url=https://www.grc.nasa.gov/WWW/K-12/airplane/propth.html|access-date=2023-03-14|website=Glenn Research Center
| publisher=NASA}}</ref><ref name=walsh>{{Cite book
| title = Gas Turbine Performance
| page = 36
| author1-first = Philip
| author1-last =Walsh
| author2-first= Paul
| author2-last=Fletcher
| isbn = 9781405151030
| publisher= John Wiley and Sons
| year = 2008
| url = https://books.google.com/books?id=DtFml9BQkEIC
}}</ref>

The propeller is coupled to the turbine through a [[reduction gear]] that converts the high [[Revolutions per minute|RPM]]/low [[torque]] output to low RPM/high torque. This can be of two primary designs, free-turbine and fixed. A [[free-turbine turboshaft]] found on the [[Pratt & Whitney Canada PT6]], where the gas generator is not connected to the propeller. This allows for [[propeller strike]] or similar damage to occur without damaging the gas generator and allowing for only the power section (turbine and gearbox) to be removed and replaced in such an event, and also allows for less stress on the start during engine ground starts. Whereas a [[Turboshaft|fixed shaft]] has the gearbox and gas generator connected, such as on the [[Honeywell TPE331]].

The propeller itself is normally a [[constant-speed propeller|constant-speed (variable pitch) propeller]] type similar to that used with larger aircraft [[reciprocating engine]]s, except that the propeller-control requirements are very different.<ref>Airscrews For Turbines, Fairhurst, Flight magazine, 10 November 1949, p.609</ref> Due to the turbine engine's slow response to power inputs, particularly at low speeds, the propeller has a greater range of selected travel in order to make rapid thrust changes, notably for taxi, reverse, and other ground operations.<ref name=":0">{{Cite book |title=A&P Powerplant Textbook |publisher=Jeppeson Company |year=2011 |isbn=978-0884873389 |edition=3rd}}</ref> The propeller has 2 modes, Alpha and Beta. Alpha is the mode for all flight operations including takeoff. Beta, a mode typically consisting of zero to negative thrust, is used for all ground operations aside from takeoff.<ref name=":0" /> The Beta mode is further broken down into 2 additional modes, Beta for taxi and Beta plus power. Beta for taxi as the name implies is used for taxi operations and consists of all pitch ranges from the lowest alpha range pitch, all the way down to zero pitch, producing very little to zero-thrust and is typically accessed by moving the power lever to a beta for taxi range. Beta plus power is a reverse range and produces negative thrust, often used for landing on short runways where the aircraft would need to rapidly slow down, as well as backing operations and is accessed by moving the power lever below the beta for taxi range.<ref name=":0" /> Due to the pilot not being able to see out of the rear of the aircraft for backing and the amount of debris reverse stirs up, manufacturers will often limit the speeds beta plus power may be used and restrict its use on unimproved runways.<ref name=":0" /> Feathering of these propellers is performed by the propeller control lever.<ref name=":0" />

The constant-speed propeller is distinguished from the reciprocating engine constant-speed propeller by the control system. The turboprop system consists of 3 [[propeller governor]]s, a governor, and overspeed governor, and a fuel-topping governor.<ref name=":0" /> The governor works in much the same way a reciprocating engine propeller governor works, though a turboprop governor may incorporate beta control valve or beta lift rod for beta operation and is typically located in the 12 o'clock position.<ref name=":0" /> There are also other governors that are included in addition depending on the model, such as an overspeed and fuel topping governor on a [[Pratt & Whitney Canada PT6]], and an under-speed governor on a [[Honeywell TPE331]].<ref name=":0" />  The turboprop is also distinguished from other kinds of [[Gas turbine|turbine engine]] in that the fuel control unit is connected to the governor to help dictate power.

To make the engine more compact, reverse airflow can be used. On a reverse-flow turboprop engine, the compressor intake is at the aft of the engine, and the exhaust is situated forward, reducing the distance between the turbine and the propeller.<ref>{{cite web |last=Martin |first=Swayne |date=2019-05-16 |title=How A Turboprop Engine Works |url=https://www.boldmethod.com/learn-to-fly/systems/this-is-how-a-turboprop-engine-works/ |url-status=live |archive-url=https://web.archive.org/web/20211106054553/https://www.boldmethod.com/learn-to-fly/systems/this-is-how-a-turboprop-engine-works/ |archive-date=2021-11-06 |access-date=2021-11-06 |website=Boldmethod}}</ref>

Unlike the small-diameter fans used in [[turbofan]] engines, the propeller has a large diameter that lets it accelerate a large volume of air. This permits a lower airstream velocity for a given amount of thrust. Since it is more efficient at low speeds to accelerate a large amount of air by a small degree than a small amount of air by a large degree,<ref name=bevil>[[Paul Bevilaqua]]. [http://www.dtic.mil/dticasd/sbir/sbir032/n184.doc The shaft driven Lift Fan propulsion system for the Joint Strike Fighter] {{Webarchive|url=https://web.archive.org/web/20110605073353/http://www.dtic.mil/dticasd/sbir/sbir032/n184.doc |date=5 June 2011 }} page 3. Presented 1 May 1997. DTIC.MIL Word document, 5.5 MB. Retrieved 25 February 2012.</ref><!--thrust formula is the same for propellers and hover jets--><ref>{{cite web |last1=Bensen |first1=Igor B. |title=How They Fly |url=http://www.gyrocopters.co.uk/html/dr_bensen_explains_all.html |access-date=31 May 2023 |archive-url=https://web.archive.org/web/20010420164042/http://www.gyrocopters.co.uk/html/dr_bensen_explains_all.html |archive-date=April 20, 2001 |url-status=unfit}}</ref> a low [[disc loading]] (thrust per unit disc area) increases the aircraft's [[Efficient energy use|energy efficiency]], and this reduces the fuel use.<ref name=wayne>{{Cite book|last=Johnson|first=Wayne|url=https://books.google.com/books?id=SgZheyNeXJIC|title=Helicopter Theory|date=1994-01-01|publisher=Courier Corporation|isbn=978-0-486-68230-3|language=en}}</ref><ref name=step>{{Cite book|last1=Stepniewski|first1=Wieslaw Zenon|url=https://books.google.com/books?id=PawbFeAAllIC|title=Rotary-wing Aerodynamics|last2=Keys|first2=C. N.|date=1984-01-01|publisher=Courier Corporation|isbn=978-0-486-64647-3|language=en}}</ref>

Propellers work well until the flight speed of the aircraft is high enough that the airflow past the blade tips reaches the speed of sound. Beyond that speed, the proportion of the power that drives the propeller that is converted to propeller thrust falls dramatically. For this reason turboprop engines are not commonly used on aircraft<ref name=glennTur/><ref name=glennThr/><ref name=lyle/> that fly faster than 0.6–0.7 [[Mach number|Mach]],<ref name=srm/> with some exceptions such as the [[Tupolev Tu-95]]. However, [[propfan]] engines, which are very similar to turboprop engines, can cruise at flight speeds approaching 0.75 Mach. To maintain propeller efficiency across a wide range of airspeeds, turboprops use constant-speed (variable-pitch) propellers. The blades of a constant-speed propeller increase their pitch as aircraft speed increases. Another benefit of this type of propeller is that it can also be used to generate reverse thrust to reduce stopping distance on the runway. Additionally, in the event of an engine failure, the propeller can be [[Feathering (propeller)#Feathering|feathered]], thus minimizing the drag of the non-functioning propeller.<ref>{{Cite web|url=https://www.experimentalaircraft.info/articles/aircraft-propeller-12.php|title=Operating Propellers during Landing & Emergencies|website=experimentalaircraft.info|access-date=2019-07-08}}</ref>

While the power turbine may be integral with the gas generator section, many turboprops today feature a free power turbine on a separate coaxial shaft. This enables the propeller to rotate freely, independent of compressor speed.<ref name=pt6nation_2>{{cite web|url=http://www.pt6nation.com/en/articles/article/history-an-engine-ahead-of-its-time/|title=An Engine Ahead of Its Time|author=<!--Staff writer(s); no by-line.-->|website=PT6 Nation|publisher=Pratt & Whitney Canada|access-date=18 April 2015|archive-date=7 June 2013|archive-url=https://web.archive.org/web/20130607032911/http://www.pt6nation.com/en/articles/article/history-an-engine-ahead-of-its-time/|url-status=dead}}</ref>