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((Short description|Aircraft propulsion component))
((more citations needed|date=September 2011))
[[File:hercules.propeller.arp.jpg|thumb|upright=1.14|The propellers of a [[C-130J Super Hercules]] military transport aircraft]]
In [[aeronautics]], an '''aircraft propeller''', also called an '''airscrew''',<ref name="beaumont">Beaumont, R.A.; ''Aeronautical Engineering'', Odhams, 1942, Chapter 13, "Airscrews".</ref><ref>((cite book |title=A Dictionary of Aviation |first=David W. |last=Wragg |isbn=9780850451634 |edition=first |publisher=Osprey |year=1973 |page=28))</ref> converts rotary motion from an [[Aircraft engine|engine]] or other power source into a swirling slipstream which pushes the propeller forwards or backwards. It comprises a rotating power-driven hub, to which are attached several radial [[airfoil]]-section blades such that the whole assembly rotates about a longitudinal axis. The [[blade pitch]] may be fixed, manually variable to a few set positions, or of the automatically variable "constant-speed" type.

The propeller attaches to the power source's driveshaft either directly or through [[Propeller speed reduction unit|reduction gearing]]. Propellers can be made from wood, metal or [[composite material]]s.

Propellers are most suitable for use at subsonic airspeeds generally below about ((cvt|480|mph)), although supersonic speeds were achieved in the [[McDonnell XF-88B]] experimental propeller-equipped aircraft. Supersonic tip-speeds are used in some aircraft like the [[Tupolev Tu-95]], which can reach ((cvt|575|mph)).((cn|date=June 2021)) 

==History==
((See also|Early flying machines))
[[File:Taketombo.JPG|thumb|A decorated Japanese ''taketombo'' [[bamboo-copter]] ]]
The earliest references for vertical flight came from China. Since around 400 BC,<ref name="Gordon">Leishman, J. Gordon. ''Principles of Helicopter Aerodynamics''. Cambridge aerospace series, 18. Cambridge: [[Cambridge University Press]], 2006. ((ISBN|978-0-521-85860-1)). ((cite web|url=http://terpconnect.umd.edu/~leishman/Aero/history.html|title=A History of Helicopter Flight|access-date=2014-07-15|url-status=dead|archive-url=https://web.archive.org/web/20140713201846/http://terpconnect.umd.edu/~leishman/Aero/history.html|archive-date=2014-07-13)) Web extract</ref> [[China|Chinese]] children have played with [[Bamboo-copter|bamboo flying toys]].<ref>[http://www.aerospaceweb.org/design/helicopter/history.shtml] "Early Helicopter History." ''Aerospaceweb.org.'' Retrieved: 12 December 2010</ref><ref name="Taking Flight: Inventing the Aerial Age, from Antiquity Through the First World War">((cite book|title=Taking Flight: Inventing the Aerial Age, from Antiquity Through the First World War|url=https://archive.org/details/takingflightinve0000hall|url-access=registration|date=8 May 2003|publisher=Oxford University Press|isbn=978-0-19-516035-2|pages=[https://archive.org/details/takingflightinve0000hall/page/n47 22]–23))</ref><ref name="china-1">Goebel, Greg. ((cite web|url=http://www.vectorsite.net/avheli_1.html|title="The Invention Of The Helicopter."|access-date=2008-11-11|url-status=bot: unknown|archive-url=https://web.archive.org/web/20110629140626/http://www.vectorsite.net/avheli_1.html|archive-date=June 29, 2011)) ''Vectorsite.net.'' Retrieved: 11 November 2008</ref> This bamboo-copter is spun by rolling a stick attached to a rotor between one's hands. The spinning creates lift, and the toy flies when released.<ref name="Gordon"/> The 4th-century AD [[Daoist]] book ''[[Baopuzi]]'' by [[Ge Hong]] (抱朴子 "Master who Embraces Simplicity") reportedly describes some of the ideas inherent to rotary wing aircraft.<ref name="china-2">Fay, John. ((cite web|url=http://www.helis.com/pioneers/1.php|title=Pioneers, Evolution of the Rotary Wing Aircraft|access-date=2007-03-21|url-status=live|archive-url=https://web.archive.org/web/20061107114307/http://www.helis.com/pioneers/1.php|archive-date=2006-11-07)) "Helicopter Pioneers&nbsp;– Evolution of Rotary Wing Aircraft." ''Helicopter History Site.'' Retrieved: 28 November 2007</ref>

Designs similar to the Chinese helicopter toy appeared in Renaissance paintings and other works.<ref>[[Donald F. Lach]]. (1977). [https://books.google.com/books?id=N0xD7BYXv_YC] ''Asia in the making of Europe. Volume II, A Century of Wonder''. p. 403</ref>

[[File:Leonardo da Vinci helicopter.jpg|thumb|[[Leonardo's aerial screw]] ]]
It was not until the early 1480s, when [[Leonardo da Vinci]] created a design for a machine that could be described as an [[Leonardo's aerial screw|"aerial screw"]], that any recorded advancement was made towards vertical flight. His notes suggested that he built small flying models, but there were no indications for any provision to stop the rotor from making the craft rotate.<ref name="flight-1">Rumerman, Judy. ((cite web|url=http://www.centennialofflight.net/essay/Rotary/early_helicopters/HE1.htm|title=Early Helicopter Technology|access-date=2014-02-02|url-status=live|archive-url=https://web.archive.org/web/20140220023307/http://www.centennialofflight.net/essay/Rotary/early_helicopters/HE1.htm|archive-date=2014-02-20)) "Early Helicopter Technology." ''Centennial of Flight Commission,'' 2003. Retrieved 12 December 2010</ref><ref name="aerial screw">Pilotfriend.com ((cite web|url=http://www.pilotfriend.com/photo_albums/helicopters/Leonardo%20Da%20Vinci's%20Helical%20Air%20Screw.htm|title=Leonardo da Vinci's Helical Air Screw|access-date=2015-02-07|url-status=live|archive-url=https://web.archive.org/web/20150924073521/http://www.pilotfriend.com/photo_albums/helicopters/Leonardo%20Da%20Vinci%27s%20Helical%20Air%20Screw.htm|archive-date=2015-09-24)) "Leonardo da Vinci's Helical Air Screw." ''Pilotfriend.com''. Retrieved 12 December 2010</ref> As scientific knowledge increased and became more accepted, man continued to pursue the idea of vertical flight. Many of these later models and machines would more closely resemble the ancient bamboo flying top with spinning wings, rather than Leonardo's screw.

In July 1754, Russian [[Mikhail Lomonosov]] had developed a small coaxial modeled after the [[Chinese top]] but powered by a wound-up spring device <ref name="Leishman 2006, p. 8">Leishman, J. Gordon (2006). [https://books.google.com/books?id=nMV-TkaX-9cC&lpg=PP1&dq=Principles%20of%20Helicopter%20Aerodynamics] Principles of Helicopter Aerodynamics. Cambridge University Press. p. 8. ((ISBN|0-521-85860-7))</ref> and demonstrated it to the [[Russian Academy of Sciences]]. It was powered by a spring, and was suggested as a method to lift [[meteorological]] instruments. In 1783, [[Christian de Launoy]], and his [[mechanic]], Bienvenu, used a coaxial version of the [[Chinese top]] in a model consisting of contrarotating [[Turkey (bird)|turkey]] flight feathers <ref name="Leishman 2006, p. 8"/> as rotor blades, and in 1784, demonstrated it to the [[French Academy of Sciences]]. A dirigible [[airship]] was described by [[Jean Baptiste Marie Meusnier]] presented in 1783. The drawings depict a ((convert|260|ft|m|adj=mid|-long)) streamlined envelope with internal ballonets that could be used for regulating lift. The airship was designed to be driven by three propellers. In 1784 [[Jean-Pierre Blanchard]] fitted a hand-powered propeller to a balloon, the first recorded means of propulsion carried aloft.<ref>Winter & Degner (1933), pp. 26–27.</ref> [[Sir George Cayley]], influenced by a childhood fascination with the Chinese flying top, developed a model of feathers, similar to that of Launoy and Bienvenu, but powered by rubber bands. By the end of the century, he had progressed to using sheets of tin for rotor blades and springs for power. His writings on his experiments and models would become influential on future aviation pioneers.<ref name="flight-1"/>

[[File:Lomonocov s Aerodynamic Machine 01.jpg|thumb|upright|Prototype created by [[Mikhail Lomonosov]], 1754]]
[[William Bland]] sent designs for his "Atmotic Airship" to [[the Great Exhibition]] held in London in 1851, where a model was displayed. This was an elongated balloon with a steam engine driving twin propellers suspended underneath.<ref>[http://nla.gov.au/nla.news-article2032274 ''Airship honours for Australia – Bland's remarkable invention more than 70 years ago.'' The Argus, September 13, 1924]</ref><ref>((cite web|url=http://www.smh.com.au/news/national/visions-of-a-flying-machine/2006/05/10/1146940613357.html|title=Visions of a flying machine - National - smh.com.au|website=www.smh.com.au|date=11 May 2006|access-date=28 April 2018|url-status=live|archive-url=https://web.archive.org/web/20171230114547/http://www.smh.com.au/news/national/visions-of-a-flying-machine/2006/05/10/1146940613357.html|archive-date=30 December 2017))</ref> [[Alphonse Pénaud]] developed coaxial rotor model helicopter toys in 1870, also powered by rubber bands. In 1872 [[Henri Dupuy de Lôme|Dupuy de Lome]] launched a large navigable balloon, which was driven by a large propeller turned by eight men.<ref>Brooks, Peter, W., Zeppelin: Rigid Airships 1893–1940, Washington, Smithsonian Institution Press, 1992, ((ISBN|1-56098-228-4)) p. 19.</ref> [[Hiram Maxim]] built a craft that weighed ((cvt|3.5|LT)), with a ((cvt|110|ft)) wingspan that was powered by two ((cvt|360|hp)) [[steam engine]]s driving two propellers. In 1894, his machine was tested with overhead rails to prevent it from rising. The test showed that it had enough lift to take off.<ref>Beril, Becker (1967). ''Dreams and Realities of the Conquest of the Skies''. New York: Atheneum. pp. 124–125</ref> One of Pénaud's toys, given as a gift by [[Milton Wright (bishop)|their father]], inspired the [[Wright brothers]] to pursue the dream of flight.<ref>((cite web|url=https://www.loc.gov/teachers/classroommaterials/primarysourcesets/flight/pdf/teacher_guide.pdf|title=Archived copy|website=[[Library of Congress]]|access-date=2017-12-29|url-status=live|archive-url=https://web.archive.org/web/20171018101652/http://www.loc.gov/teachers/classroommaterials/primarysourcesets/flight/pdf/teacher_guide.pdf|archive-date=2017-10-18))</ref> The twisted [[airfoil]] (aerofoil) shape of an aircraft propeller was pioneered by the Wright brothers. While some earlier engineers had attempted to model air propellers on [[Propeller (marine)|marine propellers]], the Wright Brothers realized that a propeller is essentially the same as a [[wing]], and were able to use data from their earlier wind tunnel experiments on wings, introducing a twist along the length of the blades. This was necessary to maintain a more uniform [[angle of attack]] of the blade along its length.<ref>((cite book|title=Pilot's Handbook of Aeronautical Knowledge|year=2008|publisher=U.S. Federal Aviation Administration|location=Oklahoma City|id=FAA-8083-25A|pages=2–7 ie page 7 of Chapter 02: Aircraft Structure|url=http://www.faa.gov/regulations_policies/handbooks_manuals/aviation/pilot_handbook/|url-status=live|archive-url=https://web.archive.org/web/20150701011444/http://www.faa.gov/regulations_policies/handbooks_manuals/aviation/pilot_handbook/|archive-date=2015-07-01))</ref> Their original propeller blades had an efficiency of about 82%,<ref>Ash, Robert L., Colin P. Britcher and Kenneth W. Hyde. [https://web.archive.org/web/20110604093014/http://www.memagazine.org/supparch/flight03/propwr/propwr.html "Wrights: How two brothers from Dayton added a new twist to airplane propulsion."] ''Mechanical Engineering: 100 years of Flight, 3 July 2007.</ref> compared to 90% for a modern (2010) small general aviation propeller, the 3-blade McCauley used on a [[Beechcraft Bonanza]] aircraft.<ref>Rogers, David F. "[http://www.nar-associates.com/technical-flying/propeller/cruise_propeller_efficiency_screen.pdf Propeller Efficiency] ((webarchive|url=https://web.archive.org/web/20141221235338/http://www.nar-associates.com/technical-flying/propeller/cruise_propeller_efficiency_screen.pdf |date=2014-12-21 ))", Figure 3. ''NAR'', 2010. Accessed: 28 August 2014.</ref> Roper<ref>((cite web|url=http://www.humanpoweredflying.propdesigner.co.uk/html/flights.html|title=Flights|first=Chris|last=Roper|website=www.humanpoweredflying.propdesigner.co.uk|access-date=28 April 2018|url-status=dead|archive-url=https://web.archive.org/web/20160313235426/http://www.humanpoweredflying.propdesigner.co.uk/html/flights.html|archive-date=13 March 2016))</ref> quotes 90% for a propeller for a human-powered aircraft.

[[Mahogany]] was the wood preferred for propellers through [[World War I]], but wartime shortages encouraged use of [[Juglans|walnut]], [[oak]], [[Prunus|cherry]] and [[Fraxinus|ash]].<ref>((cite book|last=Ayres|first=Leonard P.|title=The War with Germany|publisher=United States Government Printing Office|edition=Second|date=1919|location=Washington, DC|page=92))</ref> [[Alberto Santos Dumont]] was another early pioneer, having designed propellers before the Wright Brothers<ref>Henri R. Palmer Jr. "The birdcage parasol", ''Flying Magazine'' Oct. 1960 p. 51</ref> for his [[airship]]s. He applied the knowledge he gained from experiences with airships to make a propeller with a steel shaft and aluminium blades for his [[Santos-Dumont 14-bis|14 bis biplane]] in 1906. Some of his designs used a bent aluminium sheet for blades, thus creating an airfoil shape. They were heavily [[Camber (aerodynamics)|undercambered]], and this plus the absence of lengthwise twist made them less efficient than the Wright propellers.<ref>Physical propeller theory was at the time restricted to the [[William John Macquorn Rankine|Rankine]]–[[William Froude|Froude]] theory, also known as the "actuator disc theory" or the axial momentum theory. That theory, however adequate, does not give indication on the shape that should be given to the propeller. This would be solved regarding that theory only in the 1920s by complement of the [[Betz' law|Betz]] law (Goldstein, Betz, Prandtl and Lanchester): William Graebel, ''Engineering Fluid Mechanics'', p. 144, ((ISBN|1-560-32711-1)), John Carlton, ''Marine Propellers and Propulsion'', p. 169, ((ISBN|978-0-08-097123-0)). The Wright brothers however were equating the propeller blade to an airfoil instead,  which for they previously had already determined the aerodynamic behavioural patterns: John David Anderson, ''A History of Aerodynamics: And Its Impact on Flying Machines'', ((ISBN|0-521-66955-3))</ref> Even so, this was perhaps the first use of aluminium in the construction of an airscrew. Originally, a rotating airfoil behind the aircraft, which pushes it, was called a propeller, while one which pulled from the front was a [[tractor configuration|tractor]].<ref>Encyclopædia Britannica, 1910 edition, volume 30 (1922 supplement), in the article "Aeronautics" p. 20. "Airscrews have been described as 'tractors' and 'propellers', according as the airscrew shaft is placed in tension or in compression by the thrust, and corresponding aeroplanes are usually called by the same names. The first biplanes, those of the Wrights and the Farmans, were of the propeller type, colloquially 'pushers'; almost all monoplanes were 'tractors.'</ref> Later the term 'pusher' became adopted for the rear-mounted device in contrast to the tractor configuration and both became referred to as 'propellers' or 'airscrews'. The understanding of low speed propeller aerodynamics was fairly complete by the 1920s, but later requirements to handle more power in a smaller diameter have made the problem more complex.

Propeller research for [[National Advisory Committee for Aeronautics]] (NACA) was directed by [[William F. Durand]] from 1916. Parameters measured included propeller efficiency, [[thrust]] developed, and [[power (physics)|power]] absorbed. While a propeller may be tested in a [[wind tunnel]], its performance in free-flight might differ. At the [[Langley Memorial Aeronautical Laboratory]], E. P. Leslie used [[Vought VE-7]]s with Wright E-4 engines for data on free-flight, while Durand used reduced size, with similar shape, for wind tunnel data. Their results were published in 1926 as NACA report #220.<ref>[[William Durand]] & E. P. Leslie (1926) [https://archive.org/details/NASA_NTRS_Archive_19930091287 Comparison of tests on air propellers in flight with wind tunnel model tests on similar forms], [[National Advisory Committee for Aeronautics]] # 220</ref>

==Theory and design==
[[File:Precision air ATR72 5423a.gif|thumb|A 6-bladed [[Hamilton Standard]] 568F propeller on an [[ATR 72]] short-haul airliner]]

Lowry<ref>((cite web|url=https://www.avweb.com/news/airman/182418-1.html|title=The Bootstrap Approach to Aircraft Performance(Part Two — Constant-Speed Propeller Airplanes) - AVweb Features Article|website=www.avweb.com|date=12 December 1999|access-date=28 April 2018|url-status=live|archive-url=https://web.archive.org/web/20120818194336/http://www.avweb.com/news/airman/182418-1.html|archive-date=18 August 2012))</ref> quotes a propeller efficiency of about 73.5% at cruise for a [[Cessna 172]]. This is derived from his "Bootstrap approach" for analyzing the performance of light general aviation aircraft using fixed pitch or constant speed propellers. The efficiency of the propeller is influenced by the angle of attack (α). This is defined as α = Φ - θ,<ref name=kundu>((cite book|last=Kundu|first=Ajoy|title=Aircraft Design|publisher=Cambridge University Press|year=2010|page=346|isbn=978-0521885164))</ref> where θ is the [[helix angle]] (the angle between the resultant relative velocity and the blade rotation direction) and Φ is the [[blade pitch]] angle. Very small pitch and helix angles give a good performance against resistance but provide little thrust, while larger angles have the opposite effect. The best helix angle is when the blade is acting as a wing producing much more lift than drag. However, 'lift-and-drag' is only one way to express the aerodynamic force on the blades. To explain aircraft and engine performance the same force is expressed slightly differently in terms of thrust and torque<ref>https://archive.org/details/in.ernet.dli.2015.205354 Fig 1-8</ref> since the required output of the propeller is thrust. Thrust and torque are the basis of the definition for the efficiency of the propeller as shown below. The [[advance ratio]] of a propeller is similar to the angle of attack of a wing.

A propeller's [[mechanical efficiency|efficiency]] is determined by<ref name=Spak>[http://web.mit.edu/aeroastro/people/spakovszky.html Prof. Z. S. Spakovszky] ((webarchive|url=https://web.archive.org/web/20120628044328/http://web.mit.edu/aeroastro/people/spakovszky.html|date=2012-06-28)). "[http://web.mit.edu/16.unified/www/FALL/thermodynamics/notes/node86.html#SECTION06374300000000000000 11.7.4.3 Efficiency] ((webarchive|url=https://web.archive.org/web/20150226204713/http://web.mit.edu/16.unified/www/FALL/thermodynamics/notes/node86.html|date=2015-02-26))" ''[[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, main page] ((webarchive|url=https://web.archive.org/web/20100217180407/http://web.mit.edu/16.unified/www/FALL/thermodynamics/notes/notes.html|date=2010-02-17))</ref> 
:<math>\eta = \frac{\hbox{propulsive power out)){\hbox{shaft power in)) = \frac{\hbox{thrust}\cdot\hbox{axial speed)){\hbox{resistance torque}\cdot\hbox{rotational speed)).</math>

Propellers are similar in aerofoil section to a low-[[drag (physics)|drag]] wing and as such are poor in operation when at other than their optimum [[angle of attack]]. Therefore, most propellers use a [[#Variable_pitch|variable pitch]] mechanism to alter the blades' pitch angle as engine speed and aircraft velocity are changed.

[[File:US Navy 060817-N-1467R-042 A Sailor assigned to Assault Craft Unit Four (ACU 4) stationed at Little Creek Amphibious Base, Va., checks the propeller of a Landing Craft Air Cushion (LCAC).jpg|thumb|upright|A sailor checks the propeller of a [[Landing Craft Air Cushion]] hovercraft]]
A further consideration is the number and the shape of the blades used. Increasing the [[aspect ratio (wing)|aspect ratio]] of the blades reduces drag but the amount of thrust produced depends on blade area, so using high-aspect blades can result in an excessive propeller diameter. A further balance is that using a smaller number of blades reduces interference effects between the blades, but to have sufficient blade area to transmit the available power within a set diameter means a compromise is needed. Increasing the number of blades also decreases the amount of work each blade is required to perform, limiting the local [[Mach number]] – a significant performance limit on propellers.
The performance of a propeller suffers when [[Transonic speed|transonic]] flow first appears on the tips of the blades. As the relative air speed at any section of a propeller is a vector sum of the aircraft speed and the tangential speed due to rotation, the flow over the blade tip will reach transonic speed well before the aircraft does. When the airflow over the tip of the blade reaches its [[Drag divergence Mach number|critical speed]], drag and torque resistance increase rapidly and [[shock wave]]s form creating a sharp increase in noise. Aircraft with conventional propellers, therefore, do not usually fly faster than Mach 0.6. There have been propeller aircraft which attained up to the Mach 0.8 range, but the low propeller efficiency at this speed makes such applications rare.

===Blade twist===

[[File:Aircraft propeller twist.png|thumb|Changes in propeller blade angle from hub to tip.]]

The tip of a propeller blade travels faster than the hub. Therefore, it is necessary for the blade to be twisted so as to decrease the angle of attack of the blade gradually and therefore produce uniform lift from the hub to the tip. The greatest angle of incidence, or the highest pitch, is at the hub while the smallest angle of incidence or smallest pitch is at the tip. A propeller blade designed with the same angle of incidence throughout its entire length would be inefficient because as airspeed increases in flight, the portion near the hub would have a negative AOA while the blade tip would be stalled.<ref name=phak>((cite book |title=Pilot's Handbook of Aeronautical Knowledge |date=2016-08-24 |publisher=[[Federal Aviation Administration]] |pages=7-4 — 7-5))</ref>

===High speed===

There have been efforts to develop propellers and [[propfan]]s for aircraft at high subsonic speeds.<ref>[http://planesandpilotsofww2.webs.com/Fisher.html ''Pushing The Envelope With Test Pilot Herb Fisher''.] ((webarchive|url=https://web.archive.org/web/20140201175925/http://planesandpilotsofww2.webs.com/Fisher.html |date=2014-02-01 )) ''Planes and Pilots of World War 2,'' 2000. Retrieved: 22 July 2011.</ref> The 'fix' is similar to that of [[transonic]] wing design. Thin blade sections are used and the blades are swept back in a scimitar shape ([[scimitar propeller]]) in a manner similar to wing sweepback, so as to delay the onset of shockwaves as the blade tips approach the speed of sound. The maximum relative velocity is kept as low as possible by careful control of pitch to allow the blades to have large helix angles. A large number of blades are used to reduce work per blade and so circulation strength. [[Contra-rotating propellers]] are used. The propellers designed are more efficient than turbo-fans and their cruising speed (Mach 0.7–0.85) is suitable for airliners, but the noise generated is tremendous (see the [[Antonov An-70]] and [[Tupolev Tu-95]] for examples of such a design).

===Physics===
Forces acting on the blades of an aircraft propeller include the following. Some of these forces can be arranged to counteract each other, reducing the overall mechanical stresses imposed.<ref>((cite book|title=Airframe and Powerplant Mechanics Powerplant Handbook|publisher=Federal Aviation Administration|page=327|url=http://www.faa.gov/documentLibrary/media/Advisory_Circular/AC_65-12A.pdf|url-status=live|archive-url=https://web.archive.org/web/20140826114848/http://www.faa.gov/documentLibrary/media/Advisory_Circular/AC_65-12A.pdf|archive-date=2014-08-26))</ref><ref name="beaumont" />
;Thrust bending
:Thrust loads on the blades, in reaction to the force pushing the air backwards, act to bend the blades forward. Blades are therefore often ''raked'' forwards, such that the outward centrifugal force of rotation acts to bend them backwards, thus balancing out the bending effects.
;Centrifugal and aerodynamic twisting
:A centrifugal twisting force is experienced by any asymmetrical spinning object. In the propeller it acts to twist the blades to a fine pitch. The aerodynamic [[Center of pressure (fluid mechanics)|centre of pressure]] is therefore usually arranged to be slightly forward of its mechanical centreline, creating a twisting moment towards coarse pitch and counteracting the centrifugal moment. However in a high-speed dive the aerodynamic force can change significantly and the moments can become unbalanced.
;Centrifugal
:The force felt by the blades acting to pull them away from the hub when turning. It can be arranged to help counteract the thrust bending force, as described above.
;Torque bending
:Air resistance acting against the blades, combined with [[inertia]]l effects causes propeller blades to bend away from the direction of rotation.
;Vibratory
:Many types of disturbance set up vibratory forces in blades. These include aerodynamic excitation as the blades pass close to the wing and fuselage. Piston engines introduce torque impulses which may excite vibratory modes of the blades and cause fatigue failures.<ref>[https://archive.org/details/in.ernet.dli.2015.205354 Nelson, Wilbur C. (1944), ''Airplane Propeller Principles''] p.67</ref> Torque impulses are not present when driven by a gas turbine engine.

==Variable pitch==
((main|Variable-pitch propeller (aeronautics)))

The purpose of varying pitch angle is to maintain an optimal angle of attack for the propeller blades, giving maximum efficiency throughout the flight regime. This reduces fuel usage. Only by maximising propeller efficiency at high speeds can the highest possible speed be achieved.<ref>((cite web |title=Aerodynamic tests on propellers |url=https://archive.org/details/in.ernet.dli.2015.163729/page/n107/mode/2up?view=theater |access-date=1 July 2022))</ref> Effective angle of attack decreases as airspeed increases, so a coarser pitch is required at high airspeeds.

The requirement for pitch variation is shown by the propeller performance during the [[Schneider Trophy]] competition in 1931. The [[Fairey Aviation Company]] fixed-pitch propeller used was partially stalled on take-off and up to ((cvt|160|mph)) on its way up to a top speed of ((cvt|407.5|mph)).<ref>((cite web|url=https://www.flightglobal.com/pdfarchive/view/1931/1931%20-%201159.html|title=Archived copy|access-date=2018-03-30|url-status=live|archive-url=https://web.archive.org/web/20180331104633/https://www.flightglobal.com/pdfarchive/view/1931/1931%20-%201159.html|archive-date=2018-03-31))</ref> The very wide speed range was achieved because some of the usual requirements for aircraft performance did not apply. There was no compromise on top-speed efficiency, the take-off distance was not restricted to available runway length and there was no climb requirement.<ref>((cite web|url=https://www.flightglobal.com/pdfarchive/view/1931/1931%20-%201158.html|title=Archived copy|access-date=2018-04-01|url-status=live|archive-url=https://web.archive.org/web/20180401213701/https://www.flightglobal.com/pdfarchive/view/1931/1931%20-%201158.html|archive-date=2018-04-01))</ref>

The variable pitch blades used on the [[Tupolev Tu-95]] propel it at a speed exceeding the maximum once considered possible for a propeller-driven aircraft<ref>"Jane's All The World's Aircraft 1982-1983, Jane's Publishing Company Limited, ((ISBN|0 7106-0748-2)), p.228</ref> using an exceptionally coarse pitch.<ref>The Development Of Jet And Turbine Engines", 4th edition, Bill Gunston 2006, Patrick Stephens Limited, ((ISBN|0 7509 4477 3)), p.66</ref>

===Mechanisms===
[[File:Propeller diagram.jpg|thumb|Cut-away view of a [[Hamilton Standard]] propeller. This type of constant-speed propeller was used on many American fighters, bombers and transport aircraft of [[World War II]]]]
Early pitch control settings were pilot operated, either with a small number of preset positions or continuously variable.<ref name="beaumont" />

The simplest mechanism is the [[ground-adjustable propeller]], which may be adjusted on the ground, but is effectively a fixed-pitch prop once airborne. The spring-loaded "two-speed" VP prop is set to fine for takeoff, and then triggered to coarse once in cruise, the propeller remaining coarse for the remainder of the flight.

After [[World War I]], automatic propellers were developed to maintain an optimum angle of attack. This was done by balancing the centripetal twisting moment on the blades and a set of counterweights against a spring and the aerodynamic forces on the blade. Automatic props had the advantage of being simple, lightweight, and requiring no external control, but a particular propeller's performance was difficult to match with that of the aircraft's power plant.

The most common variable pitch propeller is the [[Variable-pitch propeller (aeronautics)#Constant-speed propellers|constant-speed propeller]]. This is controlled by a hydraulic constant speed unit (CSU). It automatically adjusts the blade pitch in order to maintain a constant engine speed for any given power control setting.<ref name="beaumont" /> Constant-speed propellers allow the pilot to set a rotational speed according to the need for maximum engine power or maximum efficiency, and a propeller governor acts as a closed-loop [[Controller (control theory)|controller]] to vary propeller pitch angle as required to maintain the selected engine speed.<ref>((Cite book |url=http://www.mccauley.textron.com/von_klip_tip_cs_propeller.pdf |title=Professor Von Kliptip Answers Your Questions About The McCauley Constant Speed Propeller Governing System |publisher=[[McCauley Propeller Systems|McCauley]] |location=Wichita, KS |archive-url=https://web.archive.org/web/20110322103306/http://www.mccauley.textron.com/von_klip_tip_cs_propeller.pdf |archive-date=2011-03-22 |url-status=dead))</ref> In most aircraft this system is hydraulic, with engine oil serving as the hydraulic fluid. However, electrically controlled propellers were developed during [[World War II]] and saw extensive use on military aircraft, and have recently seen a revival in use on home-built aircraft.((Citation needed|date=February 2016|reason=claim of recent revival needs independent citation))

Another design is the [[V-Prop]], which is self-powering and self-governing.

=== Feathering ===
<!-- Courtesy note per [[WP:RSECT]]: [[Feathering (propeller)]] links here. Other articles have section links here. -->
[[File:Airbus A400M EC-404 ILA 2012 11 (cropped2).jpg|thumb|Feathered propeller on the outboard [[Europrop TP400|TP400]] turboprop of an [[Airbus A400M]]]]
On most variable-pitch propellers, the blades can be rotated parallel to the airflow to stop rotation of the propeller and reduce drag when the engine fails or is deliberately shut down. This is called ''feathering'', a term borrowed from [[Rowing (sport)#Anatomy of a stroke|rowing]]. On single-engined aircraft, whether a powered glider or turbine-powered aircraft, the effect is to increase the gliding distance. On a multi-engine aircraft, feathering the propeller on an inoperative engine reduces drag, and helps the aircraft maintain speed and altitude with the operative engines. Feathering also prevents ''windmilling'', the turning of engine components by the propeller rotation forced by the slipstream; windmilling can damage the engine, start a fire, or cause structural damage to the aircraft.

Most feathering systems for reciprocating engines sense a drop in oil pressure and move the blades toward the feather position, and require the pilot to pull the propeller control back to disengage the high-pitch stop pins before the engine reaches idle [[revolutions per minute|RPM]]. [[Turboprop]] control systems usually utilize a ''negative torque sensor'' in the reduction gearbox which moves the blades toward feather when the engine is no longer providing power to the propeller. Depending on design, the pilot may have to push a button to override the high-pitch stops and complete the feathering process, or the feathering process may be totally automatic.

===Reverse pitch===
((main|Thrust reversal))
The propellers on some aircraft can operate with a negative blade pitch angle, and thus reverse the thrust from the propeller. This is known as Beta Pitch. Reverse thrust is used to help slow the aircraft after landing and is particularly advantageous when landing on a wet runway as wheel braking suffers reduced effectiveness. In some cases reverse pitch allows the aircraft to taxi in reverse – this is particularly useful for getting floatplanes out of confined docks.

==Counter-rotation==
((Main|Counter-rotating propellers))
[[File:Counter-rotating propellers.gif|thumb|Counter-rotating propellers]]
Counter-rotating propellers are sometimes used on twin-engine and multi-engine aircraft with wing-mounted engines. These propellers turn in opposite directions from their counterpart on the other wing to balance out the [[torque]] and [[p-factor]] effects. They are sometimes referred to as "handed" propellers since there are left hand and right hand versions of each prop.

Generally, the propellers on both engines of most conventional twin-engined aircraft spin clockwise (as viewed from the rear of the aircraft). To eliminate the [[critical engine]] problem, counter-rotating propellers usually turn "inwards" towards the fuselage – clockwise on the left engine and counterclockwise on the right – however, there are exceptions (especially during [[World War II]]) such as the [[P-38 Lightning]] which turned "outwards" (counterclockwise on the left engine and clockwise on the right) away from the fuselage from the WW II years, and the [[Airbus A400]] whose inboard and outboard engines turn in opposite directions even on the same wing.

==Contra-rotation==
((Main|Contra-rotating propellers))

A contra-rotating propeller or contra-prop places two counter-rotating propellers on concentric drive shafts so that one sits immediately 'downstream' of the other propeller. This provides the benefits of counter-rotating propellers for a single powerplant. The forward propeller provides the majority of the thrust, while the rear propeller also recovers energy lost in the swirling motion of the air in the propeller slipstream. Contra-rotation also increases the ability of a propeller to absorb power from a given engine, without increasing propeller diameter. However the added cost, complexity, weight and noise of the system rarely make it worthwhile and it is only used on high-performance types where ultimate performance is more important than efficiency.

==Aircraft fans==
((Main|Propfan|Ducted fan))
A fan is a propeller with a large number of blades. A fan therefore produces a lot of thrust for a given diameter but the closeness of the blades means that each strongly affects the flow around the others. If the flow is supersonic, this interference can be beneficial if the flow can be compressed through a series of shock waves rather than one. By placing the [[Ducted fan|fan within a shaped duct]], specific flow patterns can be created depending on flight speed and engine performance. As air enters the duct, its speed is reduced while its pressure and temperature increase. If the aircraft is at a high subsonic speed this creates two advantages: the air enters the fan at a lower Mach speed; and the higher temperature increases the local speed of sound. While there is a loss in efficiency as the fan is drawing on a smaller area of the free stream and so using less air, this is balanced by the ducted fan retaining efficiency at higher speeds where conventional propeller efficiency would be poor. A ducted fan or propeller also has certain benefits at lower speeds but the duct needs to be shaped in a different manner than one for higher speed flight. More air is taken in and the fan therefore operates at an efficiency equivalent to a larger un-ducted propeller. Noise is also reduced by the ducting and should a blade become detached the duct would help contain the damage. However the duct adds weight, cost, complexity and (to a certain degree) drag.

==See also==
*[[Advance ratio]]
*[[Axial fan design]]
*[[Blade element theory]]
*[[Blade element momentum theory]]
*[[Helicopter rotor]]
*[[List of aircraft propeller manufacturers]]
*[[Momentum theory]]
*[[Charles M. Olmsted]], early American "high efficiency" propeller designer (1909)
*[[Propeller theory]]
*[[Turboprop]]
*[[Rotor wing#Radial-lift rotors|Radial-lift rotors]]

==References==
((Reflist))

==External links==
((Commons category|Aircraft propellers))
*[https://web.archive.org/web/20111214083950/http://www.experimentalaircraft.info/homebuilt-aircraft/aircraft-propeller.php Experimental Aircraft Propellers]
* [http://howthingsfly.si.edu Smithsonian National Air and Space Museum's How Things Fly website]

((Aircraft gas turbine engine components))
((Aircraft piston engine components))
((Authority control))

[[Category:Aircraft propulsion components]]
[[Category:Propellers|Aeronautics]]

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