Editing Contra-rotating propellers

{{Short description|Two-propeller design for improving low-airspeed maneuverability}}
{{distinguish|counter-rotating propellers}}
{{About||contra-rotating marine propellers, helicopter rotors and turbine fan blades|Contra-rotating}}
[[File:Contra-rotating propellers.gif|thumb|Contra-rotating propellers]]
[[File:25 P 51XR Mustang N6WJ Precious Metal Reno Air Race 2014 photo Don Ramey Logan.jpg|thumb|300px|Contra-rotating propellers on the [[Rolls-Royce Griffon]]-powered P-51XR Mustang ''[[Precious Metal (aircraft)|Precious Metal]]'' at the 2014 [[Reno Air Races]]]]

Aircraft equipped with '''contra-rotating propellers''' ('''CRP''')<ref>{{cite journal|author1=Sasaki, N. |author2=Murakami, M. |author3=Nozawa, K. |author4=Soejima, S. |author5=Shiraki, A. |author6=Aono, T. |author7=Fujimoto, T. |author8=Funeno, I. |author9=Ishii, N. |author10=Onogi, H. |year=1998|title=Design system for optimum contra-rotating propellers|journal=Journal of Marine Science and Technology|volume=3|issue=1|pages=3–21|doi=10.1007/bf01239802|bibcode=1998JMST....3....3S |s2cid=110551942 }}</ref> '''coaxial contra-rotating propellers''', or '''high-speed propellers''', apply the maximum power of usually a single engine piston powered or [[turboprop]] engine to drive a pair of [[coaxial]] [[Propeller (aircraft)|propellers]] in [[contra-rotation]]. Two propellers are arranged one behind the other, and power is transferred from the engine via a [[Epicyclic gearing|planetary gear]] or [[Gear#Spur|spur gear]] [[Transmission (mechanics)|transmission]]. Although contra-rotating propellers are also known as ''[[counter-rotating propeller]]s'',<ref>{{cite magazine |magazine=Flight |issn=0015-3710 |author=J. M. R. |title=Enterprise in airscrews: First details of a mighty new de Havilland airscrew and the story of 21 years of achievement |url=https://www.flightglobal.com/pdfarchive/view/1956/1956%20-%200248.html |date=March 2, 1956 |volume=69 |number=2458 |pages=237–248}}</ref><ref>{{cite conference |title=Technology and benefits of aircraft counter rotation propellers |given1=W. C. |surname1=Strack |given2=G. |surname2=Knip |given3=A. L. |surname3=Weisbrich |given4=J. |surname4=Godston |given5=E. |surname5=Bradley |conference=Aerospace Congress and Exposition |date=October 25–28, 1982 |location=Anaheim, California, USA |publisher=[[NASA]] |url=https://archive.org/details/NASA_NTRS_Archive_19830002859/page/n0}} [https://ntrs.nasa.gov/search.jsp?R=19830002859 alternate url]</ref> the term is much more widely used when referring to airscrews on separate non-coaxial shafts turning in opposite directions.

== Operation==
When airspeed is low, the [[mass]] of the air flowing through the propeller disk ([[thrust]]) causes a significant amount of [[tangential]] or rotational air flow to be created by the spinning blades. The energy of this tangential air flow is wasted in a single-propeller design, and causes handling problems at low speed as the air strikes the [[vertical stabilizer]], causing the aircraft to yaw left or right, depending on the direction of propeller rotation. To use this wasted effort, the placement of a second propeller behind the first takes advantage of the disturbed airflow.

A well designed contra-rotating propeller will have no rotational air flow, pushing a maximum amount of air uniformly through the propeller disk, resulting in high performance and low induced energy loss. It also serves to counter the [[asymmetrical]] [[torque]] effect of a conventional propeller (see [[P-factor]]). Some contra-rotating systems were designed to be used at takeoff for maximum power and efficiency under such conditions, and allowing one of the propellers to be disabled during cruise to extend flight time.

==Advantages and disadvantages==
The torque on the aircraft from a pair of contra-rotating propellers effectively cancels out.

Contra-rotating propellers have been found to be between 6% and 16% more efficient than normal propellers.<ref name=vrover>{{cite report |title=Analysis of a contra-rotating propeller driven transport aircraft |given1=J. S. |surname1=Vanderover |given2=K. D. |surname2=Visser |url=https://www.rcgroups.com/forums/showatt.php?attachmentid=2815700}}</ref>

However they can be very noisy, with increases in noise in the axial (forward and aft) direction of up to 30&nbsp;dB, and tangentially 10&nbsp;dB.<ref name=vrover/> Most of this extra noise can be found in the higher frequencies. These substantial noise problems limit commercial applications. One possibility is to enclose the contra-rotating propellers [[Ducted fan|in a shroud]].<ref>{{cite conference |url=http://supersonic.eng.uci.edu/download/AIAA-2013-0217.pdf |title=Aeroacoustic testing of open rotors at very small scale |access-date=August 5, 2016 |last1=Truong |first1=Alexander |first2=Dimitri |last2=Papamoschou |edition=51st |conference=AIAA Aerospace Sciences Meeting |date=January 7, 2013 |location=Grapevine, Texas, USA |conference-url=https://arc.aiaa.org/doi/book/10.2514/MASM13}}</ref> It is also helpful if the tip speed or the loading of the blades is reduced, if the aft propeller has fewer blades or a smaller diameter than the fore propeller, or if the spacing between the aft and fore propellers is increased.<ref>{{cite book |surname1=Hager |given1=Roy V |surname2=Vrabel |given2=Deborah |title=Advanced turboprop project |date=1988 |publisher=National Aeronautics and Space Administration (NASA) Scientific and Technical Information Division |location=Lewis Research Center, Cleveland, Ohio |pages=[https://archive.org/details/NASA_NTRS_Archive_19890003194/page/n93 82], 98–100 |archive-url=https://web.archive.org/web/20170313115348/http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19890003194.pdf |url-status=live |archive-date=March 13, 2017 |url=https://archive.org/details/NASA_NTRS_Archive_19890003194 |access-date=February 2, 2019 |language=en |series=NASA SP-495 |oclc=17508419}} [https://ntrs.nasa.gov/search.jsp?R=19890003194 alternate url]</ref>

The efficiency of a contra-rotating propeller is somewhat offset by its mechanical complexity and the added weight of this gearing that makes the aircraft heavier, thus some performance is sacrificed to carry it. Nonetheless, coaxial contra-rotating propellers and [[Coaxial rotors|rotors]] have been used in several [[military aircraft]], such as the [[Tupolev Tu-95|Tupolev Tu-95 "Bear"]].

They are also being examined for use in [[airliner]]s.<ref>Kijk magazine, 1/2013</ref>

==Use in aircraft==
While several nations experimented with contra-rotating propellers in aircraft, only the United Kingdom and Soviet Union produced them in large numbers. The first aircraft to be fitted with a contra-rotating propeller to fly was in the US when two inventors from Ft Worth, Texas tested the concept on an aircraft.<ref>{{cite magazine |issn=0161-7370 |url=https://books.google.com/books?id=DygDAAAAMBAJ&pg=PA33 |title=Plane's propellers revolve in opposite directions |date=November 1931 |magazine=[[Popular Science Monthly]] |page=33 |volume=119 |number=5}}</ref>

===United Kingdom===
[[File:Supermarine Spitfire XIX vr.jpg|thumb|right|Contra-rotating propellers of a [[Supermarine Spitfire|Spitfire]] Mk XIX]]

A contra-rotating propeller was patented by [[Frederick W. Lanchester|F. W. Lanchester]] in 1907.<ref>{{cite magazine |given=F. W. |surname=Lanchester |volume=40 |number=1720 |url=http://www.flightglobal.com/pdfarchive/view/1941/1941%20-%202944.html |title=Contra-props: Recollections of early considerations by advisory committee for aeronautics: A pioneer's 1907 patent: Suggestions for further research |magazine=Flight |access-date=3 November 2015 |date=December 11, 1941 |pages=418–419}}</ref>

Some of the more successful British aircraft with contra-rotating propellers are the [[Avro Shackleton]], powered by the [[Rolls-Royce Griffon]] engine, and the [[Fairey Gannet]], which used the [[Armstrong Siddeley Double Mamba|Double Mamba Mk.101]] engine. In the Double Mamba two separate power sections drove one propeller each, allowing one power section (engine) to be shut down in flight, increasing endurance.

Another naval aircraft, the [[Westland Wyvern]] had contra-rotating propellers. The [[Martin-Baker MB 5]] test aircraft also used this propeller type.

Later variants of the [[Supermarine Spitfire]] and [[Supermarine Seafire|Seafire]] used the Griffon with contra-rotating props. In the Spitfire/Seafire and Shackleton's case the primary reason for using contra-rotating propellers was to increase the propeller blade-area, and hence absorb greater engine power, within a propeller diameter limited by the height of the aircraft's [[Landing gear|undercarriage]]. The [[Short Sturgeon]] used two Merlin 140s with contra-rotating propellers.

The [[Bristol Brabazon]] prototype airliner used eight [[Bristol Centaurus]] engines driving four pairs of contra-rotating propellers, each engine driving a single propeller.<ref>{{cite encyclopedia |section-url=http://www.aviationarchive.org.uk/stories/getobjectstory.php?rnum=g1526&enum=GE121&pnum=4&maxp=9 |section=Brabazon engine layout |url=http://www.aviationarchive.org.uk/stories/storycontents.php?enum=GE121 |title=The Bristol Brabazon - Engineering masterpiece or Great White Elephant |encyclopedia=Aviation Archive: Aviation Heritage |access-date=3 November 2015 |archive-url=https://web.archive.org/web/20150923180801/http://www.aviationarchive.org.uk/stories/storycontents.php?enum=GE121 |archive-date=23 September 2015 |url-status=dead }}</ref>

The post-war [[Saunders-Roe Princess|SARO Princess]] prototype flying boat airliner also had eight of its ten engines driving contra-rotating propellers.

===USSR, Russia and Ukraine===
[[File:Kuznetsov NK-12M turboprop on Tu-95.jpg|thumb|left|One of the four contra-rotating propellers on a [[Tu-95]] Russian strategic bomber]]

In the 1950s, the Soviet Union's [[Kuznetsov Design Bureau]] developed the [[NK-12]] turboprop. It drives an eight-blade contra-rotating propeller and, at {{convert|15000|hp|kW|adj=pre|shaft|abbr=off}}, it is the most powerful turboprop in service. Four NK-12 engines power the [[Tupolev Tu-95]] ''Bear'', the only turboprop bomber to enter service, as well as one of the [[fastest propeller-driven aircraft]]. The [[Tupolev Tu-114|Tu-114]], an airliner derivative of the Tu-95, holds the world speed record for propeller aircraft.<ref>{{cite web |url=http://records.fai.org/general_aviation/current.asp?id1=21&id2=3 |website=Fédération Aéronautique Internationale (FAI) |title=General aviation world records |archive-url=https://web.archive.org/web/20071007145139/http://records.fai.org/general_aviation/current.asp?id1=21&id2=3 |archive-date=October 7, 2007 |url-status=dead}}</ref> The Tu-95 was also the first Soviet bomber to have intercontinental range. The [[Tupolev Tu-126|Tu-126]] AEW aircraft and [[Tupolev Tu-142|Tu-142]] maritime patrol aircraft are two more NK-12 powered designs derived from the Tu-95.

The NK-12 engine powers another well-known Soviet aircraft, the [[Antonov An-22]] Antheus, a heavy-lift cargo aircraft. At the time of its introduction, the An-22 was the largest aircraft in the world and is still, by far, the world's largest turboprop-powered aircraft. From the 1960s through the 1970s, it set several world records in the categories of maximum payload-to-height ratio and maximum payload lifted to altitude.

Of lesser note is the use of the NK-12 engine in the [[A-90 Orlyonok]], a mid-size Soviet [[ekranoplan]]. The A-90 uses one NK-12 engine mounted at the top of its T-tail, along with two turbofans installed in the nose.

In the 1980s, Kuznetsov continued to develop powerful contra-rotating engines. The NK-110, which was tested in the late 1980s, had a contra-rotating propeller configuration with four blades in front and four in back, like the NK-12. Its {{convert|4.7|m|in|order=flip|abbr=off|adj=mid|propeller diameter}} was smaller than the NK-12's {{cvt|5.6|–|6.2|m|in|order=flip}} diameter, but it produced a power output of {{cvt|15665|kW|hp|order=flip}}, delivering a takeoff thrust of {{convert|177|kN|lbf|order=flip|abbr=off}}.<ref name="NK110Sheet">{{cite web |language=ru |page=48 |title=NK-110 |url=http://www.uvauga-dvig.narod.ru/nk-110.pdf |website=Ulyanovsk Higher Aviation School of Civil Aviation}}</ref> Even more powerful was the NK-62, which was in development throughout most of the decade. The NK-62 had an identical propeller diameter and blade configuration to the NK-110, but it offered a higher takeoff thrust of {{cvt|245|kN|lbf|order=flip}}. The associated NK-62M had a takeoff thrust of {{cvt|285.2|kN|lbf|order=flip}}, and it could deliver {{cvt|314.7|kN|lbf|order=flip}} of emergency thrust.<ref name="NKLargeThrust">{{cite magazine |language=ru |year=2018 |number=1 |volume=115 |pages=20–24 |given=V. A. |surname=Zrelov |title=Development of engines 'NK' large thrust on the basis of a single gas generator |magazine=Dvigatel |url=http://engine.aviaport.ru/issues/115/pics/pg20.pdf}}</ref> Unlike the NK-12, however, these later engines were not adopted by any of the aircraft design bureaus.

In 1994, Antonov produced the [[Antonov An-70|An-70]], a heavy transport aircraft. It is powered by four [[Progress D-27]] [[propfan]] engines driving contra-rotating propellers. The characteristics of the D-27 engine and its propeller make it a propfan, a hybrid between a turbofan engine and a turboprop engine.

===United States===
[[File:YB-35 42-13603 on the ramp.jpg|thumb|[[XB-35]] Flying Wing showing its quartet of pusher contra-rotating propellers. The option was later discarded due to severe vibration in flight and later changed to traditional single rotating propellers.]]
[[File:B-42 Mixmaster.jpg|thumb|[[Douglas XB-42 Mixmaster]] ]][[File:General Motors P-75 Eagle.jpg|thumb|General Motors P-75 Eagle]]

The United States worked with several prototypes, including the [[Northrop XB-35]], [[XB-42 Mixmaster]], the [[Douglas XTB2D Skypirate]], the [[Curtiss XBTC]], the [[A2J Super Savage]], the [[Boeing XF8B]], the [[XP-56 Black Bullet]], the [[Fisher P-75 Eagle]] and the [[tailsitter|tail-sitting]] [[Convair XFY]] "Pogo" and [[Lockheed XFV]] "Salmon" [[vertical takeoff and landing|VTOL]] fighters and the [[Hughes XF-11]] reconnaissance plane. The [[Convair R3Y Tradewind]] flying boat entered service with contra-rotating propellers. However, both piston-engined and [[turboprop]]-powered propeller-driven aircraft were reaching their zenith and new technological developments such as the advent of the pure [[turbojet]] and [[turbofan]] engines, both without propellers, meant that the designs were quickly eclipsed.

The US propeller manufacturer, [[Hamilton Standard]], bought a [[Fairey Gannet]] in 1983 to study the effects of counter rotation on propeller noise and blade vibratory stresses. The Gannet was particularly suitable because the independently-driven propellers provided a comparison between counter and single rotation.<ref>{{cite conference |pages=708–717 |title=Single rotation and counter rotation prop-fan propulsion system technologies |url=http://www.icas.org/ICAS_ARCHIVE/ICAS1984/ICAS-84-5.6.2.pdf |given1=B. S. |surname1=Gatzen |given2=C. N. |surname2=Reynolds |conference=Congress of the International Council of the Aeronautical Sciences |date=September 9–14, 1984 |location=–Toulouse, France |edition=14th |conference-url=http://www.icas.org/ICAS_ARCHIVE/ICAS1984/1984.html}}</ref>

===Ultralight applications===
An Austrian company, [[Sun Flightcraft]], distributes a contra-rotating gearbox for use on [[Rotax 503]] and [[Rotax 582|582]] engines on ultralight and microlight aircraft. The [[Coax-P]] was developed by Hans Neudorfer of [[NeuraJet]] and allows powered hang-gliders and parachutes to develop 15 to 20 percent more power while reducing torque moments. The manufacturer also reports reduced noise levels from dual contra-rotating props using the Coax-P gearbox.<ref>{{cite web |url=http://sunflightcraft.com/en/coaxp.php |title=COAX-P: Counter rotating propeller gearbox |website=Sun Flightcraft |access-date=July 18, 2019}}</ref><ref name="WDLA04">{{cite journal |surname1=Bertrand |given1=Noel |given2=Rene |surname2=Coulon |title=World Directory of Leisure Aviation 2003-04 |journal=World Directory of Light Aviation |pages=70, 87 |publisher=Pagefast Ltd |location=Lancaster, United Kingdom |year=2003 |issn=1368-485X}}</ref><ref>{{cite web|url=http://www.neurajet.at/englisch/home.html|archive-url=https://web.archive.org/web/20051222043916/http://www.neurajet.at/englisch/home.html|title=Willkommen bei Neura Jet|archive-date=22 December 2005|work=neurajet.at|access-date=3 November 2015}}</ref>

== See also ==
* [[Contra-rotating marine propellers]]
* [[Toroidal propeller]] ("looped propeller")

==References==
{{reflist|30em}}

==External links==
{{commons category}}
* {{cite magazine |url=https://books.google.com/books?id=hCcDAAAAMBAJ&pg=PA111 |title=Two propellers in tandem turning in opposite ways give warplanes new speed |date=December 1941 |magazine=[[Popular Science Monthly]] |page=111 |volume=139 |number=6 |issn=0161-7370}}
* {{cite web |title=Avro Shackleton |website=Aircraft.co.za |url=http://www.aircraft.co.za/Encyclopedia/A/98.php |url-status=dead |archive-url=https://web.archive.org/web/20051119175605/http://www.aircraft.co.za/Encyclopedia/A/98.php |archive-date=November 19, 2005}}
* [https://web.archive.org/web/20051105030047/http://luftfahrtmuseum.com/htmi/itk/u5.htm Luftfahrtmuseum.com – Further information and pictures of contra rotators for the Fairey Gannet and Shackleton]
* {{cite AV media |url=https://www.youtube.com/watch?v=FbgpH2nrn-A  |archive-url=https://ghostarchive.org/varchive/youtube/20211222/FbgpH2nrn-A |archive-date=2021-12-22 |url-status=live|title=Spitfire PR.XIX startup by Steve Hinton at Chino, California |website=YouTube}}{{cbignore}}
* [http://www.enginehistory.org/members/Convention/2012/Presentations/Fey/Contra-Rotating01.pdf A History of Aircraft Using Contra-Rotating Propellers (Part 1) – Aircraft Engine Historical Society]
* [http://www.enginehistory.org/members/Convention/2012/Presentations/Fey/Contra-Rotating02.pdf A History of Aircraft Using Contra-Rotating Propellers (Part 2) – Aircraft Engine Historical Society]
* [http://www.enginehistory.org/members/Convention/2012/Presentations/Fey/Contra-Rotating03.pdf A History of Aircraft Using Contra-Rotating Propellers (Part 3) – Aircraft Engine Historical Society]
* [http://www.enginehistory.org/members/Convention/2012/Presentations/Fey/Contra-Rotating04.pdf A History of Aircraft Using Contra-Rotating Propellers (Part 4) – Aircraft Engine Historical Society]

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[[Category:Aircraft engines]]
[[Category:Aircraft configurations]]
[[Category:Propellers]]