Editing Contra-rotating propellers (section)

==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>