View source for Engineering:Slowed rotor

{{Short description|Helicopter design variant}}
[[File:McDonnell XV-1 in flight.jpg|thumb|334px|right|The McDonnell XV-1 could slow its rotor from 410 to 180 RPM]]

The '''slowed rotor''' principle is used in the design of some [[Engineering:Helicopter|helicopters]]. On a conventional helicopter the [[Revolutions per minute|rotational speed]] of the [[Engineering:Helicopter rotor|rotor]] is constant; reducing it at lower flight speeds can reduce fuel consumption and enable the aircraft to fly more economically. In the compound helicopter and related aircraft configurations such as the [[Engineering:Gyrodyne|gyrodyne]] and winged [[Engineering:Autogyro|autogyro]], reducing the rotational speed of the rotor and offloading part of its lift to a fixed wing reduces [[Physics:Drag|drag]], enabling the aircraft to fly faster.

==Introduction==
Traditional helicopters get both their [[Engineering:Propulsion#Air propulsion|propulsion]] and [[Physics:Lift (force)|lift]] from the main rotor; by using a dedicated propulsion device such as a [[Engineering:Propeller|propeller]] or [[Engineering:Jet engine|jet engine]], the rotor burden is lessened.<ref name=chandler>Chandler, Jay. "[http://www.propilotmag.com/archives/2012/September%2012/A3_Rotor_p1.html Advanced rotor designs break conventional helicopter speed restrictions (page 1)] {{webarchive|url=https://web.archive.org/web/20130718171642/http://www.propilotmag.com/archives/2012/September%2012/A3_Rotor_p1.html |date=2013-07-18 }}" [http://www.propilotmag.com/archives/2012/September%2012/A3_Rotor_p2.html Page 2] {{webarchive|url=https://web.archive.org/web/20130718141528/http://www.propilotmag.com/archives/2012/September%2012/A3_Rotor_p2.html |date=2013-07-18 }} [http://www.propilotmag.com/archives/2012/September%2012/A3_Rotor_p3.html Page 3] {{webarchive|url=https://web.archive.org/web/20130718141548/http://www.propilotmag.com/archives/2012/September%2012/A3_Rotor_p3.html |date=2013-07-18 }}. ''ProPilotMag'', September 2012. Accessed: 10 May 2014. [https://web.archive.org/web/20130718171642/http://www.propilotmag.com/archives/2012/September%2012/A3_Rotor_p1.html Archive 1] [https://web.archive.org/web/20130718141528/http://www.propilotmag.com/archives/2012/September%2012/A3_Rotor_p2.html Archive 2] [https://web.archive.org/web/20130718141548/http://www.propilotmag.com/archives/2012/September%2012/A3_Rotor_p3.html Archive 3]</ref>
If [[Physics:Wing|wing]]s are also used to lift the aircraft, the rotor can be unloaded (partially or fully) and its rotational speed further reduced, enabling higher aircraft speed. Compound helicopters use these methods,{{r|rob31}}{{r|silva}}<ref name=h03-7/> but the [[Engineering:Boeing A160 Hummingbird|Boeing A160 Hummingbird]] shows that rotor-slowing is possible without wings or propellers, and regular helicopters may reduce turbine RPM (and thus rotor speed) to 85% using 19% less power.{{r|khos1}} Alternatively,<!--probably not combining with SR--> research suggests that twin-engine helicopters may decrease fuel consumption by 25%-40% when running only one engine, given adequate height and velocity well inside the safe areas of the [[Helicopter height–velocity diagram|height–velocity diagram]].<ref>Dubois, Thierry. "[http://www.ainonline.com/aviation-news/business-aviation/2015-02-14/researchers-look-single-engine-cruise-ops-twins Researchers Look at Single-engine Cruise Ops on Twins]" ''AINonline'', 14 February 2015. Accessed: 19 February 2015.</ref><ref>Perry, Dominic. "[http://www.flightglobal.com/news/articles/airbus-helicopters-promises-safe-single-engine-operations-with-bluecopter-414417/ Airbus Helicopters promises safe single-engine operations with Bluecopter demonstrator]" ''Flight Global'', 8 July 2015. [https://web.archive.org/web/20150802141742/http://www.flightglobal.com/news/articles/airbus-helicopters-promises-safe-single-engine-operations-with-bluecopter-414417/ Archive]</ref><ref>Perry, Dominic. "[https://www.flightglobal.com/news/articles/turbomeca-eyes-flight-tests-of-engine-sleep-mode-417150/ Turbomeca eyes flight tests of 'engine sleep mode']" ''Flight Global'', 25 September 2015. [https://web.archive.org/web/20150929212740/https://www.flightglobal.com/news/articles/turbomeca-eyes-flight-tests-of-engine-sleep-mode-417150/ Archive]</ref>

As of 2012, no compound or hybrid wing/rotor (manned) aircraft had been produced in quantity, and only a few had been flown as experimental aircraft,<ref>Rigsby, page 3</ref> mainly because the increased complexities have not been justified by military or civilian markets.<ref name=jHT325>Johnson HT, p. 325</ref> Varying the rotor speed may induce severe vibrations at specific resonance frequencies.<ref name=lomb1/>

[[Engineering:Contra-rotating|Contra-rotating]] rotors (as on the [[Engineering:Sikorsky X2|Sikorsky X2]]) solve the problem of lift dissymmetry by having both left and right sides provide near equal lift with less flapping.<ref name=nelms/><ref name=chandler/> The X2 deals with the compressibility issue by reducing its rotor speed<ref name=chandler/> from 446 to 360 RPM<ref name=datta/><ref name=uniX2/> to keep the advancing blade tip below the sound barrier when going above 200 knots.<ref>Walsh 2011, page 3</ref>

==Design principles==
===Speed limits of aircraft rotors===
[[File:FAA heli-manual Dissymmetry of lift.PNG|thumb|Effect of blade airspeed on lift on advancing and retreating side, when aircraft speed is 100 knots.]]
The rotors of conventional helicopters are designed to operate at a fixed speed of rotation, to within a few percent.<ref name=beck>Robert Beckhusen. "[https://www.wired.com/2012/06/hummingbird/#more-84749 Army Dumps All-Seeing Chopper Drone]" ''Wired'' June 25, 2012. Accessed: 12 October 2013. "''for standard choppers ... the number of revolutions per minute is also set at a fixed rate''"</ref><ref>The UH-60 permits 95–101% rotor RPM [http://www.usarmyaviation.com/studyguides/index.php?folder=Documents/UH-60BlackhawkSpecific&download=Uh60limits.doc UH-60 limits] ''US Army Aviation''. Retrieved 2 January 2010</ref><ref name=trimb>{{cite news |last=Trimble |first=Stephen |title=DARPA's Hummingbird unmanned helicopter comes of age |url=http://www.flightglobal.com/news/articles/darpa39s-hummingbird-unmanned-helicopter-comes-of-225070/ |work=FlightGlobal |date=3 July 2008 |accessdate=14 May 2014|quote=The rotor speed on a typical helicopter can be varied around 95-102%|archiveurl=https://web.archive.org/web/20140514181119/http://www.flightglobal.com/news/articles/darpa39s-hummingbird-unmanned-helicopter-comes-of-225070/|archivedate=14 May 2014 |url-status=live}}</ref><ref name=lomb1/> This introduces limitations in areas of the [[Engineering:Flight envelope|flight envelope]] where the optimal speed differs.<ref name=khos1>Khoshlahjeh</ref>

In particular, it limits the maximum forward speed of the aircraft. Two main issues restrict the speed of rotorcraft:<ref name=lomb1>Lombardi, Frank. "[http://www.aviationtoday.com/rw/commercial/technology/Optimizing-the-Rotor_82267.html Optimizing the Rotor]" ''Rotor&Wing'', June 2014. Accessed: 15 June 2014. [https://web.archive.org/web/20140615210037/http://www.aviationtoday.com/rw/commercial/technology/Optimizing-the-Rotor_82267.html Archived on 15 June 2014]</ref><ref name=h03-7>Harris 2003, page 7</ref><ref>Chiles, James R. "[http://www.airspacemag.com/flight-today/hot-rod-helicopters-39130976/?no-ist Hot-Rod Helicopters]" [http://www.airspacemag.com/flight-today/hot-rod-helicopters-39130976/?page=2 Page 2] [http://www.airspacemag.com/flight-today/hot-rod-helicopters-39130976/?page=3 Page 3] ''Air & Space/Smithsonian'', September 2009. Accessed: 18 May 2014.</ref><ref name=nelms/>

* [[Engineering:Retreating blade stall|Retreating blade stall]]. As forward speed of the helicopter increases, the airflow over the retreating blade becomes relatively slower, while the airflow over the advancing blade is relatively faster, creating more lift. If not counteracted by [[Engineering:Helicopter rotor#Fully articulated|flapping]],<ref>"[http://www.dynamicflight.com/aerodynamics/flapping/ Blade flapping]" ''Dynamic Flight''</ref> this would cause [[Engineering:Dissymmetry of lift|dissymmetry of lift]] and eventually retreating blade stall,<ref name=rob31>Robb 2006, page 31</ref><ref name=silva>Silva 2010, page 1.</ref><ref>"[http://www.challis-heliplane.com/v3/helicopterlimitations.php Helicopter Limitations]" ''Challis Heliplane''</ref><ref>"[http://www.dynamicflight.com/aerodynamics/retreating/ Retreating blade stall]" ''Dynamic Flight''</ref><ref name=chandler/> and blade stability suffers as the blade reaches its limits for flapping.<ref name=nelms/><ref name=jHT323>Johnson HT, p. 323</ref>
* [[Physics:Transonic|Transonic]] drag near the rotor blade tip. The faster-moving advancing blade tip may begin to approach the [[Physics:Speed of sound|speed of sound]], where transonic drag begins to rise steeply, and severe buffeting and vibration effects can occur. This effect prevents any further increase in speed, even if the helicopter has surplus power remaining, and even if it features a highly streamlined fuselage. A similar effect prevents propeller-driven aircraft from achieving supersonic speeds, although they can achieve higher speeds than a helicopter since the propeller blade isn't advancing in the direction of travel.{{r|rob31}}{{r|silva}}<ref name=chandler/><ref>Prouty, Ray. "[https://web.archive.org/web/20160304060434/https://www.aviationtoday.com/rw/military/utility/Ask-Ray-Prouty_1530.html Ask Ray Prouty: Compound Helicopters, Compressibility (archived)]" ''Rotor&Wing'', 1 May 2005. Accessed: 11 Dec 2019.</ref><ref>"[http://rotorcraft.arc.nasa.gov/tutorial/Nomenclature.htm#t Nomenclature: Transonic drag rise]" ''[[Organization:NASA|NASA]]''</ref><ref name=fili/>

These (and other)<ref>Beare, Glenn. [http://www.helis.com/howflies/maxspeed.php "Why can't a Helicopter fly faster than it does ?"] ''helis.com'' . Accessed: 9 May 2014.</ref><ref name=kras>Krasner, Helen. [http://www.decodedscience.com/why-cant-helicopters-fly-fast/22018 "Why Can’t Helicopters Fly Fast?"] ''Decoded Science'', 10 December 2012. Accessed: 9 May 2014.</ref> problems limit the practical speed of a conventional helicopter to around {{convert|160|-|200|kn|km/h}}.<ref name=chandler/><ref name=fili/><ref>Majumdar, Dave. "[http://news.usni.org/2014/03/19/darpa-awards-contracts-search-460-mph-helicopter DARPA Awards Contracts in Search of a 460 MPH Helicopter]" ''United States Naval Institute'', 19 March 2014. Accessed: 9 May 2014.</ref><ref name=wise>Wise, Jeff. "[http://www.popularmechanics.com/technology/aviation/news/the-rise-of-radical-new-rotorcraft-16850989-2 The Rise of Radical New Rotorcraft]" ''Popular Mechanics'', 3 June 2014. Accessed: 19 June 2014. [https://web.archive.org/web/20140619194953/http://www.popularmechanics.com/technology/aviation/news/the-rise-of-radical-new-rotorcraft-16850989-2 Archive] Quote: "This aerodynamic principle limits conventional helicopters to about 200 mph."</ref> At the extreme, the theoretical top speed for a rotary winged aircraft is about {{convert|225|kn|mph km/h}},<ref name=kras/> just above the current official speed record for a conventional helicopter held by a [[Engineering:Westland Lynx|Westland Lynx]], which flew at {{convert|400|km/h|abbr=on}} in 1986<ref>"[http://www.fai.org/fai-record-file/?recordId=11659 Rotorcraft Absolute: Speed over a straight 15/25 km course] {{webarchive|url=https://web.archive.org/web/20131203033038/http://www.fai.org/fai-record-file/?recordId=11659 |date=2013-12-03 }}". [[Organization:Fédération Aéronautique Internationale|Fédération Aéronautique Internationale]] (FAI). Note search under E-1 Helicopters and "Speed over a straight 15/25 km course". Accessed: 26 April 2014.</ref> where its blade tips were nearly [[Physics:Mach number|Mach]]&nbsp;1.<ref>{{citation |last=Hopkins |first=Harry |url=http://www.flightglobal.com/pdfarchive/view/1986/1986%20-%203544.html |title=Fastest blades in the world |journal=Flight International |date=27 December 1986 |accessdate=28 April 2014 |pages=24–27 |format=pdf |quote=[https://web.archive.org/web/20140429045740/http://www.flightglobal.com/pdfarchive/view/1986/1986%20-%203544.html Archive page 24] [https://web.archive.org/web/20140429120402/http://www.flightglobal.com/pdfarchive/view/1986/1986%20-%203545.html Archive page 25] [https://web.archive.org/web/20140516103440/http://www.flightglobal.com/pdfarchive/view/1986/1986%20-%203546.html Archive page 26] [https://web.archive.org/web/20140516103533/http://www.flightglobal.com/pdfarchive/view/1986/1986%20-%203547.html Archive page 27] }}</ref>

===Slowed rotors and aircraft speed===
[[File:Rotorcraft Aspect ratio (mu) diagram.PNG|thumb|Rotorcraft Aspect ratio (mu) diagram]]
[[File:Simulated drag curve for compound helicopter.jpg|thumb|Drag type curves as a function of airspeed (simulated)]]<!--move lower when drag section is expanded-->
[[File:Venn diagram of slowed rotor aircraft in high speed mode.svg|right|150px|Cruise combinations for rotor power, propeller and wings.]]
For rotorcraft, [[Engineering:Advance ratio|advance ratio]] (or Mu, symbol <math>\mu</math>) is defined as the aircraft forward speed '''V''' divided by its relative blade tip speed.<ref>"[http://rotorcraft.arc.nasa.gov/tutorial/Nomenclature.htm#mu Nomenclature: Mu]" ''[[Organization:NASA|NASA]]''</ref><ref>[http://dictionary.reference.com/browse/advance+ratio Definition of Advance ratio]</ref><ref>"[http://www.aerospaceweb.org/design/helicopter/hinges.shtml Flapping Hinges]" ''Aerospaceweb.org''. Accessed: 8 May 2014.</ref> Upper mu limit<!--not necessarily the highest--> is a critical design factor for rotorcraft,<ref name=jHT323/> and the optimum for traditional helicopters is around 0.4.<ref name=h03-7/><ref name=fili>Filippone, Antonio (2000). "Data and performances of selected aircraft and rotorcraft" pages 643-646. Department of Energy Engineering, Technical University of Denmark / Progress in Aerospace Sciences, Volume 36, Issue 8. Accessed: 21 May 2014. {{doi|10.1016/S0376-0421(00)00011-7}} [http://www.sciencedirect.com/science/article/pii/S0376042100000117 Abstract]</ref>

The "relative blade tip speed" '''u''' is the tip speed relative to the aircraft (not the airspeed of the tip). Thus the formula for Advance ratio is
 
<math>\mu = \frac {V}{u} = \frac {V}{\Omega\cdot R}</math>  where Omega (Ω) is the rotor's [[Physics:Angular velocity|angular velocity]], and R is the rotor radius (about the length of one rotor blade)<ref name=uniMu>Jackson, Dave. "[http://www.unicopter.com/B263.html#Tip_Speed_Ratio Tip Speed Ratio (Advance Ratio)]" ''Unicopter'', 6 September 2013. Retrieved: 22 May 2015. [https://web.archive.org/web/20141021201634/http://www.unicopter.com/B263.html Archived] on 21 October 2014.</ref><!--quick online source should be the first a reader goes to--><ref name=jHT323/><ref name=datta/>

When the rotor blade is [[Perpendicular|perpendicular]] to the aircraft and advancing, its tip airspeed '''V<sub>t</sub>''' is the aircraft speed plus relative blade tip speed, or '''V<sub>t</sub>'''='''V'''+'''u'''.<ref name=nelms/><ref name=hfh02>"[http://www.faa.gov/regulations_policies/handbooks_manuals/aviation/helicopter_flying_handbook/ Helicopter Flying Handbook]", [http://www.faa.gov/regulations_policies/handbooks_manuals/aviation/helicopter_flying_handbook/media/hfh_ch02.pdf  Chapter 02: Aerodynamics of Flight (PDF, 9.01 MB), Figure 2-33] page 2-18. ''FAA''-H-8083-21A, 2012. Accessed: 21 May 2014.</ref> At mu=1, '''V''' is equal to '''u''' and the tip airspeed is twice the aircraft speed.

At the same position on the opposite side (retreating blade), the tip airspeed is the aircraft speed minus relative blade tip speed, or '''V<sub>t</sub>'''='''V'''-'''u'''. At mu=1, the tip airspeed is zero.<ref name=wise/><ref>Berry, page 3-4</ref> At a mu between 0.7 and 1.0, most of the retreating side has reverse airflow.<ref name=datta/>

Although rotor characteristics are fundamental to rotorcraft performance,<ref>Harris 2008, page 13</ref> little public analytical and experimental knowledge exists between advance ratios of 0.45 to 1.0,<ref name=datta/><ref>Berry, page 25</ref> and none is known above 1.0 for full-size rotors.<ref>Harris 2008, page 25</ref><ref>Kottapalli, page 1</ref> [[Computer simulation]]s are not capable of adequate predictions at high mu.<ref>Harris 2008, page 8</ref><ref>Bowen-Davies, page 189-190</ref> The region of reverse flow on the retreating blade is not well understood,<ref>Harris 2008, page 14</ref><ref>Bowen-Davies, page 198</ref> however some research has been conducted,<ref>DuBois 2013</ref><ref>{{cite journal |title=Computational Investigation and Fundamental Understanding of a Slowed UH-60A Rotor at High Advance Ratios |journal=Journal of the American Helicopter Society |volume=61 |issue=2 |pages=1–17 |author1=Potsdam, Mark |author2=Datta, Anubhav |author3=Jayaraman, Buvana |date=18 March 2016 |doi=10.4050/JAHS.61.022002 }}</ref> particularly for scaled rotors.<ref>Bowen-Davies, page 216</ref><ref>{{cite journal|title=Streamwise oscillation of airfoils into reverse-flow |journal = AIAA Journal|volume = 54|issue = 5|pages = 1628–1636|author1=Granlund, Kenneth |author2=Ol, Michael |author3=Jones, Anya |doi=10.2514/1.J054674|year = 2016|bibcode = 2016AIAAJ..54.1628G}}</ref> The US Army [[Engineering:Aviation Applied Technology Directorate|Aviation Applied Technology Directorate]] runs a supporting program in 2016 aiming at developing transmissions with a 50% rotor speed reduction.<ref>{{cite web|url=https://govtribe.com/project/next-generation-rotorcraft-transmission-ngrt/activity |title=Contract Activity: Next Generation Rotorcraft Transmission (NGRT)|author1=Renata Y. Ellington  |author2=Laurie Pierce  |name-list-style=amp |work=[[Engineering:Aviation Applied Technology Directorate|Aviation Applied Technology Directorate]] |publisher=GovTribe |date=21 March 2016|accessdate=27 March 2016|archiveurl=https://web.archive.org/web/20160327124043/https://govtribe.com/project/next-generation-rotorcraft-transmission-ngrt/activity |archivedate=27 March 2016|url-status=live}}</ref>

The profile drag of a rotor corresponds to [[Cube (algebra)|the cube]] of its [[Physics:Rotational speed|rotational speed]].<ref name=gustaf>Gustafson, page 12</ref><ref name=Aeromech>Johnson RA, page 251.</ref>
Reducing the rotational speed is therefore a significant reduction of rotor drag, allowing higher aircraft speed<ref name=datta/> A conventional rotor such as the UH-60A has lowest consumption around 75% rpm, but higher aircraft speed (and weight) requires higher rpm.<ref>Bowen-Davies, page 97-99</ref>

A rotor disk with variable radius is a different way of reducing tip speed to avoid compressibility, but blade loading theory suggests that a fixed radius with varying rpm performs better than a fixed rpm with varying radius.<ref>Bowen-Davies, page 101</ref><!--practicality is debatable-->

===Fuel economy of slowed rotors===
Conventional helicopters have constant-speed rotors and adjust lift by varying the blade angle of attack or collective pitch. The rotors are optimised for high-lift or high-speed flight modes and in less demanding situations are not as efficient.

The profile drag of a rotor corresponds to [[Cube (algebra)|the cube]] of its [[Physics:Rotational speed|rotational speed]].<ref name=gustaf>Gustafson, page 12</ref><ref name=Aeromech>Johnson RA, page 251.</ref>
Reducing the rotational speed and increasing the angle of attack can therefore give a significant reduction in rotor drag, allowing lower fuel consumption.{{r|khos1}}

==History==
[[File:Pitcairn_Autogiro_NASA_GPN-2000-001990.jpg|thumb|Pitcairn PCA-2 autogyro. Unpowered rotor, tractor propeller, wings.]]
Technical parameters given for each type listed: 
*maximum speed.
*''μ'', the ratio of forward airspeed to rotational tip speed.
*Rotor lift as a percentage of total lift, at full speed.
*[[Engineering:Lift-to-drag ratio|Lift-to-drag ratio]] (L/D).

===Early development===

When [[Biography:Juan de la Cierva|Juan de la Cierva]] developed the [[Engineering:Autogyro|autogyro]] through the 1920s and 1930s, it was found that the tip speeds of the advancing rotor blade could become excessive. Designers such as he and Harold F. Pitcairn developed the idea of adding a conventional wing to offload the rotor during high-speed flight, allowing it to rotate at slower speeds.{{cn|date=May 2023}}

The 1932 Pitcairn PCA-2 autogyro had a maximum speed of 20-{{convert|102|kn|mph km/h}},<ref>Harris 2003, page A-40</ref> ''μ'' of 0.7,<ref>Harris 2008, page 19</ref> and L/D pf 4.8<ref>{{cite web|last=Duda|first=Holger|title=Flight performance of lightweight gyroplanes|url=https://www.icas.org/ICAS_ARCHIVE/ICAS2012/PAPERS/434.PDF|publisher=[[Organization:German Aerospace Center|German Aerospace Center]]|accessdate=5 April 2020|author2=Insa Pruter|page=5|year=2012}}</ref>

NACA engineer John Wheatley examined the effect of varying advance ratios up to about 0.7 in a wind tunnel in 1933 and published a landmark study in 1934. Although lift could be predicted with some accuracy, by 1939 the state of the art theory still gave unrealistically low values for rotor drag.<ref>Harris (2008) pp.35-40.</ref>

===Postwar projects===
Fairey Aviation in the UK worked on gyrodynes in the late 1940s and 1950s developing tip-jet propulsion which eliminated the need for countertorque. They culminated in the [[Engineering:Fairey Rotodyne|Fairey Rotodyne]], the prototype for a VTOL passenger aircraft, which could combine the vertical landing of a helicopter with the speed of a fixed wing aircraft. The Rotodyne had a single 90 ft diameter main rotor supplemented by a 46 ft wide wing with forward thrust provided by twin  turboprop engines. In forward flight the power to the rotor was reduced to about 10%.{{cn|date=May 2023}} Its maximum speed was {{convert|166|kn|mph km/h}} a record set in 1959.<ref>"[http://www.fai.org/fai-record-file/?recordId=13216 FAI Record ID #13216 - Rotodyne, Speed over a closed circuit of 100 km without payload] {{webarchive|url=https://web.archive.org/web/20150217223109/http://www.fai.org/fai-record-file/?recordId=13216 |date=2015-02-17 }}" ''[[Organization:Fédération Aéronautique Internationale|Fédération Aéronautique Internationale]]''. Record date 5 January 1959. Accessed: April 2014.</ref><ref name=anders>Anders, Frank. (1988) "[http://groenbros.com/technology/fairey-rotodyne/ The Fairey Rotodyne]" (excerpt) ''Gyrodyne Technology (Groen Brothers Aviation)''. Retrieved: 17 January 2011. [https://web.archive.org/web/20140226074803/http://www.groenbros.com/FaireyRotodyne.php Archived] 26 February 2014</ref> 0.6.<ref>Rigsby, page 4</ref> Rotor speed was 120 (high speed cruising flight as an autogyro) to 140 (flare out while landing as a helicopter) rpm<ref>{{cite magazine |url=http://www.flightglobal.com/pdfarchive/view/1962/1962%20-%201390.html |title=Requiem for the Rotodyne |work=Flight International |date= 9 August 1962 |pages=200–202 |archive-url=https://web.archive.org/web/20160306215258/http://www.flightglobal.com/pdfarchive/view/1962/1962%20-%201390.html |archive-date=6 March 2016}}</ref> During forward flight 60% of the lift came from the wings and 40% from the rotor.<ref name=LetLetLet>Braas, Nico. "[http://www.letletlet-warplanes.com/2008/06/15/the-fairey-rotodyne/ Fairey Rotodyne]" ''Let Let Let Warplanes,'' 15 June 2008. Accessed: April 2014. [https://web.archive.org/web/20130930215301/http://www.letletlet-warplanes.com/2008/06/15/the-fairey-rotodyne/ Archived] on 30 September 2013</ref>

At the same time, the US Air Force was investigating fast VTOL aircraft. McDonnell developed what became the [[Engineering:McDonnell XV-1|McDonnell XV-1]], the first of the V-designated types, which flew in 1955. It was a tip-jet driven gyrodyne, which turned off rotor thrust at high airspeeds and relied on a pusher propeller to maintain forward flight and rotor autorotation. Lift was shared between the rotor and stub wings. It established a rotorcraft speed record of {{convert|170|kn|mph km/h}}. 0.95.<ref name=rod>Anderson, Rod. "[http://contactmagazine.com/Issue83/Issue_83__3-4.pdf The CarterCopter and its legacy]" Issue 83, ''Contact Magazine'', 30 March 2006. Accessed: 11 December 2010. [https://issuu.com/contact.magazine/docs/issue_83_carter_copter Mirror]<!--primary source--></ref> 180-410<ref>Harris 2003, page 14</ref> (50%<ref>Watkinson, page 355</ref>). 85% \ 15%.<ref>Robb 2006, page 41</ref> 6.5 (Wind tunnel tests at 180 RPM with no propeller.<ref>Harris 2003, page 18. Lift forces at page A-101</ref>)

The Lockheed AH-56 Cheyenne military attack helicopter for the US Army arose out of Lockheed's ongoing research programme into rigid rotors, which began with the [[Engineering:Lockheed CL-475|CL-475]] in 1959. Stub wings and a thrust turbojet to offload the rotor were first added to an XH-51A and in 1965 this allowed the craft to achieve a world speed record of {{convert|272|mph|kph}}. The Cheyenne flew just two years later, obtaining its forward thrust from a pusher propeller. Although pre-production prototypes were ordered the program met problems and was cancelled.<ref>Munson 1973. pp.55,144-5.</ref> {{convert|212|kn|mph km/h}}.<ref>Landis and Jenkins 2000, pp. 41–48.</ref><ref>"[http://www.globalsecurity.org/military/systems/aircraft/ah-56.htm AH-56A Cheyenne]" ''Globalsecurity.org''. Accessed: April 2014.</ref> 0.8.<ref name=rod/> .. \ 20%.<ref>Harris? not 2008, not Vol1+2, page 119</ref>

The Piasecki 16H Pathfinder project similarly evolved an initially conventional design into a compound helicopter through the 1960s, culminating in the 16H-1A Pathfinder II which flew successfully in 1965. Thrust was obtained via a ducted fan at the tail.<ref>Munson 1973. pp.96,187-8.</ref>

The Bell 533 of 1969 was a compound jet helicopter. {{convert|275|kn|mph km/h}}.<ref>Robb 2006, page 43</ref><ref>Spenser, Jay P. "Bell Helicopter". ''Whirlybirds, A History of the U.S. Helicopter Pioneers'', p. 274. University of Washington Press, 1998. {{ISBN|0-295-98058-3}}.</ref>

<gallery mode="packed" heights="100" caption="Some postwar types">
File:McDonnell XV-1 on the ground 1954.jpg|[[Engineering:McDonnell XV-1|McDonnell XV-1]]. Optionally powered rotor, pusher propeller, wings.
File:Fairey Rotodyne XE521.jpg|[[Engineering:Fairey Rotodyne|Fairey Rotodyne]]. Optionally powered rotor, tractor propellers, wings.
File:YAH-56A "Cheyenne" (front) 4017.jpg|Lockheed AH-56 Cheyenne. Powered rotor, pusher propeller, wings.
File:Bell 533a.jpg|Bell 533. Powered rotor, jets, wings.
</gallery>

===Modern developments===
The compound helicopter has continued to be studied and flown experimentally. In 2010 the [[Engineering:Sikorsky X2|Sikorsky X2]] flew with coaxial rotors. {{convert|250|kn|mph km/h}}.<ref>{{cite web|url= http://www.flightglobal.com/articles/2010/09/15/347379/sikorsky-x2-hits-250kt-goal.html |title= Sikorsky X2 hits 250kt goal |author= Croft, John |publisher= Flight International |date= 15 September 2010 |accessdate= 15 September 2010 |archiveurl=https://web.archive.org/web/20110117235150/http://www.flightglobal.com/articles/2010/09/15/347379/sikorsky-x2-hits-250kt-goal.html |archivedate=17 January 2011 |url-status=live }}</ref><ref name=pm>{{cite magazine|url= http://www.popularmechanics.com/technology/aviation/news/inside-sikorskys-record-breaking-helicopter-tech?click=pm_news |title= Inside Sikorsky's Speed-Record-Breaking Helicopter Technology |last= Goodier |first= Rob |date= September 20, 2010 |magazine= Popular Mechanics |accessdate= 22 September 2010}}</ref> 0.8.<ref name=datta/> 360 to 446.<ref name=datta>Datta, page 2.</ref><ref name=uniX2>Jackson, Dave. "[http://www.unicopter.com/1465.html Coaxial - Sikorsky ~ X2 TD]" ''Unicopter''. Accessed: April 2014.</ref> No wings.<ref name=walsh>D. Walsh, S. Weiner, K. Arifian, T. Lawrence, M. Wilson, T. Millott and R. Blackwell. "[https://vtol.org/EB393010-E91A-11E0-8A940050568D0042 High Airspeed Testing of the Sikorsky X2 Technology Demonstrator]{{Dead link|date=May 2019 |bot=InternetArchiveBot |fix-attempted=yes }}" ''Sikorsky'', May 4, 2011. Accessed: October 5, 2013.</ref> In 2013 the Eurocopter X3 flew.<ref>[http://www.airbushelicopters.com/site/en/ref/X3-Demonstrator_1099.html The X3 concept] {{Webarchive|url=https://web.archive.org/web/20140512221644/http://www.airbushelicopters.com/site/en/ref/X3-Demonstrator_1099.html |date=2014-05-12 }} [https://www.youtube.com/watch?v=LxhogYKwV7Y Video1] [https://www.youtube.com/watch?v=eA7oqWXxGz0 Video2], at 2m50s ''[[Company:Airbus Helicopters|Airbus Helicopters]]''. Accessed: 9 May 2014.</ref> {{convert|255|kn|mph km/h}}.<ref name=lemonde>Thivent, Viviane. "[http://www.lemonde.fr/sciences/article/2013/06/10/le-x3-un-helico-a-472-km-h_3427392_1650684.html Le X3, un hélico à 472 km/h]" ''Le Monde'', 11 June 2013. Accessed: 10 May 2014. [http://abonnes.lemonde.fr/sciences/article/2013/06/10/le-x3-un-helico-a-472-km-h_3427392_1650684.html Possible mirror]</ref><ref name=wired2013>[https://www.wired.com/autopia/2013/06/eurocopter-x3-speed-record/ X3 Helicopter Sets Speed Record At Nearly 300 MPH] Wired</ref> 310 minus 15%.<ref name=nelms>Nelms, Douglas. "[http://aviationweek.com/awin/aviation-week-flies-eurocopter-s-x3 Aviation Week Flies Eurocopter’s X3]" ''Aviation Week & Space Technology'', 9 July 2012. Accessed: 10 May 2014. [http://aviationweek.com/Article.aspx?id=/article-xml/AW_07_09_2012_p64-472797.xml Alternate link] [https://web.archive.org/web/20140512224445/http://aviationweek.com/awin/aviation-week-flies-eurocopter-s-x3 Archived] on 12 May 2014</ref> 40<ref name=nelms/><ref name=chandler/>-80% \.<ref name=guy80>Norris, Guy. "[https://www.aviationweek.com/aw/generic/story_channel.jsp?channel=busav&id=news/awst/2012/02/27/AW_02_27_2012_p34-428686.xml&headline=Eurocopter%20X-3%20Targets%20U.S.%20Market Eurocopter X-3 Targets U.S. Market]{{dead link|date=May 2018 |bot=InternetArchiveBot |fix-attempted=yes }}" ''Aviation Week'', 28 February 2012. Accessed: 1 March 2012. [http://www.jetbrokerseurope.com/news/23567.html Mirror] {{Webarchive|url=https://web.archive.org/web/20140413143415/http://www.jetbrokerseurope.com/news/23567.html |date=2014-04-13 }}</ref><ref>Tarantola, Andrew. "[http://www.gizmodo.com.au/2013/06/monster-machines-the-new-fastest-helicopter-on-earth-can-fly-at-an-insane-480kmh/ Monster Machines: The New Fastest Helicopter On Earth Can Fly At An Insane 480km/h]" ''Gizmodo'', 19 June 2013. Accessed: April 2014.</ref>

The compound autogyro, in which the rotor is supplemented by wings and thrust engine but is not itself powered, has also undergone further refinement by Jay Carter Jr. He flew his [[Engineering:CarterCopter|CarterCopter]] in 2005. {{convert|150|kn|mph km/h}}.<ref>Wise, Jeff. [http://www.popsci.com/military-aviation-space/article/2005-01/jay-carter-jr "Jay Carter, Jr."] ''Popular Science'', 2005. [https://books.google.com/books?id=wgKpEb86UPIC&pg=PA63 Magazine]</ref> 1. 50%.<ref name=datta/> By 2013 he had developed its design into a [[Engineering:Personal air vehicle|personal air vehicle]], the Carter PAV. {{convert|175|kn|mph km/h}}. 1.13. 105<ref>Warwick, Graham. "[http://aviationweek.com/defense/carter-hopes-demo-src-rotorcraft-military-0 Carter Hopes To Demo SR/C Rotorcraft To Military]" ''Aviation Week'', 5 February 2014. Accessed: 19 May 2014. [https://web.archive.org/web/20140519054955/http://aviationweek.com/defense/carter-hopes-demo-src-rotorcraft-military-0 Archived on 19 May 2014]</ref> to 350.<ref name=aopa2015>Moore, Jim. "[http://www.aopa.org/News-and-Video/All-News/2015/May/21/Carter Carter seeks factory]" ''Aircraft Owners and Pilots Association'', 21 May 2015. Accessed: 28 May 2014. [https://web.archive.org/web/20150522112841/http://www.aopa.org/News-and-Video/All-News/2015/May/21/Carter Archived] on 22 May 2015.</ref>

The potential of the slowed rotor in enhancing fuel economy has also been studied in the [[Engineering:Boeing A160 Hummingbird|Boeing A160 Hummingbird]] UAV, a conventional helicopter. {{convert|140|kn|mph km/h}}. 140 to 350.<ref name=pmA160>Hambling, David. "[http://www.popularmechanics.com/technology/military/robots/the-rise-of-the-drone-helicopter#slide-3 The Rise of the Drone Helicopter - A160T Hummingbird]" ''Popular Mechanics''. Accessed: April 2014.</ref>

<gallery mode="packed" caption="Recent slowed rotor aircraft" heights="100">
File:Boeing A160 Hummingbird VTOL-UAS.jpg|[[Engineering:Boeing A160 Hummingbird|Boeing A160 Hummingbird]] <br/> No wings, no propeller.
File:Sikorsky X2 World Record Speed Demonstrator RSideFront MacDill AirFest 5Oct2011 (14513000689).jpg|[[Engineering:Sikorsky X2|Sikorsky X2]] <br/> Powered rotor, pusher propeller, no wings.
File:Eurocopter X3 F-ZXXX ILA 2012 2.jpg|Eurocopter X3 <br/> Powered rotor, tractor propellers, wings.
File:CarterPAV.jpg|Carter PAV <br/> Unpowered rotor, pusher propeller, wings.
</gallery>

==See also==
* [[Engineering:Gyrodyne|Gyrodyne]]
* [[Engineering:Convertiplane|Convertiplane]]

==References==

===Citations===
{{Reflist|25em}}

===Bibliography===
{{Refbegin}}
* Berry, Ben & Chopra, Inderjit. ''[http://fcc.vtol.org/wp-content/uploads/2014/03/UMD_Slowed-Rotor-Wind-Tunnel-Testing.pdf Slowed Rotor Wind Tunnel Testing at UMD]'' University of Maryland, 19 February 2014. Size: 3MB.   
* Bowen-Davies, Graham M. ''[http://drum.lib.umd.edu/bitstream/handle/1903/16506/BowenDavies_umd_0117E_15997.pdf?sequence=1 Performance and loads of variable tip speed rotorcraft at high advance ratios]'' (dissertation) University of Maryland, 25 June 2015. [http://drum.lib.umd.edu/handle/1903/16506 Header]. DOI:10.13016/M2N62C . Size: 313 pages in 7MB
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* DuBois, Cameron J. (2013). ''[http://rave.ohiolink.edu/etdc/view?acc_num=osu1366253477 Flow Control on an Airfoil Under Reversed Flow Conditions Using Nanosecond Dielectric Barrier Discharge Actuators]'' (dissertation abstract) ''[[Organization:Ohio State University|Ohio State University]]''. Accessed: 4 December 2014.  Size: 86 pages in 6MB
* Floros, Matthew W. & Wayne Johnson (2004). ''[https://web.archive.org/web/20130408132645/http://www.dtic.mil/cgi-bin/GetTRDoc?AD=ADA529853 Stability Analysis of the Slowed-Rotor Compound Helicopter Configuration]'' (1MB). Defense Technical Information Center, 2004. [http://apps.dtic.mil/dtic/tr/fulltext/u2/a519768.pdf Alternate version], 8MB
* Gustafson, F. B. ''[http://apps.dtic.mil/dtic/tr/fulltext/u2/a800614.pdf Effect on helicopter performance of modifications in profile-drag characteristics of rotor-blade airfoil sections]''  [[Organization:NASA|NACA]], August 1944.
* Harris, Franklin D. (2003). ''[https://web.archive.org/web/20130219172002/http://rotorcraft.arc.nasa.gov/publications/files/Harris_NASA-CR-2003-212799.pdf An Overview of Autogyros and the McDonnell XV–1 Convertiplane]'' [[Organization:NASA|NASA]]/CR—2003–212799, 2003. [https://ntrs.nasa.gov/search.jsp?R=20080022367&hterms=212799&qs=N%3D0%26Ntk%3DAll%26Ntt%3D212799%26Ntx%3Dmode%2520matchallpartial%26Nm%3D123|Collection|NASA%2520STI||17|Collection|NACA Header] [https://web.archive.org/web/20111017102331/http://halfdome.arc.nasa.gov/publications/files/Harris_NASA-CR-2003-212799.pdf Mirror1], [https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20080022367_2008020986.pdf Mirror2]. Size: 284 pages in 13MB
* Harris, Franklin D. (2008). ''[https://web.archive.org/web/20130218223606/http://rotorcraft.arc.nasa.gov/publications/files/NASA%20CR%202008-215370%20Harris.pdf Rotor Performance at High Advance Ratio: Theory versus Test]'' [[Organization:NASA|NASA]]/CR—2008–215370, October 2008. [https://ntrs.nasa.gov/search.jsp?R=20090005978&hterms=20090005978&qs=N%3D0%26Ntk%3DAll%26Ntt%3D20090005978%26Ntx%3Dmode%2520matchallpartial Header] [https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20090005978.pdf Mirror]. Accessed: 13 April 2014. Size: 521 pages in 5MB
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* {{cite book|last=Johnson|first=Wayne|title=Rotorcraft Aeromechanics|url=https://books.google.com/books?id=bByX9gHpOJYC|year=2013|publisher=Cambridge University Press|isbn=978-1-107-02807-4}}
* Khoshlahjeh, Maryam & Gandhi, Farhan (2013). ''[https://vtol.org/store/product/helicopter-performance-improvement-with-rotor-speed-variation-and-chord-extension-morphing-8692.cfm Helicopter Rotor Performance Improvement with RPM Variation and Chord Extension Morphing]'' American Helicopter Society. Accessed: 9 June 2014.
* Kottapalli, Sesi et al. (2012). ''[http://rotorcraft.arc.nasa.gov/Publications/files/Kottapalli_AHS_SFO_2012_908.pdf Performance and loads correlation of a UH-60A slowed rotor at high advance ratios] {{Webarchive|url=https://web.archive.org/web/20140517120533/http://rotorcraft.arc.nasa.gov/Publications/files/Kottapalli_AHS_SFO_2012_908.pdf |date=2014-05-17 }}'' [[Organization:NASA|NASA]] ARC-E-DAA-TN4610, June 2012. [https://ntrs.nasa.gov/search.jsp?N=0&Ntk=All&Ntt=20120010553&Ntx=mode%20matchallpartial Header]  Size: 30 pages in 2MB
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* Munson, Kenneth (1973); ''Helicopters: And Other Rotorcraft Since 1907'', London, Blandford, Revised edition 1973.
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* Silva, Christopher ; Yeo, Hyeonsoo ; Johnson, Wayne. (2010) ''[http://rotorcraft.arc.nasa.gov/Publications/files/SilvaC_AHS_2010.pdf Design of a Slowed-Rotor Compound Helicopter for Future Joint Service Missions]'' [[Organization:NASA|NASA]] ADA529322. [https://web.archive.org/web/20160304051636/http://www.dtic.mil/docs/citations/ADA529322 Mirror] Size: 17 pages in 4MB
* D. Walsh, S. Weiner, K. Arifian, T. Lawrence, M. Wilson, T. Millott and R. Blackwell. ''[https://vtol.org/EB393010-E91A-11E0-8A940050568D0042 High Airspeed Testing of the Sikorsky X2 Technology Demonstrator]{{Dead link|date=May 2019 |bot=InternetArchiveBot |fix-attempted=yes }}'' Sikorsky, May 4, 2011. Accessed: October 5, 2013. Size: 12 pages in 3MB
* {{cite book|last=Watkinson|first=John|title=The Art of the Helicopter|year=2004|publisher=Elsevier Butterworth-Heinemann|isbn=07506-5715-4}}
{{Refend}}

==External links==
{{ external media
|float=right|width=250px
| image1 =[http://www.vstol.org/wheel.htm Some previous attempts at high-speed VTOL] <small>only works in Microsoft Internet Explorer</small> }}

[[Category:Aerospace engineering]]
[[Category:Helicopter aerodynamics]]
[[Category:Rotorcraft]]

{{Sourceattribution|Slowed rotor}}