{{Short description|Helicopter with two horizontal rotor assemblies}}{{Hatnote|This article is about tandem-rotor aircraft. For synchropters, see [[intermeshing-rotor helicopter]].}}[[File:CH-46 Sea Knight Helicopter.jpg|thumb|Boeing Vertol [[CH-46 Sea Knight]]]] [[File:Tandem rotor.gif|thumb|Animation of the rotors spinning]]
A '''tandem-rotor aircraft''' is an [[aircraft]] with two large [[helicopter rotor]] assemblies mounted one in front of the other in the [[horizontal plane]]. This configuration is mainly used for large cargo helicopters.<ref name="cavalry">{{cite web |url=http://www.cavalrypilot.com/fm1-514/Ch3.htm |title=FM 1-514 CHPTR 3 - Rotor System Operation |accessdate=2017-12-03 |url-status=dead |archiveurl=https://web.archive.org/web/20140218111928/http://www.cavalrypilot.com/fm1-514/Ch3.htm |archivedate=2014-02-18 }}</ref>
== Design ==
Single-rotor helicopters need a mechanism to neutralize the [[Aircraft principal axes|yawing]] movement produced by the single large rotor. This is commonly accomplished by a [[tail rotor]], [[coaxial rotor]]s, and the [[NOTAR]] systems. Tandem-rotor helicopters, however, use [[counter-rotating rotor]]s, with each cancelling out the other's [[torque]]. Therefore, all of the power from the engines can be used for [[lift (force)|lift]], whereas a single-rotor helicopter uses some of the engine power to counter the torque.<ref name="cavalry"/> An alternative is to mount two rotors in a [[Coaxial rotors|coaxial]] configuration. The first successful tandem-rotor helicopter was built by [[Nicolas Florine]] in 1927.
Advantages of the tandem rotor system are a larger [[Center of gravity of an aircraft|center of gravity]] range and good [[longitudinal stability]]. Disadvantages of the tandem rotor system are a complex [[transmission (mechanics)|transmission]]<ref name="cavalry"/> and the need for two large rotors.
The two rotors are linked by a transmission that ensures the rotors are synchronized and do not hit each other, even during an engine failure.<ref>Art of the helicopter by John Watkinson, p. 13</ref>
Tandem-rotor designs achieve [[Flight dynamics (fixed-wing aircraft)|yaw]] by applying opposite left and right [[Helicopter flight controls|cyclic]] to each rotor, effectively pulling both ends of the helicopter in opposite directions. To achieve [[Flight dynamics (fixed-wing aircraft)|pitch]], opposite [[collective pitch#Collective|collective]] is applied to each rotor; decreasing the lift produced at one end, while increasing lift at the opposite end, effectively tilting the helicopter forward or back.<ref>{{cite web|url=https://fas.org/irp/doddir/army/fm3-04-203.pdf|title=US Army FM 3-04.203 Fundamentals of Flight; page 1-33, para 1-81, 1-82}}</ref>
Tandem-rotor helicopters have the advantage of being able to hold more weight with shorter blades, since there are two sets. However, the rear rotor works in the aerodynamic shadow of the front rotor, which reduces its efficiency. This loss can be minimized by increasing the distance between the two rotor hubs, and by elevating one hub over the other.<ref>Principles of helicopter aerodynamics by J. Gordon Leishman p. 73.</ref><ref name="wzs">Rotary Wing Aerodynamics, W.Z. Stepniewski, p. 197.</ref> Tandem-rotor helicopters tend to have a lower [[disk loading]] than single-rotor helicopters.<ref>Rotary Wing Aerodynamics, W.Z. Stepniewski, p. 185</ref>
Tandem-rotor helicopters typically require less power to hover and achieve low-speed flight as compared to single-rotor helicopters. Both configurations typically require the same power to achieve high-speed flight.<ref>Rotary Wing Aerodynamics, W.Z. Stepniewski, p. 200.</ref>
== Examples == [[File:PH-10171a.jpg|thumb|[[Columbia Helicopters]] [[Boeing Vertol 107-II]] and [[Boeing 234]]]] [[File:Piasecki H-21 (modified).jpg|thumb|A [[Piasecki H-21]]B in the early 1960s]] [[File:Yakovlev Yak-24 at Central Air Force museum pic 7.JPG|thumb|A [[Yakovlev Yak-24]] at [[Central Air Force Museum]]]]
* [[Bell HSL]] (1953) * [[Boeing CH-47 Chinook]] (1961) - most-produced tandem-rotor helicopter (over 1,200 built) * [[Boeing Model 360]] (1987) * [[Boeing Vertol 107-II]] (1958) ** [[Boeing Vertol CH-46 Sea Knight]] (1960), military version of Vertol 107-II * [[Boeing Vertol XCH-62]] (1970s - not completed) * [[Boeing Vertol Model 234]] (1981) * [[Bristol Belvedere]] (1958) * [[Bristol Type 173]] (1952) * [[Chu CJC-3]] (1952) * [[Filper Research Beta|Filper Beta 200]] (1966) * [[Filper Research Beta|Filper Beta 400]] (1967) * [[Filper Research Beta|Filper Helicopter]] (1965) * [[HERC JOV-3]] (1948) * [[Hunt Rotary Aeroplane]] (1910) * [[McCulloch H-30|Jovair Sedan 4A]] (1963) * [[McCulloch H-30|McCulloch MC-4]] (1951) * [[Piasecki H-16]] (1953) * [[Piasecki H-21]] (1949) * [[Piasecki H-25]]/HUP Retriever (1952) * [[Piasecki HRP Rescuer]] (1945) * [[Piasecki PV-14]] (1948) * [[Rotorcraft XR-11]] (1947) * [[Yakovlev Yak-24]] (1952)
== See also ==
* [[Coaxial-rotor aircraft]] * [[Intermeshing-rotor helicopter]] * [[Rotorcraft]] * [[Tiltrotor]] * [[Transverse-rotor aircraft]]
== References ==
{{refs}}
== External links == * {{Commons cat inline}}
[[Category:Helicopter components]] [[Category:Tandem rotor helicopters|*]]