right|thumb|Traditional tail rotor of an [[Aérospatiale Puma.]]
The '''tail rotor''' of a main rotor. It is primarily used in order to counteract the yaw motion and the torque that a rapidly turning disk naturally produces. The tail rotor in simple terms is a propeller that pushes the body of the helicopter in the opposite direction of the main rotor, preventing loss of control.

==Significance==

The tail rotor, stabilising the helicopter as it does, is an extremely crucial element of the helicopter, and loss or serious damage often means immediate loss of flight control, often resulting in serious and fatal crashes. 

However, the loss does not ''necessarily'' mean all control has to be lost - if the aircraft can be kept moving in a forward direction, it can sometimes still be flown. In one case, a New Zealand helicopter pilot with extensive flight experience managed to lose his Robinson R44 helicopter's tail rotor at 4500 ft due to object strike (likely from something that had fallen from the open cabin), and then nursed the aircraft for over 25 minutes to an agricultural airstrip in Fiordland, because no other landing sites were available in the mountaineous terrain. He was forced to land at a speed of 80 knots and skidded for 45 m before coming to a rest, unharmed and with the helicopter in a repairable state.<ref>''[http://www.nzherald.co.nz/section/1/story.cfm?c_id=1&objectid=10493099 Pilot's 'exceptional flying' saves $540,000 helicopter]'' - ''The New Zealand Herald'', Monday 18 February 2008</ref>

==Design variations==
There are two major variations to traditional tail rotor design concerning the placement of the tail rotor and the surrounding structure. Some companies such as Eurocopter enclose the rotor within a fantail assembly. Such design - called fenestron - protects the tail rotor from foreign object damage better than the traditional outer mounted design but complicates the design of the tailcone to account for the enclosed mechanisms. 

==New developments==
In some more recent helicopter designs, the tail rotor has been mounted tangential to the furthest back point of the top rotor.  That is to say that it looks much like an old propeller plane, only at the back of the helicopter instead of the front of a wing.  In these new designs the rotor spins in a direction opposite to the top rotor (i.e. counter-clockwise if the rotor spins clockwise and vise-versa).  This in effect, cancels the spin and has the added benefit of producing forward thrust. 

thumb|200px|Rotor anticouple de [[Puma (hélicoptère)|Puma]]
thumb|200px|Fenestron de [[Gazelle (hélicoptère)|Gazelle]]

Le '''rotor anticouple''' est un rotor auxiliaire de petite taille situé à l'arrière d'un pales est actionné par les pédales d'un lacet :
* en vol de translation pour contrer le couple induit par le rotor principal qui tend à faire tourner la cellule dans le sens opposé,
* en vol stationnaire pour effectuer un mouvement de giration.

Il ne participe pas à la sustentation proprement dite et peut être de différents types :  

* rotor conventionnel bipale sur les hélicoptères légers (type Écureuil d'Eurocopter) pouvant aller jusqu'à 5 pales sur les hélicoptères lourds (Super Puma). Remarque: les hélicoptères lourds possèdent une liaison mécanique sur les commandes de vol, entre la chaine "lacet" et la chaine "collectif", qui permet de corriger la chaine lacet en fonction de l'application de pas en collectif

* rotor multipale (8 à 13) caréné de type "Dauphin, l'EC 135 d'Eurocopter ou encore le Cabri G2 d'Hélicoptères Guimbal. Ce type de rotor plusieurs avantages :
** réduction de traînée en vol de croisière : le rotor étant masqué dans la dérive il ne crée pas de traînée. Notons par ailleurs, que le fenestron type Gazelle, est dissymétrique et il soulage le R.A.C à vitesse de translation élevée
** meilleure sécurité : le rotor étant carréné, il est plus difficile d'accès pour les mains et le câbles imprudents
** moindre signature radar : une fois encore le rotor est masqué et renvoie donc un plus faible écho radar (le Comanche était équipé de ce type de rotor)

thumb|left|Rotor anticouple conventionnel de l'[[BK 117|EC 145 ]]