page_title: Helicopter
section_title: Anti-torque
hierarchical_section_title: Helicopter / Design characteristics / Anti-torque
caption_attribution_description: English: Belgium Police 520N helicopter
context_page_description: A helicopter is a type of rotorcraft in which lift and thrust are supplied by horizontally-spinning rotors. This allows the helicopter to take off and land vertically, to hover, and to fly forward, backward and laterally. These attributes allow helicopters to be used in congested or isolated areas where fixed-wing aircraft and many forms of VTOL aircraft cannot perform.
  The English word helicopter is adapted from the French word hélicoptère, coined by Gustave Ponton d'Amécourt in 1861, which originates from the Greek helix "helix, spiral, whirl, convolution" and pteron "wing". English language nicknames for “helicopter” include "chopper", "copter", "heli", and "whirlybird". In the United States military, the common slang is “helo” pronounced with a long "e".
  Helicopters were developed and built during the first half-century of flight, with the Focke-Wulf Fw 61 being the first operational helicopter in 1936. Some helicopters reached limited production, but it was not until 1942 that a helicopter designed by Igor Sikorsky reached full-scale production, with 131 aircraft built.
context_section_description: Most helicopters have a single main rotor, but torque created by its aerodynamic drag must be countered by an opposed torque. The design that Igor Sikorsky settled on for his VS-300 was a smaller tail rotor. The tail rotor pushes or pulls against the tail to counter the torque effect, and this has become the most common configuration for helicopter design, usually at the end of a tail boom.
  Some helicopters use other anti-torque controls instead of the tail rotor, such as the ducted fan (called Fenestron or FANTAIL) and NOTAR. NOTAR provides anti-torque similar to the way a wing develops lift through the use of the Coandă effect on the tail boom.
  The use of two or more horizontal rotors turning in opposite directions is another configuration used to counteract the effects of torque on the aircraft without relying on an anti-torque tail rotor. This allows the power normally required to be diverted for the tail rotor to be applied fully to the main rotors, increasing the aircraft's power efficiency and lifting capacity. There are several common configurations that use the counter-rotating effect to benefit the rotorcraft:
  Tandem rotors are two counter-rotating rotors with one mounted behind the other.
  Transverse rotors are pair of counter-rotating rotors transversely mounted at the ends of fixed wings or outrigger structures. Now used on tiltrotors, some early model helicopters had used them.
  Coaxial rotors are two counter-rotating rotors mounted one above the other with the same axis.
  Intermeshing rotors are two counter-rotating rotors mounted close to each other at a sufficient angle to let the rotors intermesh over the top of the aircraft without colliding.
  Quadcopters have four rotors often with parallel axes (sometimes rotating in the same direction with tilted axes) which are commonly used on model aircraft.
  Tip jet designs let the rotor push itself through the air and avoid generating torque.
caption: MD Helicopters 520N NOTAR
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