Helicopter Anti-Torque Systems

In this article, we discuss systems that counter main rotor torque and provide yaw control for helicopters. We’ll cover

• the traditional (open) tail rotor,
• the ducted (Fenestron) tail fan,
• electric tail rotors, and
• the NOTAR (no tail rotor) system.

Why are anti-torque systems necessary?

Before diving into the types of systems, let’s see why they are needed. They are required in single main rotor helicopters. A power source (typically a turboshaft engine) provides torque to keep the main rotor spinning. An equal and opposite torque is applied back to the helicopter. If not countered by an anti-torque system, this will spin the helicopter in the opposite direction of the main rotor.

Viewed from above, a typical American helicopter main rotor spins counterclockwise (CCW). Without an anti-torque system, this helicopter would spin clockwise (CW) as shown in the diagram below. The anti-torque system typically supplies a thrust at the tail of the aircraft to stop the CW motion, as shown.

Anti-torque must be able to increase/decrease quickly for two reasons. First, various maneuvers like a “quick stop” cause sharp changes in main rotor torque. Without a simultaneous change in anti-torque, the helicopter would yaw undesirably. Second, even when main rotor torque is held constant, the anti-torque system is used to provide yaw control. For example, in a hover, a pilot may need to turn the helicopter from facing north to east before landing. Adjusting the magnitude of anti-torque facilitates this.

Now that we know what anti-torque systems are used for, let’s look at specific designs.

Open tail rotor

The most common anti-torque system is the open tail rotor. This rotor produces horizontal thrust at the tail of the aircraft, which results in a yaw moment and anti-torque. The rotor typically spins at a constant speed in operation, and the blades of this rotor are feathered to control thrust / yaw moment. The pilot controls tail rotor feathering with pedals, and this anti-torque system provides very good, responsive yaw control.

An open tail rotor presents some hazards as it may strike people or objects on the ground. For example, in 2014, a Navy pilot was struck in the head by a tail rotor when he exited his helicopter to check the refueling process. Many incidents of tail rotors striking objects/people on/near the ground have been reported.

Another disadvantage of open tail rotors is noise. Particularly at closer distances, the higher frequency noise emitted by a tail rotor can be very annoying.

The tail rotor is geared to the main rotor and spins at a speed proportional to the main rotor (typically about 5x faster). Even when the engine(s) fail, if main rotor speed is maintained (via autorotation), then tail rotor speed is also maintained.

Ducted tail rotor

Aside from being enclosed in a duct, these tail rotors behave like open tail rotors. They produce a yaw moment via a lateral thrust at the tail of the aircraft, modulated by blade feathering (linked to the pilot’s pedals). Advantages of ducted rotors, over open rotors, include noise reduction, safer operation, and reduced aerodynamic interference. A ducted rotor can have a smaller diameter than an open rotor due to enhanced aerodynamic efficiency. Disadvantages include the cost, weight and drag associated with the duct.

Ducted tail rotors are sometimes called fan-in-tail, fantail or Fenestron systems. While an open tail rotor typically has 2 to 6 blades, ducted tail rotors typically have 7 to 18 blades. The Fenestron name was trademarked by Eurocopter, which incorporated this design on many helicopters.

Ducted tail rotors were investigated by a British company named G & J Weir Ltd. in the late 1930s or early 1940s. Their aim was to increase safety and performance of tail rotors. They never flew a ducted tail rotor, but the French aircraft manufacturer Sud Aviation incorporated a fantail on their second prototype SA 340 helicopter, which flew in April 1968. A refined design was put into production on the SA 341 Gazelle in the early 1970s.

A larger, all-composite, fantail was incorporated into the SA 360 Dauphin and larger helicopters. Despite efforts, a consensus formed among engineers that such fantail designs were not suitable for larger aircraft. For example, it was reportedly investigated and then abandoned for the SA 330 Puma.

Eurocopter produced Fenestrons in the 1990s with unevenly spaced blades, which reportedly further reduced noise levels. They incorporated such a design into the EC135 and later many other helicopters. More recently, Airbus (formerly Eurocopter) developed a Fenestron for the new H160 model. This Fenestron is mounted with a 12-degree tilt from vertical (called a cant angle), directing its thrust slightly upward to assist the main rotor, which improves hover and low speed performance.

Many other helicopter manufacturers have produced Fenestron designs, including Boing-Sikorsky, Kamov and Kawasaki.

Electric tail rotors

Many companies have investigated the use of electric rotors. A whole new industry called urban air mobility (UAM) is rising around this concept. In 2020, Bell Flight (formerly Bell Helicopter) built a prototype model 429 with four small, electric-powered tail rotors, shown below.

The shaft that traditionally spans the tail boom to power the tail rotor instead powers two generators within the tail boom. These generators provide power to the four electric motors, which were produced by Safran.

Like other tail rotors, the pilot controls the thrust of these rotors via pedals in the floor of the cockpit. While the above rotors translate pedal movement into tail rotor feathering, this design uses tail rotor speed. A computer converts the pedal position (likely with other information) into electric motor speed commands.

There are low torque scenarios where tail rotors must provide thrust opposite to the normal direction. For example, a traditional tail rotor producing right thrust may produce left thrust maneuvering in autorotation. This is quickly accomplished with feathering in traditional rotors, but this system would need to ramp rotor speeds down to 0 and then up in the opposite direction. This would presumably give rise to a large range of RPM in which the tail rotor generates negligible thrust gradient, and could be a weakness of this design.

NOTAR (No tail rotor)

A more unique anti-torque system is the NOTAR, which stands for no tail rotor. In this design, the fan/rotor is located within the tail boom, just aft of the fuselage. Air is blown aft and some of it exits laterally to directly produce lateral thrust, near the aft tip of the tail. The rest of it exists downward directed slots, all on one side of the tail. The latter air induces flow (from relative movement or rotor wash) around one side of the tail, which puts that side at a lower air pressure and pulls the tail in that direction. This phenomenon is known as the Coanda effect.

This design is the quietest and safest (in terms of blade strikes) of the ones discussed here. However, as you might expect, controllability is weaker.

Other resources

If you'd like to read more about anti-torque systems or helicopters in general, we recommend "Helicopter Performance, Stability, and Control" by Ray Prouty.