🔵​✈️ 105. Helicopter: Antitorque System and Antitorque Drive Systems 🚁🎞️

Source
: FAA-H-8083-21B, Helicopter Flying Handbook  -  Pag: 4-7

Helicopter: Antitorque System and Antitorque Drive Systems

Antitorque System

Helicopters with a single, main rotor system require a separate antitorque system. This is most often accomplished through a variable pitch, antitorque rotor or tail rotor. Pilots vary the thrust of the antitorque system to maintain directional control whenever the main rotor torque changes, or to make heading changes while hovering. 

Most helicopters drive the tail rotor shaft from the transmission to ensure tail rotor rotation (and hence control) in the event that the engine quits. Usually, negative antitorque thrust is needed in autorotations to overcome transmission friction.

Fenestron

Another form of antitorque system is the Fenestron or “fanin-tail” design. This system uses a series of rotating blades shrouded within a vertical tail. Because the blades are located within a circular duct, they are less likely to come into contact with people or objects.

NOTAR®

Using the natural characteristics of helicopter aerodynamics, the NOTAR® antitorque system provides safe, quiet, responsive, foreign object damage (FOD) resistant directional control. The enclosed variable-pitch composite blade fan produces a low pressure, high volume of ambient air to pressurize the composite tailboom. 

The air is expelled through two slots which run the length of the tailboom on the right side, causing a boundary-layer control called the Coanda effect. The result is that the tailboom becomes a “wing,” flying in the downwash of the rotor system, producing up to 60 percent of the antitorque required in a hover. The balance of the directional control is accomplished by a rotating direct jet thruster. 

In forward flight, the vertical stabilizers provide the majority of the antitorque; however, directional control remains a function of the direct jet thruster. The NOTAR® antitorque system eliminates some of the mechanical disadvantages of a tail rotor, including long drive shafts, hanger bearings, intermediate gearboxes and 90° gearboxes.

Antitorque Drive Systems 

The antitorque drive system consists of an antitorque drive shaft and a antitorque gearbox mounted at the end of the tail boom. The drive shaft may consist of one long shaft or a series of shorter shafts connected at both ends with flexible couplings. 

This allows the drive shaft to flex with the tail boom. The tail rotor gearbox provides a right-angle drive for the tail rotor and may also include gearing to adjust the output to optimum tail rotor rpm. Tail rotors may also have an intermediate gearbox to turn the power up a pylon or vertical fin.

System Type PDF	Operating Principle PDF	Key Components PDF	Safety and Environmental Benefits PDF	Directional Control Mechanism PDF	Drive System Characteristics PDF
NOTAR (No Tail Rotor)	Uses a fan to pressurize the tailboom and expel air through slots via the Coanda effect, combined with a direct jet thruster.	Variable-pitch composite blade fan, composite tailboom with slots, rotating direct jet thruster, and vertical stabilizers.	High safety for ground personnel, low noise levels, and resistance to Foreign Object Damage (FOD); eliminates many mechanical disadvantages.	The Coanda effect produces up to 60 percent of required torque in hover; the remaining balance is provided by a rotating direct jet thruster.	Eliminates the need for long drive shafts, hanger bearings, intermediate gearboxes, and 90 degree gearboxes.
Conventional Tail Rotor	Variable pitch rotor blades produce horizontal thrust to counteract the torque generated by the main rotor.	Variable pitch blades, transmission-driven drive shaft, and 90 degree gearbox.	Ensures directional control remains available during engine failure scenarios (autorotation).	The pilot varies the thrust of the antitorque rotor to maintain or change the aircraft heading.	Driven from the main transmission via a drive shaft to ensure continuous rotation if the engine fails.
Fenestron (Fan-in-tail) / Antitorque Drive Systems (General)	A series of rotating blades shrouded within a circular vertical tail duct. Mechanical power transmission from the main transmission system to the tail assembly.	Rotating blades, circular duct, and vertical tail. Antitorque drive shaft (single or series), flexible couplings, intermediate gearbox, and 90 degree tail rotor gearbox.	Increased safety as blades are less likely to come into contact with ground personnel or external objects. Flexible couplings allow the drive shaft to flex in alignment with the movement of the tail boom.	Thrust produced by the shrouded fan (implied).	The drive shaft connects to a gearbox providing a right-angle drive and adjustment to the optimum RPM.