All single rotor helicopters need some way to counteract the torque that is created by the rotor blades spinning around the mast. The most common anti-torque system used on helicopters is the Tail Rotor System. The Tail Rotor System is a relatively small rotor and transmission attached at the end of the tail boom that is driven from a shaft coming from the main engine and transmission (ASA, Helicopter Flying Handbook 1-5).
Another anti-thrust system used less frequently is the Fenestron system. It is driven in a similar way to the standard tail rotor system but instead of two rotor blades at the end of the boom there is a series of rotating blades that are enclosed in a protective shroud, thus adding a degree in safety by protecting the tail rotor blades from ground contact (ASA, Helicopter Flying Handbook 4-7). The anti-torque system I want to discuss in greater detail is called the “NOTAR” system.
The NOTAR system is dramatically different in design as it does not require another rotor at the end of the tail boom to create thrust and in losing that tail rotor this system has a number of advantages, added safety being one of the crucial benefits. The NOTAR system uses the natural characteristics of aerodynamics along with thrust from pressurized air exiting the tail boom to provide the thrust needed to counter the torque being produced by the main rotor (ASA, Helicopter Flying Handbook 4-7).
It does this using the following components that are built into the design of the helicopter: air intake, fan, tail boom the can contain and control airflow, tail thruster cone, and two vertical stabilizers at the end of the tail boom. The first component of this system is the air intake, or a large opening on top of the rear fuselage. This intake is covered by a fine mesh screen designed to keep foreign objects from getting sucked into the system (Wagtendonk 190).
The intake pulls air into the second component of this system: an enclosed variable-pitch composite blade fan. This fan’s purpose is to create a low pressure and high volume of ambient air that is sent into the tail boom, pressurizing it in the process. The fan blades are variable-pitch meaning their pitch, or pitch angle, can be increased or decreased creating more or less volume of air that is being introduced into the tail boom (Wagtendonk 190).
The fan is located just behind the main transmission where the tail boom connects to the fuselage and is driven directly by the main rotor gearbox, this ensures that the fan is always providing directional control including when in auto rotation (Wagtendonk 190). The tail boom is the third and very crucial component of the NOTAR system. It looks similar to a standard tail boom but has a bigger circumference, is made from composite material and is completely hollow on the inside.
The tail boom is designed with two parallel slots that run the length of the right side that allow the fan air (low pressure) to flow out and downwards (Wagtendonk 190). This movement of airflow energizes, or speeds up, the boundary layer of downwash flow that is created by the main rotor. This is called the Coanda effect (Wagtendonk 190). This essentially makes the tail boom a wing in relationship to the airflow created by the main rotor- low pressure on the right side and high pressure on the left side creating lift/thrust in the opposite direction of the torque from the main rotor.
The Coanda effect is most effective when the helicopter is at a hover and can produce up to 60% of the needed anti-torque force. When forward speed is gained or in windy conditions the main rotor downwash begins to angle away from the tail boom reducing the Coanda effect (ASA Helicopter Flying Handbook 4-7). At the end of the tail boom we have another component to this system that provides the remaining force needed to produce enough anti-torque: the rotating direct jet thruster cone.
The direct jet thruster is basically a nozzle at the end of the tail boom that directs the flow of the pressurized fan driven air. When the airflow reaches the nozzle, it first hits baffles located inside the rotating nozzle, which helps direct the airflow out the rectangular opening on the cone (Wagtendonk 191). The pilot can control the orientation of the cone by making pedal inputs- pressing the left pedal points the opening on the cone to the left side creating more anti-torque while right pedal turns the cone to the right reducing the anti-torque thrust (Wagtendonk 191).
The final component to the NOTAR system is the twin vertical stabilizers that are attached on each end of the horizontal stabilizer. These stabilizers provide most of the anti-torque once the helicopter is in forward flight (ASA Helicopter Flying Handbook 4-7). Unlike the standard helicopter vertical stabilizer the left stabilizer actually moves and acts like a rudder, moving in unison with the rotation of the direct jet thruster (Wagtendonk 192).
The right stabilizer is more like a “yaw damper” and is hooked up to a Yaw Stability Augmentation System (YSAS) (Stephens, “NOTAR: More Than What It Appears To Be”). The YSAS consists of a small electro-mechanical actuator that moves the right stabilizer based off of information coming from a yaw rate gyro and lateral accelerometer that is installed in the cockpit (Stephens, NOTAR: More Than What It Appears To Be”). There are some distinct advantages of the NOTAR system over the more conventional tail rotor and Fenestron anti-torque systems.
One obvious advantage when comparing the NOTAR system to any other helicopter in flight is the amount of noise level reduced due to the lack of another added rotor (Abdollahi 6). In fact the MD 900 (which uses NOTAR) boasts the lowest noise levels of comparable helicopters (Abdollahi 6). Another advantage the NOTAR system has over the conventional tail rotor design is added safety. With no tail rotor, the NOTAR system eliminates the hazards of tail rotor strike, foreign object damage, and eliminates hazards involving people walking into the tail rotor (Wagtendonk 189).
Also, the ability to control the heading in crosswind conditions is improved, and tail rotor blade stalls are eliminated (Wagtendonk 189). Though the NOTAR system is not widely used in the helicopter industry it is proven to be a highly effective, safer, anti-torque system. Its simple design using the natural characteristics of aerodynamics adds to its advantages, as does the additional safely gained regarding passengers and the pilot by eliminating the need for a tail rotor.