The use of jet thrust for anti-torque is not a new one, having been used on the Cierva-Weir W-9 in 1945 and also in other designs. However, the design required high power and directional sensitivity was poor. The idea was also used on prototype helicopters built by Hiller in the US and Nord in France. The NO TAil Rotor or NOTAR concept uses a similar yet different approach to anti-torque and yaw control. The design is discussed by Logan (1978) and Sampatacos et al. (1983), and is reviewed by Winn & Logan (1990). Here, anti-torque capability comes from a circulation control concept, which results in a distributed side-force along the entire tail boom assembly. As shown in Fig. 6.37, a high – velocity jet (or jets) of air from a pressurized tail boom is blown tangential to the surface out of narrow slots that run lengthwise on one side of the tail boom. In combination with the downwash velocity produced by the main rotor, these jets cause the flow to remain attached to the tail boom surface by means of the Coanda effect. Downstream of the slots, a powerful suction pressure is produced on one side of the tail boom and a side (anti-torque) force results. The magnitude of this side-force depends on the jet velocity out of the slot. This is controlled by adjusting the pressure inside the tail boom by means of a variable pitch fan and compressor. A small auxiliary nozzle at the end of the tail boom provides a pressurized jet to improve yaw control rates and overcome the inherent lag in the circulation control system. The nozzle in the jet thruster is rotated by the conventional action of the pilot’s foot pedals.
In forward flight, the circulation control becomes less effective as the main rotor down – wash moves further along the tail boom. Fixed aerodynamic stabilizers and the jet thruster
combine to produce the necessary anti-torque and yaw control. The NOTAR concept has proved attractive to operators because of its low noise, safety for ground personnel, and freedom from blade strikes when operating in confined locations. Also, for a military helicopter this system is attractive because of the absence of a vulnerable tail rotor assembly, and it has a high level of redundancy in the event of any tail boom damage.