NOTAR is an acronym for an anti-torque system created by McDonnell Douglas Helicopter Systems for use on helicopters. The actual acronym stems from the words "NO TAil Rotor," which describes how the system works. When employed, the NOTAR system completely nullifies the use of a tail rotor and, instead, uses a system of fans inside the tailboom to create a boundary layer of air flow; this is made possible by the Coanda effect, which is the tendency for fluid jet propulsion to be attracted to surfaces in a nearby proximity.
The variable pitch composite blade fan is the driving force behind the NOTAR system, which allows the helicopter to produce simultaneous low-pressure and high-volume ambient air. This production of air is what pressurizes and stabilizes the tailboom. This collected air is then expelled through two slots alongside the tailboom, which is what initiates the Coanda effect. Effectively, this turns the tailboom into a "wing" of sorts, giving the aircraft 60 percent of the momentum needed for hover. In the midst of actual onward flight, the vertical stabilizers of the NOTAR system provide the majority of the momentum, while the directional control is still handled by the helicopter's standard direct jet thrusters.
The effects of such a system were recorded to be tested and analyzed as early as 1945 in the British helicopter "Cierva W.9". Actual development of what is now known as NOTAR began in 1975 by engineers at Hughes Helicopters. The first aircraft to fly with a modernized version of the NOTAR system was the Hughes OH-6 Cayuse, in December 1981. By the time McDonnell Douglas Helicopter Systems acquired Hughes Helicopters, a modified prototype of the OH-6 Cayuse had also been developed. That historic helicopter now has a place at the U.S. Army Aviation Museum.
The modern application of the NOTAR system is controlled entirely by the company that developed it, McDonnell Douglas Helicopter Systems. The MD 520N, MD 600N, and the MD Explorer were the only production helicopters that made use of the NOTAR system in 2011. According to McDonnell Douglas Helicopter Systems, the use of NOTAR completely removes the disadvantages of a tail rotor, which include long drive shafts, hanger bearings, and unnecessary gearboxes. Helicopters equipped with NOTAR equipment are said to be the quietest in the industry.