A conventional tail rotor arrangement dates back to the first helicopters designed and produced. However, in addition to the added complexity of drive shafts, bearings, and gearboxes, tail rotors are noisy and susceptible to foreign object damage. In the 1970s engineers at Hughes Helicopters began working on concepts to eliminate the tail rotor for the US Army. They used the acronym NOTAR for NO Tail Rotor. In December 1981, Hughes flew an OH-6A fitted with a version of the NOTAR design for the first time.
The technology that made this possible is based on the Coanda Effect. Discovered in 1932 by Henri Coanda, it is the tendency of a fluid jet to adhere to a solid wall even when the wall curves away from the jet’s axis or direction. The NOTAR system takes advantage of this effect by using an enclosed fan driven by the transmission to force low-pressure air through two slots in the tail boom. This causes the rotor downwash to hug the contour of the tail boom creating a lateral lift vector that counteracts the majority of the rotor torque. The remaining rotor torque and directional control is managed by a direct jet thruster, which is controlled by pedal input.
The system works well and reduces the possibility of loss of directional control from tail rotor strikes in confined areas. Safety of offsite landings where personnel are walking around an operating helicopter is also enhanced. In addition, the system helps reduce maintenance costs and extends the life of structural components by reducing the vibrations levels that are caused by a high-speed tail rotor.
Three years after the NOTAR helicopter’s first flight Hughes sold its helicopters business to McDonnell Douglas. In May of 1990, McDonnell Douglas flew the first civil production NOTAR model, the 520N. In 1997 McDonnell Douglas merged with Boeing to become the Boeing Company. In 1999, Boeing sold the former MD commercial helicopter product lines to MD Helicopter Holdings Inc., but maintained the rights to the NOTAR system.
MD Helicopters Holdings Inc. was acquired in July 2005 by Patriarch Partners, LLC, an investment fund. The company was recapitalized as an independent company and is based in Mesa, Arizona. It currently manufactures several models with the NOTAR system. More information can be found at MD helicopters website.
Tags: Tim McAdams