The MD Helicopters MD-520N introduced a revolutionary advance in helicopter design as it dispensed with a conventional tail rotor in favour of the Hughes/McDonnell Douglas' developed NOTAR (NO TAil Rotor) system.
Development of the revolutionary NOTAR system dates back to late 1975 when Hughes engineers began initial concept development work. In December 1981 Hughes flew an OH-6A fitted with NOTAR for the first time. This prototype was modified several times, notably in 1985 when it was fitted with the more streamlined MD-500E forward fuselage, an Allison 250-C20B turboshaft, a second blowing slot and a new fan. By then Hughes had become McDonnell Douglas.
Although the concept, which uses the Coanda effect, took some time to refine, the NOTAR system is simple in theory and works to provide directional control the same way a wing develops lift. Low pressure air is forced through two slots on the right side of the tailboom, causing the downwash from the main rotor to hug the tailboom, producing lift, and thus a measure of directional control. This is augmented by a direct jet thruster and vertical stabilisers.
NOTAR system benefits include far lower external noise (the MD-520N is the quietest certificated helicopter in the world), increased safety due to the lack of a tail rotor, improved handling and performance, reduced vibration and easier maintainability.
McDonnell Douglas originally intended to develop the standard MD-520N alongside the more powerful hot and high optimised MD-530N (both were launched in January 1989 and were based on the conventional MD-500E). The MD-530N was the first to fly, on December 29 1989, the MD-520N first flew on May 1 1990. Development of the MD-530N was suspended when McDonnell Douglas decided that the MD-520N met most customer requirements for the 530N. Certification for the MD-520N was awarded on September 13 1991, and the first was delivered on December 31 that year.
Following the 1997 Boeing/McDonnell Douglas merger, Boeing sold the former MD civil helicopter lines to MD Helicopters in early 1999.
At the 2000 Farnborough Airshow MD Helicopters announced enhancements to the MD-520N including an improved Rolls-Royce 250-C20R+ engine with 3-5% more power for better performance on warm days, and, with changes to the diffuser and fan rigging, increased range and payload capability.
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