Background The Hughes 500/MD 500 series can be traced back to the early 1960s and the issuing of a requirement for a
Light Observation Helicopter (LOH) for the
United States Army. Following a competitive tendering process, Hughes' Model 369 was selected as the winning bid, triumphing over rival submission from the helicopter manufacturers
Bell and
Hiller. On 27 February 1963, under the service designation of
OH-6 Cayuse, the
maiden flight of the type was performed. The MD 500 series features shock-absorbing landing skid struts, a turboshaft engine mounted at a 45-degree angle toward the rear of the cabin pod, a fuel tank cell under the floor and the battery in the nose. The engine exhaust port is located at the end of the cabin pod underneath the tailboom. It has a short-diameter main rotor system and a short tail, which gives it an agile control response and also makes it less susceptible to weather-cocking. It had a distinctive atypical teardrop-shaped fuselage, a feature that sometimes led to personnel referring to it as the "flying egg". Hughes had allegedly succeeded in the LOH contest with its OH-6 helicopter by submitting a very low and aggressive price per airframe (without an engine), to the point where the company allegedly lost money . Due to price escalations for both the OH-6 and spare components, the U.S. Army opted to reopen bids for the programme in 1967. Seeking to profitably produce the type, Hughes offered the machine at a more realistic unit price of $56,550, however, this bid was undercut by the redesigned Bell OH-58 Kiowa, a militarised version of the
JetRanger series. The 500D was replaced by the 500E from 1982 with a pointed nose and various interior improvements, such as greater head- and legroom. The 530F was a more powerful version of the 500E optimized for
hot and high work, being furnished with an enlarged main rotor and more powerful Allison 250-C30 engine, capable of producing up to 425 hp. During January 1984,
Hughes Helicopters, the original manufacturer of the 500 series, was acquired by the American aircraft manufacturer
McDonnell Douglas. Accordingly, starting in August 1985, the 500E and 530F were rebranded as the MD 500E and MD 530F Lifter respectively. Military variants are marketed under the
MD 500 Defender name.
MD 520N The MD 520N introduced a revolutionary advance in helicopter design, dispensing with a conventional anti-torque tail rotor in favor of the Hughes/McDonnell-Douglas-developed
NOTAR system. It also eliminated the vulnerable exposed tail rotor blades, eliminating the possibility of persons being injured or killed on the ground and the cause for many confined area manoeuvring accidents. McDonnell Douglas originally intended to develop the standard MD 520N alongside the more powerful hot-and-high optimized 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, and 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. In 2000, MD Helicopters announced enhancements to the MD 520N, including an improved Rolls-Royce 250-C20R+ engine with 3% to 5% more power for better performance on warm days, and changes to the diffuser and fan rigging, also increased range. ==Operational history==