In 1960, the US Department of Defense issued Technical Specification 153 for a Light Observation Helicopter (LOH) capable of fulfilling various roles: personnel transport, escort and attack missions, casualty evacuation and observation.
Twelve companies took part in the competition and Hughes submitted the Model 369, nicknamed the "flying egg" on account of its shape. The Hiller and Bell aircraft were selected as finalists, but the US Army later included the Hughes helicopter as well — which was offered at an exceptionally low price — under the designation OH-6, and five prototypes were ordered in spring 1961. The Bell was subsequently eliminated from the contest and the Hughes 369 was redesignated OH-6A "Cayuse". The first of the five prototypes flew on 27 February 1963 and was delivered to Fort Rucker air base the following November, to begin trials for the Army.
The Hughes aircraft was very interesting from a structural point of view. For example, the fully-articulated rotor had four blades of constant chord, consisting of an extruded light alloy spar, to which a single sheet of light alloy was bonded to form the profile of the blade. The fuselage had a light alloy, semi-monocoque structure and its remarkable robustness afforded the occupants good protection even in heavy landings. However if the compactness of the fuselage made the Hughes OH-6 light and sturdy, with low drag, it reduced the helicopter's versatility, as its small internal volume was clearly restrictive, despite being perfectly in accordance with Technical Specification 153.
The aircraft had an Allison 250 turbine engine, which was very light and compact, with a maximum power derated from 400 to 282shp, which afforded obvious advantages in terms of service life and safety. The sophisticated avionics included a track indicator, VHF and UHF transceivers and ADF. A wide choice of weapons fits was available.
Following trials, the choice of the OH-6A for large-scale production was announced in May 1965 with an initial order for 714, which was later increased to 1300 with an option on another 114. Production reached a maximum of 70 helicopters in the first month. In all 1434 were built, the last of which were delivered in August 1970.
The Cayuse established no fewer than 23 world records in March-April 1966: 2800km closed circuit; 3561km in a straight line; 227.7km/h over a 2000km closed circuit and 8601m altitude in horizontal flight. It won other records in the various classes for helicopters of a variety of weights, including a speed record of over 277km/h.
Apart from the five prototypes built for the US Army, Hughes built four others for its own research purposes, one of which was converted into a civil version designated Model 500, which flew at the beginning of 1967. The subsequent Model 500C variant had a 405shp Allison 250-C20 turbine. One OH-6A was later modified under a research programme for ARPA (the advanced research office of the US Department of Defense) to reduce noise levels and has been nicknamed "The Quiet One". It introduced a five-blade main rotor, four-blade anti-torque rotor, exhaust silencer and various noise blanketing devices on the air intakes. The new rotor has 67 per cent of the r.p.m. of the original one, allowing 270kg more payload to be carried at a maximum speed of 278km/h. A second experimental prototype, the OH-6C, with an Allison 250-C20 turbine, reached a speed of 322km/h during a test flight from Edwards Air Force Base.
The H-500MC was a military export version of the Hughes 500C, with major improvements including a more powerful main rotor of larger diameter transmission capable of withstanding higher torque values, an all-metal tail rotor and a maximum fuel capacity of 242 liters. The side windows are also different and high skid landing gear can be fitted. The armament is similar to that of the OH-6A.
Hughes OH-6A helicopters have been exported to the Brazilian Air Force (9) and Navy (6), and the Japanese Ground Self-Defense Force (29). The 500M version has been supplied to the Colombian Air Force (4), Italian customs (over 60), the Spanish Navy (6 in the ASW version), Denmark, Argentina, Bolivia, Mexico and the Philippines.
A later version, the 500D (500MD military) is distinguished externally from earlier models by a T-tail and five-blade rotor. The powerplant is also different — an Allison 250-C20B delivering an absolute maximum of 411shp and 355shp maximum continuous power. The increase in power and adoption of the new tail unit have led to a general improvement in structural robustness, particularly of the landing gear, the lower part of the cabin and tail boom, plus, of course, the transmission.
Current production includes the 500E, which introduces a lengthened and redesigned cabin among other improvements, and the 530F which introduces an uprated Allison 250-C30 engine for high altitude operations. Military variants of these new versions were introduced in 1984.
One of the most important orders for Hughes helicopters was for 29 of the Model 369HM built under license by Kawasaki and delivered to the Japanese Army in January 1972. The Japanese OH-6J is virtually identical to the American OH-6A, except for its performance and weight, which are conditioned by the 318shp Mitsubishi-Allison 250-C18A powerplant.
In Italy, Breda-Nardi have held the license to build and market Hughes Model 300 and 500 helicopters since 1969. Apart from various commercial orders, Breda-Nardi has supplied the NH-500M to the Italian customs. Its characteristics are the same as those of the American aircraft.
G.Apostolo "The Illustrated Encyclopedia of Helicopters", 1984