Virtual Museum of the Vietnam War

Hughes OH-6A "Cayuse"

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History of development

The US Army announced its new program for the development of Army Aviation for the sixties in 1959. One of the points of this program was to develop a new design that would replace the aging helicopters H-13 and H-23, but also the L-19 observation aircraft (later designated the O-1). The American aviation industry responded by creating a large number of project studies, taking a form of aircraft and helicopters, but also autogyros and more unconventional designs, like a tilt wing, for example. The Army board has reviewed all the incoming proposals and concluded, that a technologically advanced helicopter would best satisfy the Army requirements. The board also recommended to develop more than one new design and then a comparison of the prototypes of each of the new designs.

In 1960 the Army approved the board recommendations and developed the requirements for the new design. It was to be a helicopter utilizing a gas turbine as a power source and so it was expected to be much more capable than the existing piston-driven helicopters then in use. As a response to those requirements 14 different designs were submitted to the Army. Another Army board reviewed those designs and recommended further development and building prototypes of the Bell and Hiller helicopters. After some internal controversies inside the Army board it was decided to further extend the program by nominating Hughes to also develop a prototype. The Hughes design was at first deemed too futuristic, but then it was concluded that it is the only program potentially promising a technological breakthrough. In November 1961 the three companies received contracts to build five prototypes each. Those prototypes were to be ready in December 1963. They were supposed to be powered by the Allison T-63 gas turbine. After some delays encountered by all three contractors the prototypes were delivered and tested in many military installations under various climate conditions until July 1964. Those tests resulted in rejection of the Bell design (the OH-4). Hiller (OH-5) and Hughes (OH-6) designs were comparable as to the technical parameters and so the US Army decided to let the companies bid for the lowest unit price. This bid was won by Hughes and so they won the contract to build over 1000 helicopters on 6th of May .1965.

The new helicopter, designated Model 369 by the Hughes company was at the moment of its inception a very modern design. Smaller and simpler to build (in terms of part count) than the machines it was replacing (OH-13 and OH-23) it was at the same time faster and could carry more weight. The OH-6 was one of the first helicopters with its fuselage aerodynamics optimized at the very beginning of its design process. This favourably increased the top speed and decreased the fuel consumption, meaning better range and smaller empty weight (meaning greater usable payload). The design process also concentrated on crash behavior and crew survivability. This led to a design that gave the crew exceptionally good chance in case of a ground impact – which was many times proven during the Vietnam War. This design consisted of an integral “safety cage” around the crew seats, to which all other components, like the engine, tail boom, main rotor and its mast were fastened in such ha way that they broke off in case of a crash, absorbing the energy and at the same time stopping to pose a threat for the people in the cabin. The OH-6 design also contained several features intended to decrease the workload necessary for everyday maintenance of the helicopter. One example of such a feature was using a stainless steel “strap pack” instead of the more conventional bearings in the main rotor. This not only reduces the part count, but also the number of necessary maintenance activities, like lubricating and checking the bearings. Other example could be the system developed for dynamically balancing the tail rotor, executed in such a way that the tail rotor could be balanced without removing it from the helicopter, it also eliminated the necessity to have a special test stand to do the balancing on. The control system consisted exclusively of pushrods (eliminating control cables as more maintenance intensive) and it also didn’t need hydraulic boost, again reducing part count, decreasing the weight and price and facilitating maintenance. Such a solution also enhanced safety in combat by eliminating a system that would otherwise be sensitive to combat damage. The engine was mounted low in the rear of the fuselage, under large two-part doors, which results in excellend engine access directly from the ground, without the need to use working platforms and such, again simplifying maintenance and repairs. One person is suficcient to change the engine under field conditions. The tail rotor shaft is designed to be simple, straight, with no U-joints nor angle gearboxes – once again reducing price and part count and making the maintenance easier.

Between March and April 1966, when the YOH-6A was tested at Edwards, AFB, it set 23 international records of speed, range, climb and so on. One of those records – a non-stop flight from Culver City in California to Ormond Beach in Florida, during which a modified YOH-6A travelled a distance of 6 561,55 km – deserves special attention, as it was not bested by any other helicopter at the moment of writing this article (September 2018) – over 52 years! The collection of records set by the YOH-6A in 1966 shows how revolutionary its design was back in those days.

There was also a financial controversy connected to the OH-6A. At the very beginning of its life, when the LOH competition was still undecided, the Hughes company, or more precisely its owner, Howard Hughes decided to sell his new helicopter to the Army at a lowered price (19 860 dollars in comparison to Hiller’s 29 415 dollars, both prices exclude the engine), and any potential loss that would effect from that were to be covered from a profit made by selling helicopters to the civilian market. A problem occurred when the Army increased its order using up all the manufacturing capabilities of the Hughes company, so that there was nothing more left to be sold on the civilian market, and this put the company in trouble. Additionally the Hughes’ practices became known to the US Army and the competition, and they started to insist on repeating the bid for the lowest price when it came to ordering a new batch of helicopters for the US Army. The new bidding was conducted in 1968, with Bell and Hughes companies participating. Hughes bid 56 660 dollars and Bell bid 54 200 dollars and thus became the new supplier of light observation helicopters to the US Army. Reportedly Howard Hughes decided on his own to increase the price by 3 000 dollars directly before placing the bid and by doing this caused the company to lose the contract.

The OH-6A helicopters got to Vietnam in 1966 and were enthusiastically received by the crews dissatisfied with the OH-13 and OH-23 helicopters they flew before, due to their obsolete construction and lacking performance. Other soldiers initially were sceptic toward the new design, calling it “tinker toy” and making fun about its “beer can construction”, but they soon had to change their minds as they witnessed the new helicopter performing a lot better than heavier and much more expensive machines. OH-6A was quickly becoming a legend. The crews used to say “if you ABSOLUTLY have to crash, do it in a Loach” (as the OH-6A were called in Vietnam). The aviators were walking away from crashes that would surely killed everyone onboard if they happened in any other helicopter.

Just such capabilities as excellent performance, reliability and safety were required to fulfill the very dangerous mission that the Army assigned to most of the OH-6A helicopters in Vietnam. This mission was scouting – meaning doing reconnaissance for the Bell AH-1 Cobra attack helicopters. Standard operation required the Loach to fly low and slow, looking for targets. As soon as the target was found, the Loach was supposed to mark the target and immediately disengage, while the high flying Cobra executed its fire run. The scout mission was usually assigned to young pilots, characterized by exceptional initiative and risk tolerance. Consequently, the losses were relatively high. 842 machines out of the 1419 built for the US Army were destroyed in Vietnam, most of them as a result of the enemy action. The majority of the helicopters never made it to the factory for the planned 1600 hours overhaul – many of them returned a lot earlier to be repaired and returned back to the flight line.

The model shown on the photos above depicts a helicopter flown by the scout platoon of the Troop C, First Squadron, Ninth Cavalry Regiment, First Cavalry Division (Airmobile) in 1970. The unit designated as 1/9 Cav was the most decorated unit of the Vietnam War. Different units using OH-6A in Vietnam with time developed varying tactics for use of their helicopters. The scout platoon of the C/1/9 in 1970 were crewing their helicopters with three people – pilot, observer and the crew chief, the latter two also serving as machine gunners with the M60 machine guns. The pilot sat in front, on the right side, the observer beside him, on the left side, and the crew chief in the rear cabin on the right side. All of the doors of the helicopter were left at the base to save weight, enhance the field of view and fields of fire of the crew’s individual weapons and also to make the egress in case of a crash easier. The C/1/9 didn’t use 7,62mm Miniguns on their scout helicopters because the commanders thought the role of the scout is scouting and not engaging in firefights – that was the job of the Cobra helicopters.

construction report

Specifications ot the Hughes OH-6A Cayuse helicopter

Length (fuselage): 7 010 mm
Height: 2 477 mm
Main rotor diameter: 8 025 mm
Empty weight: 493 kg
Maximum weight: 1 227 kg
Propulsion: One 236 kW Allison T63-A-5A turboshaft engine
Armament: One M60 7,62 mm machine gun and XM27 weapons kit consisting of 7,62mm M134 Minigun or two M60 7,62 mm machine guns
Range: 550 km, ferry even 2 590km
Max speed: 237 km/h
Crew: 2 or 3 people (Pilot, crew chief and optionally additional observer)


Donald J. Porter „The McDonell Douglas OH-6A Helicopter”, 1990 TAB BOOKS Inc.

Wayne Mutza "Aeroscouts in Vietnam", Combat Chronicles 76003, Squadron/Signal Publications


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