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The OH-6A was designed for use as a military scout during the Vietnam war to meet the U.S. Army's need for an extremely maneuverable light observation helicopter (LOH program). Initially fielded in Vietnam in early 1968, the Hughes OH-6A was used for command and control, observation, target acquisition, and reconnaissance. The Cayuse was organic to division, brigade, and battalion size units. The four-passenger teardrop shaped "Flying egg" (six-passenger with rear seats folded-down) was a small, light, sturdy, maneuverable helicopter, with very low drag.

The OH-6A Cayuse was quite effective when teamed with the AH-1G Cobra attack helicopter as part of what were known as "Pink Teams". The OH-6A "Loach" (for "LOH") would find targets by flying low, "trolling for fire", then marking the target with colored smoke to lead in a Cobra, or "Snake", to attack. The Cayuse could absorb an extensive amount of small arms fire and still bring the crew home safely. The OH-6A could be armed with the M27 armament subsystem, the port (left) side mounting M134 six-barrel 7.62mm "Minigun" or a 40mm grenade launcher on the XM8 armament subsystem. In addition, an M60D 7.62mm machine gun could be mounted in the rear starboard (right) door opening. The OH-6A replaced the Korean era OH-13 Sioux and OH-23 Raven light observation helicopters. The Hughes (model 500M) international military version was sold in ten countries and built under license in Italy and Japan. The Cayuse had a single articulated four-bladed main rotor and a metal two-bladed tail rotor. The OH-6A had a cruising speed of 144 mph (125 knots). Two special operations versions of the OH-6A are the Hughes (model 500C) AH-6 "Little Bird" armed variant, and the MH-6 transport/utility version. These advanced versions feature a quieter five-bladed main rotor, minimal-cavitation tail rotor, and a distinctive T-shaped tail.

Pics

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This is the OH-6A #65-12962, which served with the 3-17 Spurs in Vietnam.  In 1970 it was crewed by C.E. Gary Swartz and had over 2,000 hours logged when it departed A-Troop, 3/17 Air Cav in 1970.

962 at Cu Chi - 1970

Swartz's "962" is now on display outside the main entrance to the U.S. Army Aviation Museum ( Fort Rucker, AL) in it's original 1/4 Cav markings:

962 at Ft. Rucker
Photo courtesy of Ft. Rucker

It's power-to-weight ratio and four-blade rotor system made the Cayuse highly maneuverable. By 1970 the U.S. Army had received 1,434 military versions of the Hughes 500. The Cayuse had a max takeoff weight of 2,700 pounds.

An Allison T-63-A-5A turboshaft engine was de-rated to produce 285 shaft horsepower. This basically same engine was used in the later OH-58A Kiowa and produced a minimum of 317 shaft horsepower (T-63-A-700). 

 Drawbacks with the OH-6A was the thin skin of their rotor blades (flying in heavy rains would destroy the leading edge of the blades which wasn't very practical in Vietnam) and the circuit breaker panel mounted on the pedestal which was always being damaged by dirt. The OH-6A had mechanical flight controls while the OH-58A featured hydraulic flight controls.

Both the OH-6A and OH-58A's (which started replacing the reliable OH-6A in 1970) could be armed with a 7.62 mm mini-gun mounted on the left side from the passenger compartment. The pilot was seated in the front right and the Observer in the front left seat. In another common configuration the mini-gun was removed and the Crew chief often flew in the rear to provide firepower and a pair of eyes to the right side of the Loach.

Ceramic armor offered some protection from the bottom, rear and sides for the Observer and pilot. "Chicken plates" and a well-positioned .38 or .45 offered some protection from the front.