Tag Archives: Light Observation Helicopter

27 February 1963

First flight of the Hughes YOH-6A prototype, N9696F, 27 February 1963. (Hughes Tool Company)
First flight of the Hughes YOH-6A prototype, N9696F, 27 February 1963. (Hughes Tool Company)
Raleigh Ellsworth Fletcher

27 February 1963: Hughes Tool Company, Aircraft Division, test pilots Raleigh Fletcher and James A. Vittitoe made the first flight of the prototype Model 369, serial number 13-0002. The helicopter received F.A.A. registration N9696F on 5 April 1963.

The Hughes Model 369 was built in response to a U.S. Army requirement for a Light Observation Helicopter (“L.O.H.”). It was designated YOH-6A, and the first aircraft received U.S. Army serial number 62-4211. It competed with prototypes from Bell Helicopter Company (YOH-4) and Fairchild-Hiller (YOH-5). All three aircraft were powered by a lightweight Allison Engine Company turboshaft engine. The YOH-6A won the three-way competition and was ordered into production as the OH-6A Cayuse. It was nicknamed “Loach,” an acronym of the initials, “L O H.”

Initially, the prototype had a tail boom with an airfoil-shaped cross section. Though this performed well in forward flight, it limited sideways flight to just 5 miles per hour. When replaced with a symmetrical cross section tail boom, sideways flight increased to 60 miles per hour (97 kilometers per hour).

The first prototype Hughes YOH-6A, N9696F, in final configuration. (Hughes Tool Company, Aircraft Division)

The YOH-6A was a two-place light helicopter, flown by a single pilot. It had a four-bladed, articulated main rotor which turned counter-clockwise, as seen from above. (The advancing blade is on the helicopter’s right.) Stacks of thin stainless steel “straps” fastened the rotor blades to the mast and also allowed for flapping and feathering. Hydraulic dampers controlled lead-lag. Originally, there were blade cuffs around the main rotor blade roots in an attempt to reduce aerodynamic drag, but these were soon discarded. A two-bladed semi-rigid tail rotor was mounted on the left side of the tail boom. Seen from the left, the tail-rotor rotates counter-clockwise. (The advancing blade is on top.)

Overhead photograph of a Hughes YOH-6. Note the blade cuffs. (U.S. Army)
Interesting overhead photograph of a Hughes YOH-6. Note the blade cuffs. (U.S. Army)

The YOH-6A was powered by a T63-A-5 turboshaft engine (Allison Model 250-C10) mounted behind the cabin at a 45° angle. The engine was rated at 212 shaft horsepower at 52,142 r.p.m. (102% N1) and 693 °C. (1,279 °F.) turbine outlet temperature for maximum continuous power, and 250 shaft horsepower at 738 °C. (1,360 °F.), 5-minute limit, for takeoff. Production OH-6A helicopters used the slightly more powerful T63-A-5A (250-C10A) engine. The T63-A-5A is a 2-spool, reverse-flow, turboshaft engine with a 6-stage axial-flow, 1-stage centrifugal-flow compressor and 4-stage axial-flow turbine. 2 turbine stages (gas generator) drive the compressor section, while the other 2 stages (power turbine) drive the engine’s output shaft through a gear reduction section.

Hughes YOH-6A 62-4211 in its configuration during the three-way LOH competitive testing. (U.S. Army)
Hughes YOH-6A 62-4211 in its configuration during the three-way LOH competitive testing. (U.S. Army)
The Boeing AH-6 "Little Bird" (formerly mcDonnell Douglas helicopter Company) is a direct development of the Hughes Model 369.
The Boeing AH-6 (formerly McDonnell Douglas Helicopter Company) is a direct development of the Hughes Model 369.

© 2017, Bryan R. Swopes

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8 December 1962

Bell YOH-4 N73999 (U.S. Army serial number 62-4202. (U.S. Army)
Bell Model 206 N73999 (U.S. Army YOH-4-BF 62-4202) at the Bell Helicopter Company plant, Hurst, Texas. (Bell Helicopter Co.)
Bell YHO-4-BF. (U.S. Army)
Bell YHO-4. (Bell Helicopter Co.)

8 December 1962: At the Bell Helicopter Company plant at Hurst, Texas, the first Model D-250, N73999 (YHO-4-BF 62-4202) made its first flight.

The United States military had requested proposals from 25 aircraft manufacturers for a Light Observation Helicopter (LOH) to be powered by a gas turbine engine. Eventually, helicopters proposed by three companies were selected for flight testing. These were the Bell YHO-4, the Fairchild Hiller FH-1100, designated as YHO-5, and the Hughes Aircraft Company Model 369, designated YHO-6.

In 1962, U.S. military aircraft designations were standardized between services, and the three helicopters were redesignated YOH-4, YOH-5 and YOH-6. Bell Helicopter had also changed its internal company designation for their proposal from D-250 to Model 206. All three were powered by an Allison T63-A-5 turboshaft engine rated at 250 shaft horsepower (Allison 250-C18).

Bell 206 N73999 (YHO-4-BF-62-4202). (U.S. Army)
Bell 206 N73999 (YHO-4-BF 62-4202). Note the stabilizer bar. (U.S. Army)
A prototype Bell YOH-4 Light Observation Helicopter hovers in ground effect. The vertical fin has been changed from the original ventral configuration. (U.S. Army)

After the fly-off, the Hughes OH-6A Cayuse was selected for production. With the LOH classification, the OH-6 earned the nickname “Loach.”  Modern variants of the OH-6, now the AH-6 and MH-6 “Little Bird,” remain in service with United States special operations forces.

Bell Helicopter tried to market their Model 206 as a light civil aircraft, but its utilitarian appearance made it a hard sell. The helicopter was redesigned as the Model 206A and given the name JetRanger. This became one of the most successful aircraft ever built and it remained in production until 2011.

The first Bell 206A JetRanger, N8560F. (Bell Helicopter Co.)
The first Bell 206A JetRanger, N8560F. (Bell Helicopter Co.)

As the Vietnam War escalated, the need for helicopters increased. Hughes Aircraft had limited production capacity so the U.S. Army ordered a version of the redesigned Bell YOH-4 as the OH-58A Kiowa (Bell Model 206A-1). Though similar in appearance to the civil Bell 206A JetRanger, the OH-58A has significant differences and few parts are interchangeable between models. The Kiowa’s main rotor blades and tail boom are longer than the JetRanger’s. The rotor system turns at a slower r.p.m. Landing skids are mounted differently. The OH-58A has a lower maximum gross weight. There are internal differences as well, for example, the main transmission of the OH-58A has only three planetary gears while the 206B uses four, giving it a greater torque capacity.

The OH-58 Kiowa was continuously upgraded to the OH-58D Kiowa Warrior, with advanced targeting and communications capabilities. The D model uses a composite four-bladed “soft-in-plane” main rotor. Military variants of the civil Bell 206B-3 JetRanger III have been used as training helicopters for the U.S. Navy (TH-57 Sea Ranger) and U.S. Army (TH-67 Creek). The U.S. Army has now retired all of its OH-58s. The final flight of an OH-58D Kiowa Warrior took place in September 2017.

The first production Bell OH-58A-BF Kiowa, 68-16687. (U.S. Army)
The first production Bell OH-58A-BF Kiowa, 68-16687. (Bell Helicopter Co.)

The YOH-4A prototype is in storage at the U.S. Army Aviation Museum, Fort Rucker, Alabama. Because of an error is assigning serial numbers, this aircraft carries a manufacturer’s data plate with the military serial number 62-4201,¹ however, the correct serial number, 62-4202, is painted on the airframe exterior.

A flight of Bell OH-58D Kiowa Warriors scouting in a war zone. (U.S. Army)
A flight of Bell OH-58D Kiowa Warriors scouting in a war zone. (U.S. Army)

¹ Serial number 62-4201 had already been assigned to a Lockheed C-140B-LM JetStar.

© 2017, Bryan R. Swopes

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