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).
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.
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 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.
¹ Serial number 62-4201 had already been assigned to a Lockheed C-140B-LM JetStar.
© 2017, Bryan R. Swopesby