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Aviation History
1964
1964 - 2193.PDF
210 FLIGHT International, 6 August 1964 Hugging the ground near its makers' plant, an OH-4A demonstrates that it meets the US Army's requirement of outstanding manoeuvr- ability BELL OH-4A A Turbine-engined Light Helicopter Planned for Maximum Performance and Minimum Maintenance FIRST American-designed, lightweight, turbine-powered heli-copter to be awarded a type certificate by the US FederalAviation Agency is the OH-4A, manufactured by Textron's Bell Helicopter Company. The OH-4A is Bell's entry in the US Army's LOH (light observation helicopter) evaluation programme in which two other firms—Hiller and Hughes—are competing. Each manufacturer has produced five prototypes now being evaluated at Fort Rucker, Alabama, and at Edwards Air Force Base, California. Bell was also the first to build all five test helicopters. Looking in retrospect at Bell's LOH effort (as this journal did at Hiller's on May 7), there are many unique and interesting aspects behind the development of the OH-4A. In 1960 the US Army asked aircraft manufacturers to prepare design proposals for a lightweight, turbine-powered helicopter it wanted in great quantity. It specified a single-rotor, four-place helicopter, capable of carrying a 4001b payload in addition to pilot and fuel at speeds to 1 lOkt. Its mission would be observation, target-acquisition, reconnaissance and command control, tasks now being done with two helicopters and one aircraft. Basic requirements were for a helicopter to be produced at mini- mum cost, and that it possess acceptable mission capability and performance, easy maintainability and high reliability. Powerplant would be the Allison T63 turbine engine flat-rated at 250 s.h.p. Seventeen proposals were submitted by 12 manufacturers in early 1961. Contracts later were awarded to Bell and their two rivals. These were some of the basic concepts behind the OH-4A: minimum size consistent with mission requirements; all-round visibility, with particular emphasis downward, for both crew and passengers; rapidly-convertible passenger/cargo area, with low deck height for easy loading and sliding doors which can remain open in flight for protruding cargo; use of robust metal box construction, with high design crash load-factors to provide energy absorption for crash safety; and quick weapons-kit installation capability, including provisions for isolating the crew compartment. To meet the design objective, conflicting priorities such as aerodynamics, size, weight, cost, configuration and simplicity had to be balanced. Basic component of the fuselage structure is the lower shell assembly, consisting of a lin ahiminium honeycomb sandwich. Outer skin of this sandwich establishes the external contour of the lower half of the forward fuselage, and the unit provides the primary load-carrying structure for the useful load and landing gear. This lower shell assembly is the base for primary controls and other components, and permits complete interchangeability of sub- assembly parts. Sandwich roof panel assemblies, which provide longitudinal stiffness in the upper portion of the aft and forward fuselage areas, also serve as maintenance walkways. Upper and lower shell assemblies are joined by a sturdy bulkhead structure between the forward and rear cabin areas. The simplicity of the OH-4A fuselage minimizes use of double- curvature skins and adapts the design to high-volume production. A rectangular-section beam in the centre of the fuselage turnover bulkhead extends vertically to the roof and serves to route the controls to the rotor and to transmit loads from the engine and transmission directly to the lower airframe. Extensive use of drilling and assembly fixtures, with locators and holding devices, has brought about the highest degree of component interchangeability and accuracy of sub-assembly mating ever achieved at Bell. Basic fuselage shape and structural design of the OH-4A were selected to minimize the frontal area and parasite drag in a con- figuration that offered mission suitability and low manufacturing cost. A full-scale model adapted from the original mock-up was wind-tunnel tested at the NASA Ames Research Center in Cali-
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