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Aviation History
1987
1987 - 0156.PDF
Boeing Vertol, which has some of the most sophisticated windtunnel facilities of any helicopter manufacturer, spent many hours refining the Model 360 fuselage and reduced its drag to half that of the CH-46. To reduce drag the tricycle landing gear is retractable, and all fuel for the 200 n.m. design mission radius is carried internally. The afterbody, with its upswept tail and rear loading ramp, was carefully designed to avoid airflow separation and download, and a 400hr windtunnel effort resulted in an almost 60 per cent reduction in after body drag. The rotor hubs are even greater sources of drag, however. While the drag of a four- blade hub is inherently higher than that of a three-blade unit, the low-profile composite hub used on the 360 represents a 20 per cent reduction in drag over an equivalent four-blade metal design. The fuselage and hub refinements together reduce overall drag by 35 per cent over that of the CH-46. While high speed was one of the Model 360's design objectives, another was to maximise the use of weight-saving composites. As a result Boeing Vertol has achieved an overall weight reduction of around 15 per cent, and an airframe weight saving in excess of 25 per cent. The aircraft is also much simpler. The composite hubs have 60 per cent fewer parts, and the all-composite airframe 83 per cent fewer parts and 93 per cent fewer fasteners. The airframe is remarkably simple. Carbonfibre frames are located only where major loads enter the fuselage, and between these frames are large composite skin panels, up to 6ft-square in size. These panels have damage-tolerant Kevlar skins sandwiching a Nomex honeycomb core which imparts stiffness, removing the need for stringers. Cutouts are reinforced with carbonfibre. Frame flanges and panel edges are thickened to accept widely spaced bolts which carry the 3g design load. To carry the 4 • 5g ultimate load the joins are also bonded, the bolts applying pressure during room-temperature curing of the bonds. Any single frame can be battle-damaged and the helicopter will still take the full 3g design load, says Boeing Vertol. The structure carrying the rotor lift loads, the three cargo hooks, and the undercarriage is also carbonfibre. The fore and aft hooks can each carry 18,0001b, while the centre hook has a 25,0001b capacity. The landing gear itself is also composite, for a 10 per cent weight saving, using a mixture of glassfibre and carbon- fibre for stiffness, fail safety, and impact resistance. Although it lacks the strength of carbon fibre, glassfibre has better failure charac teristics and continues to bear a load even after failing. By combining both materials a blend of strength and "soft" failure can be achieved, and this mixed-modulus approach, first used on rotor blades, has been extended to much of the Model 360's dynamic system. The design philosophy is fail-safe, rather than safe-life, says Boeing Vertol, and the emphasis is on damage resistance and damage tolerance. The composite blades themselves are similar in construction to those developed for the CH-46 and 47, with a unidirectional spar carrying bending loads and a cross-ply skin carrying torsion loads, and are 10 per cent lighter than metal blades. To enable the Model 360 to reach higher speeds high-modulus carbon fibre is used in the blade skins to increase torsional stiffness. Initially glassfibre, the rotor hub is now also mixed-modulus, carbonfibre having been added to give the required stiffness. The weight-saving over a metal hub is around 25 per cent. The high cruise speed dictated a fully articulated hub, says Boeing Vertol, but lubrication-free elas- tomeric bearings and dampers are used, Carbonfibre fuselage frames were installed around a single central fixture. The large, lightweight, self-stiffened skin panels were bolted, and then bonded to these frames "" 'iW*^ FLIGHT INTERNATIONAL, 18 April 1987
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