AIRFRAME DESIGN OF THE S-76 HELICOPTER

The need for a light transport helicopter capable of carrying twelve passengers with a crew of two, over a distance of 400 nautical miles, to support a variety of utility missions including off-shore oil operations, prompted Sikorsky Aircraft to design and develop a helicopter that would economically fulfill this requirement. The design of the S-76 airframe incorporates several unique design features. The fuselage shell is fabricated using sandwich construction where panels of aluminum face sheets and honeycomb core replace the conventional shell of skin stringer construction. In this way, the number of parts and hence manufacturing costs are kept to a minimum. Because of the compound curved surface, the outer face sheet of the panel is stretch formed to contour. The sheet is then chem-milled to reduce the thickness of the middle portion to the minimum structural requirements. The basic sheet thickness is selected to allow for the installation of flush attachments through the edge members. A single stage cure cycle bonds the face sheets to the aluminum core with film adhesive and co-cures the fiberglass edge closeout numbers. Kevlar® fabric is used to great advantage to minimize the weight of secondary structural components such as doors, access panels and fairings. To suit the particular design need, Kevlar material is laid up in a pre-preg laminate or used as the skin covering in a single stage cure cycle honeycomb sandwich panel. Resin systems are selected to minimize property degradation due to heat and moisture. The canopy is fabricated from molded fiber-glass in a two stage cure cycle operation. The skin and skeleton assemblies are cured separately and then bonded together to form the entire canopy structure. Uni-directional "S" glass is used in corner post areas to increase stiffness and strength. The S-76 also features a horizontal stabilator made entirely of advanced composite laminates. The skins are Kevlar /epoxy over Nomex® honeycomb core, with graphite/epoxy spar caps and a Kevlar /epoxy torque tube. The manufacturing process is greatly simplified with reduced parts count resulting in low assembly cost and overall reduction in factory labor hours. The stabilator represents the first primary production component made from advanced composite materials scheduled to be approved by the FAA in the type certification of a transport rotary wing aircraft. It was recognized early by the FAA and the aviation industry that with advanced composites, being new materials which behave differently as compared to traditional metals, new certification guidelines must be developed. To that end, numerous meetings and discussions with the FAA and industry have taken place over the past several months. The proposed guidelines are comprehensive in addressing every phase of advanced composite application to rotary wing aircraft providing a level of safety at least as high as that currently required for metal structure. The certification guidelines for civil composite rotary wing structures establishes the requirements for: (1) Adequate design data base, accounting for environmental effects on the design properties of the material system. (2) The static strength of the composite design demonstrated by full-scale subcomponent or component ultimate load tests with environmental effects accounted for. (3) Fatigue substantiation of safe-life components by full-scale component fatigue tests accounting for the effects of the appropriate environment with demonstration that component strength has not degraded below the design ultimate requirement. (4) Fail-safe components to demonstrate capability of supporting the expected repeated loads, including effects of the environment, during the period in which failure progresses to obvious proportions and prior to detection by inspection. Advanced composites provide substantial payoffs to the airframe and stabilator of the S-76. For the airframe components affected, a 30 percent weight savings was achieved. This paper describes the design features of all the airframe components with specific emphasis on the low weight achievable by using composites and how the specification weight requirements were met while staying within strict design-to-cost targets. It also describes the Certification Program associated with the primary composite stabilator structure.