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Chapter One APPLICATIONS Navy Fighter Aircraft (F-18A) The wing skins of the F-18A represented the first widespread use of graphite/epoxy in a production aircraft. The skins vary in thickness up to one inch, serving as primary as well as secondary load carrying members. It is interesting to note that the graphite skins are separated from the aluminum framing with a fiberglass barrier to prevent galvanic corrosion. The carrier- based environment that Navy aircraft are subjected to has presented unique problems to the aerospace designer. Corrosion from salt water surroundings is exacerbated by the sulfur emission from the ship’s exhaust stacks. Osprey Tilt-Rotor (V-22) The tilt-rotor V-22 is also a weight sensitive craft that is currently being developed by Boeing and Bell Helicopter. Up to 40% of the airframe consists of composites, mostly AS-4 and IM-6 graphite fibers in 3501-6 epoxy (both from Hercules). New uses of composites are being exploited on this vehicle, such as shafting and thick, heavily loaded components. Consequently, higher design strain values are being utilized. Helicopters Rotors Composite materials have been used for helicopter rotors for some time now and have gained virtually 100% acceptance as the material of choice. The use of fibrous composites offers improvements in helicopter rotors due to improved aerodynamic geometry, improved aerodynamic tuning, good damage tolerance and potential low cost. Anisotrophic strength properties are very desirable for the long, narrow foils. Additionally, a cored structure has the provision to incorporate the required balance weight at the leading edge. The favorable structural properties of the mostly fiberglass foils allow for increased lift and speed. Fatigue characteristics of the composite blade are considerably better than their aluminum counterparts with the aluminum failing near 40,000 cycles and the composite blade exceeding 500,000 cycleswithoutfailure. Vibratorystraininthissametestingprogramw a5s10minch inch for aluminum and 2400m inch inch for the composite. TM Sikorsky Aircraft of United Aircraft Corporation has proposed a Cross Beam Rotor (XB,R) which is a simplified, lightweight system that makes extensive use of composites. The low torsional stiffness of a unidirectional composite spar allows pitch change motion to be accommodated by elastic deformation, whereas sufficient bending stiffness prevents areoelastic instability. Figure 1-52 shows a configuration for a twin beam composite blade used with this system. Structure and Components The extreme vibratory environment that helicopters operate in makes composites look attractive for other elements. In an experimental program that Boeing undertook, 11,000 metal parts were replaced by 1,500 composite ones, thus eliminating 90% of the vehicle’s fasteners. Producibility and maintenance considerations improved along with overall structural reliability. 60PDF Image | Marine Componsites
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