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Appendix J GREATER AIRCRAFT FUEL EFFICIENCY BY TEAMING GRAPHITE-EPOXY AND TITANIUM The OPEC oil price escalation that started in 1973 initiated a sequence of events that has resulted in greater use of titanium in both air engines and airframes because: I. High fuel cost requires higher fuel efficiency. 2. Higher fuel efficiency requires lower fly weight. 3. Lower fly weight requires higher strength-weight. 4. Graph~te-epoxy gives the highest strength to weight airframe; however, it is too notch-sensitive to be used for frames around openings or for attachments to massive structures like landing gear, wing-to-fuselage, and engine mounts. Only steel, aluminum, or titanium are candidates for such transition structures. Titanium is the preferred choice because of its higher strength to weight ratio, its corrosion resistance, and its compatible expansion coefficient and electromotive force potential with graphite. The commercial aircraft of the 1970s and 1980s, therefore, have featured increasing proportions of titanium, even with graphite-epoxy yet to be used in airframe production. The airframes of the l990s may feature graphite-epoxy with so much titanium in opening frames and massive attachments that airframe titanium use may double. Such developments will increase the demand for titanium alloy strip over sheet. Changes are forced by economics even though prior systems are entirely adequate technically. Thus, OPEC started the above inexorable economic sequence. As a result, during the late 1970s it proved cost-effective to replace steel with titanium at costs of up to several hundred dollars per pound of flying weight saved. The first reaction to graphite-epoxy's higher strength to weight might be to consider it a lethal blow to titanium airframe usage. There are two reasons of controlling importance why, in fact, the opposite is true. Both reasons become operative because graphite-epoxy has high strength-to-weight properties in tension and compression but not in shear 205
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206 (hence the gibe, "Drill a hole in it and it falls apart "' ~ . to this problem is to frame the holes in graphite-epoxy structures with metal both tough and strong in tension, compression, and shear. Titanium is the choice over steel because of its higher strength to weight and The solution corrosion resistance. 1. 2. tt tantum i s also the choice over aluminum because: The high electromotive force between graphite and aluminum rapidly corrodes the latter. The voltage generated between titanium and graphite is close to zero and galvanic corrosion accordingly is nil. The coef f icient of expansion of aluminum ~ s much greater than that of titanium, which approximates that of graphite. The resulting thermal stresses with aluminum require that titanium be used. As a result, the 4 to 7 percent titanium use in today's predominantly aluminum commercial airframe (operating weight, empty, without engines) may well double in the next decade' s graphite-epoxy airframe--which, it must be emphasized, will be substantially lighter than today' s comparable plane. In absolute numbers, however, the weight of titanium per plane will increase .
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