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OCR for page 205
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
OCR for page 206
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 .
Representative terms from entire chapter:
lower fly
