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Structure
Composite materials, such as
fiberglass and carbon fiber, are
easily molded into complex and
aerodynamic shapes. They can
create very strong structures
tolerant to wide variations in
temperature and requiring little
maintenance. Today, the use of
composites has replaced that of
aluminum in many areas and is
found in all areas of aviation,
from airliners to Light Sport
Aircraft.
The use of composites, though,
introduces different (not
necessarily more complex)
structural analysis and testing
methods that are often not well
understood.
There are two primary purposes
for testing aircraft structures:
the first is to validate
computer models and confirm that
the aircraft meets structural
strength objectives (discussed
above); the second, more
sophisticated objective is to
simulate the effects of
long-term use on the airplane
structure and develop a “useful
life.”
Depending on the materials used
this takes one of two
directions: aluminum airplanes
emphasize fatigue testing (metal
structures typically develop
cracks after extended use); the
composite counterpart is
damage-tolerance testing
(composites don’t develop
fatigue cracks but can have
hidden damage).
In both cases major aircraft
assemblies are cycled through
simulated takeoffs, cruise,
turbulence, landings (some good,
some not so good!) and then
taken to destruction and shown
to be strong enough to be
“certified” even after this
simulated lifetime. Premature
failures can be dealt with by
regular stru
ctural inspections (if the parts
involved can be replaced).All
this structural testing, both
for strength and damage
tolerance, on a Cirrus is
performed with major components
built from sub-standard parts.
This may not be intuitive but
the idea is to test the
structure with all parts being
at the minimum end of the
acceptable range in
manufacturing - and then damaged
further to allow for field
incidents not visible during a
pre-flight inspection. You can
be confident that an average
Cirrus airplane comes from the
factory with much better
components than the items we use
for our testing.
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