Posted for "Valin & Allyson Thorn"
<thorn@starflight.aero>:
Hello Stan,
I'm not sure how your question about air conditioning external air
vents
became part of the thread for this tragic accident... Anyway, to address
these questions, I agree that everyone
should be very cautious about making
any penetrations in an airframe that weren't part of the structure's
original design and
testing. Without analysis and testing otherwise, any
holes have to be reinforced to at least restore the strength lost from
the
cutout. But strength isn't the only consideration - stiffness is very
important too. At least the same
stiffness has to be preserved or the
structure's natural frequency could change such that it might be vulnerable
to resonance from
various forces the structure sees in flight - which can
lead to structural failure.
I'm a space systems engineer and
project manager -- certainly not a
composite structures specialists. I do feel I have enough familiarity,
though, with
the principles involved to handle a simple structural
reinforcement for relatively small circular holes in a composite
sandwich
structure. For the air conditioning external air vents for our system I
took a conservative, cost effective
approach to their reinforcement to
ensure the strength and stiffness in this area was at least as good as
before the
cutouts. The tradeoff in not optimizing the reinforcement is
that the reinforcement is not as light as it could be - but, it saves
the
expense of conducting a new ground vibration survey to ensure the frequency
of the structure did not go any
lower. Note, the stiffer the structure the
higher its natural frequency and higher structural frequencies are in
the
safer direction for avoiding structural interaction/resonance issues.
The photos you included of your
installation are small so I'm not sure I'm
interpreting them correctly. From what I can see it does not look like
the
hole reinforcement guidelines provided in the installation guide were
followed. The composite sandwich structure in
that area of the Legacy (just
behind the aft wing spar) is what's called "2 core 2 sandwich" - two plys of
carbon, a Nomex core, and
another two plys of carbon. The intended layup
plan bonds the outer and inner plys of the sandwich construction
together,
with multiple plys of carbon cloth, which results in greater strength and
stiffness in this immediate area than without
the holes. From your photos
it looks like the reinforcement is only applied to the inner skin with micro
replacing the
core. This could be a problem.
On the positive side, it will only help the strength and stiffness if
you
decided to bond the lower case of the condenser to the floor. But, if you
did not directly bond the outer and inner
plys of the fuselage sandwich
structure to each other I'm concerned about the resultant strength and
stiffness of the
reinforcement.
I know this is not the best place to work this out and it will be easier to
explain on the
phone. I'll give you a call soon - but, I'm tied up this
Wednesday through Friday in a spacecraft design review that's scheduled
for
at least 10 hours each day - so it might be this weekend...
Thanks,
Valin
Starflight,
Inc.
1145 Timber Lane
Boulder, CO 80304
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