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<< Lancair Builders' Mail List >>
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John Wright wrote:
<< Brent, Would you give me your opinion on the iv-p window blow out
problem? My iv-p is down for painting and now would be the ideal time to
modify anything if I am going to.>>
Hello John:
I agree with Lance. The window problem is not a design problem but an
implementation problem. Proper prior preparation of the Plexiglas panels is
a prerequisite ;) If this is the case then, in my opinion, the 1.5X pressure
test, per part 23 of the FARs, is a good idea.
Take the wing spar example. The factory tested the design to failure then
built all subsequent spars and performed the critical bondings required to
ensure that the wings would have the required strength. Since the same is
not done for the pressure vessel then every kit is a "prototype" and a proof
pressure test is
needed. When I say "needed" I mean "prudent" as I don't know, for a fact,
that it is "required".
If you are going to damage the airplane at 7.5 PSI then damage it, but do it
on the ground. Otherwise I would not regularly operate more than 66% of the
maximum demonstrated pressure. I know of two pressure bulkheads that failed
due to improper fabrication at just over 6.5 PSI. Had they not been on the
way to 7.5 PSI then these defects would never have been found.
Also keep in mind that the "Proof Pressure Test" of 7.5 PSI is not the same
as the "Burst Pressure Test" of 10 PSI. The cabin should be able to
withstand proof pressure without damage (yield) and Burst pressure without
rupture (but possibly some permanent deformations). Testing to 7.5 PSI
without damage demonstrates that the cabin is of comparable strength as the
original factory test cabin. The test cabin was CYCLE TESTED though
thousands of pressure cycles to demonstrate fatigue strength. Failure to
demonstrate to 7.5 PSI without yield means that you are not sure if you have
the required fatigue strength. You could just be flying along one day and
BOOM, something breaks. Sound familiar?
I did not implement the "fix" as it is not a guarantee of safety, as has
been demonstrated, and has the potential of doing more harm than good. I was
very careful during fabrication regarding surface preparation, drilling etc.
N170BR has 600 hours TT, been to 30,000 feet twice, regularly flies at 5.0 -
5.3 PSI AND was pressure tested to 7.5 PSI.
If it has been tested and it works, don't f**k with it.
As I have said before, I am an idiot. Assume everything I say is wrong. Form
your own opinions through research and testing or risk becoming an idiot
yourself..... or worse, a dead idiot.
Regards
Brent
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Please send your photos and drawings to marvkaye@olsusa.com.
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