nuckolls.bob(at)aeroelect
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 Posted: Thu Jul 14, 2016 11:03 am Post subject: Speaking of 'drain holes' . . . |
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While I was working at Cessna, back in the dark ages,
the single engine cowls were modified to mount onto
brackets riveted to the firewall. These brackets were
fitted with rubber shock mounts with Dzuse sockets.
The idea was to 'float' the cowl off the fuselage structure
for reduction of engine noise in the cabin.
[img]cid:.0[/img]
Prior to the floating cowl, a contiguous cowl mounting
flange extended forward off the firewall.
After the floating cowl upgrade, fielded aircraft began
experiencing contactor failures due to moisture ingress.
It seems that an airplane getting a cooling rain
on a warm day experienced water splash on cowl mounted
equipment.
A whisky barrel contactor's temperature was dropped
by the cool water, internal pressure went down
and liquid water was literally sucked inside. The
same water had to get back out through successive
atmospheric 'breathing' which could take days of
cycles. In the mean time, the 100% humidity
inside the contactor wreaked havoc with the
devices innards.
The first attempt to 'fix' the problem involved
manual application of epoxy around all the obvious
ingress gaps in the enclosure. Not sure what the
effectiveness of this fix was but it was not total . . .
Any of the tiniest holes still provided a moisture
entry point while further restricting egress through
atmospheric breathing.
Had a similar problem I wrestled with on an amateur
radio repeater on the 1200' platform at KTVH, Hutchinson,
KS. A brand new Motorola Micor repeater would go off
the air every time it rained and stayed off for hours
until it dried out. We set out to 'seal' the cabinet
but attempts to close it only made things worse . . .
the more we 'sealed' the longer it took to dry out.
Recalling my observations years earlier at Cessna,
we opened the cabinet up. Installed baffled ducts for
air inlet and mounted fans to maintain a good
positive airflow.
The repeater might still go off the air during
a really big frog strangler . . . but would be
back on minutes after the rain ceased.
Moral of the story: Unless your enclosure is
HERMETICALLY sealed, then any attempts improve
moisture control with goo, glue, gaskets, etc may
prove ineffectual if not antagonistic.
In the case of the whisky barrel contactor 'drain'
hole, I suggest it is more of a VENT hole.
It prevents atmospheric cooling inside from reducing
pressure and drawing the moisture inside in the
first place. That contactor mounted vent hole up
was a contactor waiting to be trashed. Mounted
vent hole down, it would probably have died
of old age having ingested NO liquid water over
its lifetime . . . and if it did . . . it would
not have risen high enough to wet the innards.
It would have been readily ejected during subsequent
atmospheric breathings.
Bob . . .
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