nuckolls.bob(at)aeroelect
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 Posted: Sun Sep 05, 2010 10:18 am Post subject: IFR in with a JAB3300 + SD8 |
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At 01:38 AM 9/5/2010, you wrote:
Thanks Bob
It would only be occasional penetration of cloud for getting in or
out not planning to cruise in IMC. My load analysis is cruise about
15 amps but on approach in Imc with lights off (all leds) and pitot
on load is about 17 so not much margin with the J3300 alternator (I'm
estimating about 6 amps for a dual screen Dynon skyview system ) Just
don't want a discharge light on half way through an approach !
Maybe its overkill to go dual Alternators ?
Not 'overkill' assuming that the alternators are
evaluated for performance while paralleled with
each other. What we're discussing is risk reduction.
If you're crafting a system where intended use
demands high order of reliability for BOTH alternators,
it's not clear to me that the Jab 3300 paralleled
with an SD8 is a good choice.
Pitot heat is not a particularly useful feature
on a small airplane. If it's your intent to spend
some time in the clouds at temperatures conductive
to ice accumulation, then having a pitot heater
is "whistling in the dark".
Forgive me an anecdotal example: Late in my career
as a student, I was wanting to go do some
'real' IFR. One weekend found me at ICT unable
to go flying by myself . . . but there was a real
cool, 2000 foot layer that looked like a good
practice opportunity.
After some badgering, my instructor relented.
I planed and filed an IFR to Hutchinson some 40
miles N.E. of Wichita. We punched through out
of Wichita but the Hutch controller put me down into
the clouds for crossing the VOR fix several miles
S.W. of Hutch. I eagerly dived in, kept all the
needles mostly centered to the VOR, turned out
to the marker, crossed the marker, captured the
LOC outbound and did the procedure turn. Was really
proud of myself for keeping all the needles centered
until I broke out.
Grinning ear to ear I finished the approach
and just as I started the flare to put wheels
on the ground, the airplane fell out of sky and
plopped down pretty hard.
I looked over at the instructor who was smiling.
"Taxi in he says, and let's talk".
When we got parked, we pile out of the airplane
and he showed me what he had been observing while
I was busy grinning at the gages. During our 12
or so minutes in the clouds, a thin strip of
VERY hard ice was building on the stagnation
point of the wing's leading edge. Nothing
serious from the standpoint of weight or
drag but significant in terms of flying qualities.
The airplane didn't have pitot heat so he
had been watching for signs of IAS indication
to degrade. He was of the opinion that it
would also have offered a good teaching moment.
He admonished me that I COULD have requested
and would have been granted permission to do
all the maneuvers to final above the clouds.
All that preliminary stuff is just to get you
on the beams at ANY appropriate altitude. He
also made a good point that just because
you don't see stuff on the windshield doesn't
mean it isn't on the prop and leading edges.
The ice was so hard that I had to put the airplane
into a hangar and warm it up. Further, since
I reported ice to ground control, I had
essentially closed the airport for my departure
until at least several more flights arrived behind
me without reporting ice.
That's ONE way to turn a 2 hour adventure into
a 4 hour study in aviation realities.
I'm thinking that your dual alternator installation
is just fine where the SD-8 is the occasional
backup for the main alternator. Your main
alternator is rated for 22A intermittent (which
you could probably boost to continuous duty
with some well considered cooling). If you can
give up the pitot heater, then you MAY be
home free.
Also, go find out how much the the built in alternator
and the SD-8 will do for output at RPMs appropriate to
approach configurations. Don't guess, go get the numbers.
Then modify plan-A, plan-B contingencies to match.
Having said all that, it's a really good idea
to go do practice approaches in the same configuration
you plan to use for real approaches.
Bob . . .
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