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rckol
Joined: 14 Nov 2008
Posts: 36
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 Posted: Sun Mar 29, 2009 7:35 am Post subject: SD-8 Components on Firewall? |
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The B&C drawing for the SD-8 regulator (BC219) contains note 3 that states:
"Max ambient temperature 120 degrees F"
and their wiring diagram (504-500) contains note 1 that states:
"Regulator and capacitor should be mounted in a cool place and should be mounted on the cockpit side of the firewall".
This drawing also shows a 15 amp in-line fuse on one of the Dyno output legs, which I assume is intended to protect the circuit extending into the cockpit.
From the grounding locations of the SD-8 components on Z-13/8-Q (firewall) and the absence of the fuse on the dyno output, I infer that the regulator, capacitor, diode and relay are intended to be mounted on the engine side of the firewall with the rest of the DC supply and fat wire switching components.
Are these changes from the manufacture's recommendations based on upgrades to the components, providing blast tube cooling, bench testing, good empirical field results or ?
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nuckolls.bob(at)aeroelect
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| Posted: Sun Mar 29, 2009 11:40 am Post subject: SD-8 Components on Firewall? |
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At 10:35 AM 3/29/2009, you wrote:
| Quote: |
The B&C drawing for the SD-8 regulator (BC219) contains note 3 that states:
"Max ambient temperature 120 degrees F"
and their wiring diagram (504-500) contains note 1 that states:
"Regulator and capacitor should be mounted in a cool place and
should be mounted on the cockpit side of the firewall".
This drawing also shows a 15 amp in-line fuse on one of the Dyno
output legs, which I assume is intended to protect the circuit
extending into the cockpit.
>From the grounding locations of the SD-8 components on Z-13/8-Q
(firewall) and the absence of the fuse on the dyno output, I infer
that the regulator, capacitor, diode and relay are intended to be
mounted on the engine side of the firewall with the rest of the DC
supply and fat wire switching components.
Are these changes from the manufacture's recommendations based on
upgrades to the components, providing blast tube cooling, bench
testing, good empirical field results or ?
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Probably none of the above. There's been a lot of SD-8 hardware
installed on the engine side of the firewall. At the same time,
to my knowledge there has been no detailed thermal study of
SD-8 regulator/rectifier performance. If the 120F value is indeed a
"hard" limit, then I would be reluctant to install this device
in any aircraft. The LOWEST practical operating domain for
TC aircraft calls for continuous operation at 55C (131F) and
a short time operating temperature at 70C(158F). This is called
out for temperature controlled compartments and not exceeding
15,000' pressure altitude.
Out on the firewall of single engine airplanes we are advised
to go with DO-160 Category B3 where the manufacturer tailors
his design to meet airframe design goals. This is generally
70C continuous and 100C short time operating.
If one starts with a clean piece of paper to meet design
goals, it is a rigorous but not difficult task. I'm guessing
but it's a fair bet that B&C (like many other manufacturers)
is recommending the most benign environment possible for
the installation of their products. Unfortunately, the most
benign benign location in the aircraft is probably in the
pilot's lap. Lacking hard design data to offer comfort in
the most practical locations (firewall fwd or behind panel
where all the convection heat gathers), the next best bet
is not to ask for the pilot to hold it in his lap and WAG it.
Now, there are probably places on the forward side of
your firewall that are COOLER than places on the back
side. This is because once airborne, there's a lot of
air movement (which can make 100F air a better cooling
medium than 60F still air). If it were my airplane,
I'd ask B&C to give me a maximum allowable case temperature
for full load operations. Then thermocouple the case
on a firewall forward installation and see how it
goes. Add an offset for maximum anticipated hot day
operations.
The vast majority of the time, I believe you'll find
that you're okay on the firewall. Just for grins,
if anyone has their SD-8 rectifier/regulator installed
on the aft side of the firewall, it would be interesting
to get a thermocouple measurement of free air vs. case
temperature at THAT location too.
If this sounds like a long answer that says, "I don't
know" . . . you're correct. Without specific knowledge
of design goals for the device combined with real data
confirming as-installed temperature conditions, all else is
hot-air and/or wishful-thinking.
Bob . . .
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rckol
Joined: 14 Nov 2008
Posts: 36
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| Posted: Mon Mar 30, 2009 12:19 pm Post subject: Re: SD-8 Components on Firewall? |
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Thanks Bob, interesting food for thought.
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