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Good point on the anodizing Tracy; also a .005” layer of
silicone isn’t conductive either, even with bare aluminum – conduction
is made through the bolts and not the entire mating surface of the plate. The math
checks out too. I’m still curious about their specific setups though; I
can see how a stack of aluminum/iron/aluminum, etc with a layer of hylomar in
between each stack might not be the best conductor – the sub-mount plate
has to help.
In my setup the battery is behind the cabin for weight and
balance considerations. Ground is made to a steel tube frame with about 1 foot
of #1 welding cable; Positive runs through Master relay under the floor to
firewall with about 6 feet of same #1 welding cable, then another 2.5 feet to
starter. The engine ground is via stainless braid from front iron to engine mount
hardware at the firewall/airframe. So if the starter current path is from post
to post on the battery, then in my case it has to be close to 16 feet! I’ve
never had any starting problems, but it would be interesting to see if an extra
ground strap from starter bracket back to the airframe makes the starter spin
faster or engine fire sooner … will test that and post results.
Jeff
From: Rotary motors in
aircraft [mailto:flyrotary@lancaironline.net] On Behalf Of Tracy Crook
Sent: Tuesday, November 24, 2009 9:43 AM
To: Rotary motors in aircraft
Subject: [FlyRotary] Re: No start
I doubt anodizing is a
factor. Keep in mind that the starter we use is a light duty one but its
rating is 1.2 KW. With a typical cranking voltage of 10 volts,
that's 120 amps. With that kind of current, the only logical routing of
the ground cable is as directly to the load as possible.
Do the math. At that current level, it only takes about .008 ohms
to drop your working voltage a full 1 volt. That's 10% of your cranking
power lost. Rotaries are especially sensitive to cranking speed because
they lose more compression than piston engines at low speed. Low
compression makes starting more difficult. Losses in this circuit must be
minimized for many reasons. Also note that the ground connection
point that many have said they are using goes very near the crank sensor and
its wiring. This proximity makes inductive coupling into the CAS circuit
more likely.
Add all these factors up and it is not surprising that many builders have
starting problems.
Tracy
On Tue, Nov 24, 2009 at 8:52 AM, Jeff Whaley <jwhaley@datacast.com> wrote:
I’m curious about the engine mount
style you guys are using with this voltage drop situation.
I can’t imagine this problem
occurring with the bed-mount/aluminum plate between the oil pan and engine
block configuration.
Surely a 3/8” or ½” plate of
aluminum and 20 bolts will conduct from housing to housing? - unless
maybe it was anodized?
Jeff
From: Rotary
motors in aircraft [mailto:flyrotary@lancaironline.net] On Behalf Of Mark
Steitle
Sent: Monday, November 23, 2009 4:41 PM
To: Rotary motors in aircraft
Subject: [FlyRotary] Re: No start
Glad
you got things figured out (we were running out
of suggestions). Sure gives one a good feeling after battling a
problem for so long. BTW, I have two grounds on my engine. One is
to the front cover, one to the front side housing. I'll check into
moving one up to the starter.
On
Mon, Nov 23, 2009 at 3:33 PM, Steve Brooks <cozy4pilot@gmail.com>
wrote:
Al
& Mark,
I would say that many people may have their ground to the engine block. Mine
has been that way since I built the plane, and worked fine for three or four
years before I started having an issue.
I had to order a couple of new crimp connectors, so that I can make a longer
ground cable. Hopefully they will arrive before Thanksgiving, so that I can get
it all hooked up over the long weekend.
Steve Brooks
Al Gietzen wrote:
Steve,
I read your other post on what you discovered regarding your bad ground. I
wonder how many of us have our engine grounds tied to a bolt near the front
cover or end housing? (Mine does...)
Mark S.
Mine does as well – bolts to the front cover. I have never had a starting
issue, but I have followed this discussion with interest because a couple of
years ago I did a starting spark check and found the sparks considerably weaker
than the mode 8 (or whatever) check. I just assumed it was due to the voltage
drawdown when the starter was running. Now it’s worth another look.
I recall giving it some thought when doing the wiring (My EC/EM manuals
apparently predated Tracy recommended wiring diagram). I concluded that the
various things bolted to the engine, like the manifolds, would provide good
conduction paths regardless of the sealing between layers – and the 20B
has a couple more layers. Both my intake and exhaust flanges are continuous and
connect all the layers, and there are other things making the connection to the
front cover, so this would be less of an issue compared to an installation
having separate flanges to each housing.
Anyway, Steve; I’m glad you found the problem. And thanks; it’s a
help to the rotary community, and again proves the value of a list such as
this.
Al G
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