nuckolls.bob(at)aeroelect
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 Posted: Sat Oct 19, 2013 9:43 am Post subject: Sizing the wires |
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A question:
In your "sharp pencil work", you used 200 amps as a load for
cranking..I'm astonished that the load could be so high...even on a
cold morning. At the moment, I'm seeking the rating of my starter
motor, and have been anticipating to use that rating to determine
the max. current flow in order to size the wire to it.
Is that not the way to do it?
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In the best of all worlds, you bet. But like the
quest for OPERATING numbers for electro-whizzies
on your engine, the manufacturers of those devices
are generally ignorant of operating conditions.
The 'ratings' on the box or data sheets usually
speak to limits. "Operate my gizmo at or below these
numbers and you may expect it to perform as I have
otherwise detailed in this data sheet . . . and do
it for a long time."
So a rating on your starter motor will probably
speak to some torque-RPM curve at some voltage . . .
or perhaps a family of voltages. It will also
set a limit on temperature rise. The wizard
Charles Kettering first demonstrated how you
can flog a 1 hp motor to 5 hp of output . . .
as long as you don't expect it perform for
more than a few seconds with plenty of time
to cool off between cycles.
Someplace in that array of torque-rpm vs. voltage
curves exists an envelope of demands that your
engine may place on the motor. The system
integrator that selected the motor looked at
the data, purchasing looked at the price
and between them decided to try it. It "did
the job" but it's almost a given that nobody
plotted the full family of starter demands based
on battery condition from new to soggy, plugs
from new to perhaps past recommended life, temperature,
mixtures and fuel flows all over the charts,
oil, etc. etc.
To be sure, if you plot likelihood of extremes
for those combinations, you'll get a kind of
bell shaped curve for severity of demand where
the vast majority of starts are well inside the
motor's design limits and the engine starts in
one or two blades. But the point to be made here
is that there is little or no connection between
'ratings' on your starter motor and how the
system is going to perform.
I've got a high speed data acquisition system
that I'm planning to apply to the fleet of
vehicles in my drive to get some energy demands
in real life situations. Maybe I can get that
set up before the next -10F overnight temps
hit this winter.
Given that we have no "real" numbers for your
airplane, the next best data for doing the
sharp pencil work is to assume a worst case.
Yeah, that number should be astonishing
and it's probably for come condition that
lies out at the worst case limits on the bell
curve. If we could PROVE that those limits
are un-realistically large, cool. But we
have to move forward with what we have and
strive to refine it as we go.
I've often theorized about the possibility of
doing a "Watt-Second Study" on a constellation
of flying hardware. Maybe at a series of fly-ins.
Of course, those always tend to be fair-weather
events. I'd still need to get data somewhat
displaced from the center of the bell curve.
It would be nice to have such data to relieve
the task of sizing components or predicting
performance with numbers no better than throwing
darts at a data sheet.
Having an engine that is artfully managed with
electronics is a BIG plus for a battery.
Artful software automatically accounts for many
variables in the cranking mode. Yeah, all those
byte thrashers may give you a bit better fuel
consumption but it also offers an opportunity
for getting 5+ years out of a battery.
A Beech Skipper I used to rent was pretty easy
to figure out the combination of pre-flight
prop swing, throttle setting and seconds of
primer needed to light it off very quickly.
But I recall more than one neophyte pilot flogging
that airplane's battery and starter to near
exhaustion/destruction because they had not
acquired that man-machine connection that makes
life so much simpler.
So add another variable to the constellation
of influences on starter performance . . . pilot
skills.
Bob . . .
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