7 watts seems low. Does your engine use a high pressure
pump? I would
have guessed at least 3A, or about 40 watts..
More about electrically dependent engines (mostly gleaned from Bob
Nuckolls' aeroelectric list):
You want fault tolerance in the electrical system - the failure
of any
single component shouldn't cause the system to crap out. There are
lots
of ways to accomplish this. Most design ideas have already
been thought
of. No reason to invent new ones because the existing ones are
quite
good..
Fault tolerance may imply a certain amount of redundancy in the
system
(for components that have a relatively high failure rate).
As I said before, you want enough electrical capacity to maintain
powered flight until the fuel tanks are empty. There WILL be
flights
where having to land short of your intended destination is not
a viable
option.
My opinion is that to have enough electrical endurance will
mandate a
second alternator of some sort. Here's why: When you do the load
analysis of an electrically dependent engine, you will come up
with
something on the order of 5-8 Amps of current demand. A typical
aircraft battery when new will have a nominal capacity of 20Ah -
measured at low current demand - typically something like 10%
of the
capacity in Ah - maybe at 2A on a 20Ah battery. At 5A draw,
you won't
get 20Ah from the battery. Maybe you'll get 15Ah.
On a brand new, fully charged battery, 15Ah at 5A is 3 hours of run
time. Add a difficult start, some idle time on the ground, and
some
other flight loads like radios and lights, and you may be lucky
to get
1.5hours of run time from the battery. That's not enough in my
book,
and certainly won't allow you to use up all of your fuel.
So, you may say that you'll just add another battery... There are
downsides. Obviously, that directly adds significant
weight. It also
is an item that must be periodically maintained/replaced. In
not very
much time, the cost of swapping batteries will catch up with
the cost of
adding a second alternator. You might say that you won't fly
enough
hours to warrant this, but batteries wear out on the calendar
as much as
they do from flight hours. And even with the battery, the
airplane has
a finite limit of endurance that isn't driven by how much fuel is
available.
Another thing to consider is adding engine support systems that
are able
to operate independent of a DC power source. Gravity
feed/engine driven
fuel pump, and a magneto are the classics, and hard to beat in
terms of
reliability. Each can coexist with its modern replacement..
Back to your regularly scheduled program... :)
Matt-
I followed this same logic, but came to a different conclusion. In my
case, I asked myself what would make my primary alternator stop
generating current. There were two obvious scenarios that lept to mind
-- either the alternator itself died, or the engine is not spinning the
alternator. If the alternator died, then the only critical element is
to be aware that it is offline. I can then plan a no-stress landing at
a nearby airport. If the engine is not spinning the alternator ( i.e.,
the engine has conked out) then having a second alternator makes no
difference whatsoever -- it is also not generating power & thus I am
back to the batteries. In this case, I am immediately aware there is a
problem (gee, it's suddenly very quiet up here) and endurance on the
electrons is only needed for however long it takes gravity to return me
to the ground.
So - based on that thinking, I decided the dual-battery arrangement was
better than the dual alternator arrangement. As per the AEC, I jettison
one battery every annual. Thus I am guaranteed one "new" battery that
is storing close to spec'd AH. At $35 per battery, I consider it cheap
insurance compared to the $250 per battery I was paying for my old
Cessna. I never used cost of the second alternator in eliminating the
dual alternator arrangement - just the logic above. At 36 AH (18 AH +
18AH) I will have more electrons than fuel in any case. My current draw
on the endurance bus is around 6 Amps as you suggest above -- so even if
the second battery is down by half (9 AH) I still have 27AH to get
me to
the ground. If I chew through that and am somehow still aloft, then I
have very badly neglected my responsibilities as PIC.
I might have decided the above differently if I was building a
long-distance platform where getting there on-time & on-schedule was
part of the mission requirements. In that case, I could easily see
redundancy of alternators in case one craps out or a belt breaks. But
in my case (as a fair-weather flier with all the time in the world to
get from here to there) that wasn't a driver.
My 2-cents, YMMV, and all the rest.
Mark Supinski
I agree with your analysis. I just decided that I don't want battery
capacity to be a factor when deciding where I need to land. I have
fantasies of someday doing a fair amount of IFR and maybe some overwater
stuff. To each there own.
Regards,
Matt-
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