Generators / Alternators Off for Start?

[Colonel]

Just watched a video, Juan Browne was starting his twin with the generators on ...

It's always been a habit of mine, to start with the generators / inverters / alternators
(whatever) off. I figure I don't want any drag from them (trying to make power) when
I'm trying to spin the engine up, and I don't want any voltage surges from them, which
may damage electrical equipment.

I like to wait after start, and the RPM has settled down, before I power up the charging
circuits .. with the avionics/buses disconnected.

Is this just me?

The hotrod L29 I flew had a dual-purpose generator/starter. You had to ensure that you
manually selected it's mode from starter to generator after start, otherwise the excessive
RPM would destroy it. IIRC it had two sets of gears: a "low" for start which would allow
it to spin the motor up, and a "high" after start. Any Hawker drivers?

[Nark]

I saw that when he started the 310. Didn’t think much of it.

Now you have me thinking.

My 180 has a battery master, and a separate Gen switch. (Not the typical Cessna split switch) I always leave the Gen switch on… regulate power via battery master, as appropriate.

I’ve replaced the Gen about 2 years ago.

The 182’s electrical situation was abysmal… all of the panel lights were running in series through 2 rheostats… and off one CB… WTF? I fixed that as soon as I got it. Also the strobes were tied in to the position lights…. But get this… also had a dedicated switch on the panel for strobes that was connected to a cut wire, formally the belly beacon. Whoever wired the plane before me should be shot.

So now, much like the 180, but with an alternator, I use straight battery power and turn off the avionics master for start and shutdown, as appropriate.


Maybe I’ll change my ways with the 180, and turn off the Gen.. ? I’ll report back.

[Colonel]

Not all mechanics are electrical wizards (ahem)

I'm always on the lookout on ways to get better starts, and make it easier
on the equipment. Not sure this really matters much - I suspect you can
find POH's on little airplanes that do it both ways.

[cgzro]

Off for me. The energized alternator is about 1hp I guess to turn over so its an extra unnecessary drag on the starter and a tiny drain on the battery so a double whammy.

[digits]

I also like to keep them off.

People think it's weird. Then I ask them why they switch it on during start. "It's a habit". Great response!

While I was figuring out how to start a 310 properly, I actually did start with them on a couple of times. I only used the starter for a few seconds, but it was often followed by 20 seconds of farting and rotating while I was over/under priming it to get it working. Figured that those rotations would give me more power than I would have lost during the first seconds of the start procedure.

Then again, how much power would an alternator really generate at 400 ish rpm. Likely useless anyway.

[Colonel]

The Pitts S-2B manual specifies alternator field off while cranking, and
alternator field on after start, so I'll probably keep doing that.

I remember the 172's I used to instruct on (shudder) had a split master
switch, but I don't recall anyone ever splitting it for a normal start, with the
master on and the alt field off ... everyone starts a 172 with the alternator
energized ie both sides of the red master up. And there are more 172's
out there, and they've performed more starts that I can possibly imagine,
so it can't be that bad.

There are really two issues caused by cranking with the charging system enabled:

1) undesirable drag created by the (enabled) charging system during crank
which is going to slow down your engine start attempt, and

2) voltage spikes during cranking / failed starts, perhaps causing damage
to equipment which is turned on during cranking. Would voltage spikes
damage the master and starter relays? I doubt it - they're pretty simple.
Some people have the strobes on with the master, before cranking - voltage
spikes might shorten their electronics (or not) depending upon how paranoid
the EE was that designed them.

Not sure this really matters. All I know for sure is that ideally all electronic
equipment is disconnected until after the start, and the voltage settles down
after the charging system stabilizes.

NB This is what my voltage looks like, using the POH procedure. You can
see the master on (battery voltage), then cranking, and then the ALT field is
manually turned on after successful start:



Note that my very expensive engine monitor - I have spent less on cars than
I have on a single engine monitor (and I obviously have more than one) - is
directly connected to the master - as we can see above, it's obviously running
during cranking - and I really don't want to see how well it handles voltage
spikes during cranking, with the charging system enabled.

So, I will probably keep on doing what the POH says:

ALT FIELD off during cranking - running on battery power only, until the RPM
has settled down, then ALT FIELD on.

Note the strong initial voltage. Every battery I have, has a Battery Tender
put on it, as soon as I shut it down. Cars, boats, motorcycles, airplanes,
they're all the same.

I really don't know why people like weak batteries, failed starts, increased
wear on their overheated starter motors, risking an engine fire at start,
frequent expensive battery replacement, and batteries cracking and peeing
acid on the bottom of their fuselage in winter, and weak batteries failing in
flight, maybe at night, in cloud.

$30 at Amazon. Gee, that's a lot to pay, to avoid the above.

-- EDIT --

Correction. $25 at Walmart.



I don't get it. Why are people such cheap fucks? This is incredible bang for the
buck. Ever watch a Jay Leno YouTube video of his garage at the Burbank airport?
Every one of his 200-ish cars and 100-ish motorcycles with a battery, has a cord
running to it. Are both Jay Leno and I morons?

In most cases, a 12 volt lead-acid battery, at 100% SOC, will have a rest voltage between 12.8 and 13.1 volts. That means an effective float voltage need only be as high as 12.9 to 13.2 volts. However, most Battery Tender® battery chargers have float voltages between 13.3 and 13.5 volts. The important thing is that the float voltage should be higher than the fully charged rest state battery voltage and it should be lower than the gassing voltage which is about 13.8 volts.

This is like Camguard, which I pay less than $25/pint from Spruce in Corona. Owner
logic is to save the $25 at the oil change, and replace the $25,000 engine instead.

I really don't get it.

This is not something new. My father used to carry around a spare quill shaft and
a socket to change it, when he flew the T-33 and they had to use a hard start.

The squadron commander had to give him one of these ...

[anofly]

My little twin comanche has a set of plane power alternators. You start it with the alternators off, that way it does two things, you get to check the alternator idiot lamp comes on, indicating not working , and goes off , after you switch the field on after engine start, then for the other engine, you can start with the first engine alternator online to "help" with the second start. You still get to check the idiot lamp on the second alternator is on, and goes out after second engine is running and you turn on the field of the second alternator.

Not sure if the 310 has generators... but those put out no electrical power till the engine hits about 1000 rpm. I used to have to taxi the generator equipped plane I flew with brakes on and engine rpm increased to get enough voltage to make the new solid state radio to work. they did not work with lower voltage and were designed for alternator equipped planes.

[Slick Goodlin]

This thread is making me realize all my time is spent these days on turbines that figure their own crap out or airplanes without electrical systems.

I’m cool with that.

[Colonel]

I am unsure that an electrical system (battery, cables, starter, alternator, regulator, circuit breakers)
is always a good choice for a light aircraft, if it can be hand-propped and is flown day-only.

You can save an awful lot of money and weight without that stuff. Do you really need more than a
portable intercom, a couple headsets/PTT, and a portable vhf comm with an external antenna?

I've flown several elderly aircraft configured like that, and they worked pretty well as trainers.

For nav, just use your phone. I've flown across the entire continent using my phone for nav
(admittedly plumbed with external power). I think I loaded the Garmin software and maps and
databases onto it? The most important thing is to plumb your music from your phone, either via
cable to the ICS or BT to headset (solo, no ICS).

Just drive across the country with the comm turned off, skirting the class B and listening to your
tunes. There are much worse ways to spend a day. Turn the comm back on when you're 15 miles
or so from your destination. Get the ATIS/AWOS and lurk on CTAF/TWR, call 10 miles out inbound.

Yes, you can fly an airplane like that.

[Colonel]

generators... but those put out no electrical power till the engine hits about 1000 rpm

That's a very good point. Alternators can put out the amps at low RPM, but generators can't.

If you look at this from a thermodynamic closed systems standpoint ....

Cranking with the charging system energized will put X amps into the starter motor
of which Y amps is lost to powering the charging system, where Y is some fraction of X.

Now, of that Y amps, some of it will be lost to the field losses of the starter motor,
the friction of the mechanism which transfers rotational power to the charging
system, and finally the field losses of the charging system.

So if we put X amps into the starter motor, some fraction Y amps is lost to powering
the charging system, but the charging system will generate some fraction (perhaps Y/2
amps RMS) back into the electrical bus. As pointed out above, this fraction is dependent
upon the technology of the charging system.

Likely the losses aren't that bad. Again, I worry more about transient voltage spikes
damaging electrical equipment which is powered on during cranking.