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AeroElectric-List Digest: 24 Msgs - 08/23/18

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PostPosted: Sun Aug 26, 2018 11:50 am    Post subject: AeroElectric-List Digest: 24 Msgs - 08/23/18 Reply with quote

At 11:54 AM 8/26/2018, you wrote:
>> The Dynamo will not charge while taxiing and if you have any load at all your battery could be down when you try to leave.

Which begs the question . . . by how much? Successful
adoption of the e-bus philosophy requires a study
of the ENERGY required to complete each phase of flight.
This task is no different than the days before alternators.
Generators produced very little or no useful output
at taxi/idle speeds. Starting an engine should take
no more than a couple percent of the battery's capacity.
So after start up and pre-flighting all the electro-whizzies,
one could drop to the e-bus for taxi out and hold
for take off.

Most of my experience was off small, uncontrolled fields
where time from start up to take-off roll was on the
order of 5 minutes or less. So yes, just how badly will
be depleted before full engine driven power is available?

All these things are known or easily discovered. It
does require due diligence to run the numbers . . .
but like weight and balance, it's rarely calculated
again once the common configuration has been shown
to meet design goals.

If starting the engine depletes the engine so much that the battery is crippled by it . . .

It doesn't. Cranking the engine has some
high POWER demands but the starter is
on of the lowest energy consumers on the
airplane. Back before LEDs, a 3 hour flight
with three navigation lights illuminated at
2A each was 14v x 6a x 3hr x 3600 sec/hr
yields about 910,000 watt-seconds. The starter
is 12v x 200a x 10sec for a paultry 24000 watt-

An e-bus load of say 6a x 12v x 10min x 60 sec/min
= 43200 watt-seconds.

The battery sized and maintained to carry a 6a e-bus
for 2 hours stores 12v x 6a x 2hr x 3600 sec/hr =
518400 watt-seconds.

So if you burn up 24000 + 43200 = 67200 watt-seconds
getting airborne . . . you have only demanded
67200/518400 = 0.13 or 13% of your battery's total

This is purely a hypothetical to illustrate the
data needed to crunch your own numbers. Bottom line
is that if that 1954 TriPacer in which I took dual
instruction could do it, there is no reason you cannot craft
a system and operating philosophy that lets you do
as well or better in a Rotax powered Kitfox.

If your low voltage light or other charge indicator doesn't show the charger working during run-up are you going to take off anyway? If the indicator shows the charger has gone south on climb out, didn't you plan for enough battery to carry you to the destination anyway? Well, now the destination should be the one directly below. If the plan was for the battery to carry the electron pushing load for hours, why would the new "fly the few minutes of the pattern and land now" plan cause any concern even with the slight depletion due to starting?

Is the concern better spent elsewhere?

Agreed . . .

Bob . . .

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