Today's Message Index:
----------------------
1. 03:20 PM - Re: Load Analysis Feedback Please... (Jeff Page)
Message 1
| INDEX | Back to Main INDEX |
| NEXT | Skip to NEXT Message |
| LIST | Reply to LIST Regarding this Message |
| SENDER | Reply to SENDER Regarding this Message |
|
| Subject: | Re: Load Analysis Feedback Please... |
Matt,
I am also using the Z13-8 architecture as the basis for my design.
From a load analysis perspective, the interesting value is the longer
term average load. This tells you whether the backup alternator (eg.
SD-20 on the vacuum pad) is sufficient to carry the reduced load for
the remainder of the flight, or to predict the discharge time of the
battery with that load. So realistic current consumption is of
interest, and is often much less than brief surge currents, such as
when you activate the transmitter to talk with ATC. If necessary, try
to measure your devices, if the specifications provide only maximum
ratings.
If you have a backup alternator, then the battery discharge time is of
less concern, since it will not be discharged during the remainder of
the flight and should have lots of juice for gear or landing lights
when you arrive at the destination. If you don't plan a backup
alternator, then selecting a battery and a test/maintenance schedule
for it is important, so when the main alternator dies, you can switch
to endurance loads only and start your stopwatch, knowing how long you
have electrical functions before landing.
If you rely on your EFIS for flight instruments, and perhaps
electronic ignition, those can be powered by the standby alternator if
you have one. If not, the battery sizing, test and maintenance
schedule become critically important. Since you have many electrical
goodies, I am assuming you are planning on a 20A standby alternator.
So My load analysis was based on what was on which bus, and did not go
into detail for phases of flight. With everything working, the
alternator can meet the load in any phase of flight. I consider the
failure of the main alternator an emergency situation if I am flying
IFR. I have enough equipment on the endurance bus to finish the
flight, although in hard IMC I would elect to land at a much earlier
time, since the pilot workload is higher with less equipment (eg.
autopilot). A low voltage indicator will warn me if the standby
alternator is not keeping up. You can look at my load analysis here:
http://www.qenesis.com/tundra/Electrical/Load_Analysis.pdf
Sizing wiring and fuses is done differently. Average loads indicate
how much heat might be created in the wires during use. Wires are
sized to be well below melting the insulation. For a surge current,
such as running a flap motor, it won't be run long enough to be
expected to overheat a wire (expect perhaps in the event of a fault).
However, the resistance of the wire factors into it as well, since
sufficient size the wire is necessary to ensure that sufficient
voltage arrives at the motor. The wasted drop along the wire is based
on the current and the resistance of the wire.
You can't change the current draw (expect perhaps by purchasing a more
efficient motor), but you can reduce the resistance of the wire by
changing to a thicker one. This is discussed in The Aero-Electric
Connection chapter 8.
Rather than recalculate for every circuit, I created a little table
that listed the currents for 5 degree temperature rise (for continuous
loads) and 10 degree rise (for intermittent loads), and what the
maximum wire lengths are for that size wire for a 5% voltage drop.
This is the chart I used:
http://www.qenesis.com/tundra/Electrical/WireSizes.pdf
I looked at the current in each circuit and what the length of the
wire was expected to be and selected the size from my chart. Most of
the wires were going very short distances, so size was related only to
temperature for the current. In a few cases (HID lights at the ends
of the wing and hydraulic gear pump) the size of the wire was related
to the voltage drop instead.
Once the appropriate wire size has been selected, then selecting the
appropriate rating for the circuit protection is straightforward,
since it is based on the size of the wire that is being protected (not
the load). People worry about fuses blowing at inappropriate times.
However, with sufficient size wire, the fuse rating can be easily high
enough that this is very unlikely to be a problem, even using
conservative currents to produce 10 degree rise temperatures. I used
the current for a 35 degree rise to select my fuses.
Jeff
> Time: 11:30:59 AM PST US
> Subject: AeroElectric-List: Re: Load Analysis Feedback Please...
> From: "idleup" <matt@mattandmel.com>
>
>
> Jeff,
>
> I did end up moving the lighting off the battery bus and onto the
> endurance bus.
> I also moved the landing light and flaps to the main bus as per your
> suggestions.
> I was thinking in an electrical emergency at night I would need the landing
> light and flaps, but you are correct that those really are not needed during
> the "endurance" phase of flight, just the landing. Hopefully there would not
> be a problem that turning back on the master switch would aggrevate when I do
> need them.
>
> The main reason I used the maximum currents is because I was trying
> to plan for
> worst case scenarios. As per Peter's recommendation I really should have two
> sheets (or multiple columns) so one set of loads can be used to
> determine battery
> endurance and the other for fuse and wire sizing. I have not seen
> this however
> in any of the templates I used for the load analysis so I did not do it that
> way.
>
> Also, my architecture is using Bobs Z-13/8.
>
> Thanks,
>
> Matt
>
>
> Tundra10 wrote:
>> Matt,
>>
>> You definitely put a lot of work into this :-)
>>
>> You have used maximum currents, although in many cases, actual runtime
>> currents will be much lower. The radios are a good example. The
>> SL-40 is about 0.4A during receive. The Garmin 430 is about 2A for
>> receive + nav. The audio panel probably uses less than 2A too.
>>
>> You decide what really needs to be on the endurance buss, but I would
>> suggest moving the flap motor and landing light to the main buss.
>> Once you have successfully arrived at the airport, you can turn the
>> main buss back on and use the battery reserve to lower the flaps and
>> maybe use the landing light (or just land without it).
>>
>> I would put the lighting controller and compass light on the endurance
>> buss. You are less likely to leave them on accidentally draining the
>> battery. More importantly, if you have an electrical fire, you want
>> to be able to disconnect everything (except the ignition) by turning
>> off the master switch.
>>
>> Using more typical currents, your endurance buss will be below 18A,
>> allowing extended flight with a 20A backup alternator. You didn't
>> mention what your architecture is.
>>
>> Jeff Page
>> Dream Aircraft Tundra #10
>>
>> > From: "idleup"
>> >
>> > Would you guys be so kind as to review my Load Analysis worksheet
>> > and tell me if
>> > you see anything that looks incorrect or have recommendations to
>> > make it better?
>> > I have spent quite a bit of time on this but at the end of the day still
>> > question whether I know what I am doing and would appreciate some
>> > peer review...
>> > Thanks.
>> >
>> >
>> > Of if you would prefer to download the PDF:
>> > http://www.mattandmel.com/rv/Master_Load_Analysis_v1.pdf
>> >
>> > Thank you much.
>> >
>> > Matt
Other Matronics Email List Services
These Email List Services are sponsored solely by Matronics and through the generous Contributions of its members.
-- Please support this service by making your Contribution today! --
|