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Tundra10
Joined: 14 Jun 2010
Posts: 102
Location: Scarborough, Ontario
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 Posted: Sun Feb 05, 2012 9:20 am Post subject: Z11 with rear battery in a Cub |
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A friend of mine is building a Smith Cub and has just decided he wants
to put the battery in the rear due to weight and balance considerations.
Wiring architecture is Z11. This is what I think he has to do. Please
correct any misunderstandings I have !
Main contactor at the rear as close as possible to the battery.
This feeds a heavy conductor to the firewall.
This conductor cannot be practically circuit protected, so should be
contained in a conduit.
This cable is connected to the main bus at the firewall, and continues
from there to the starter contactor.
The main bus has the usual stuff, plus an amphib gear hydralic pump.
A 4AWG wire does not easily connect to the Bussman fuse panel.
Perhaps a junction at the firewall where the starter cable goes
through and a 10AWG wire to the main bus which can now be more
conveniently located under the panel ?
Concern: this is a lot of wire running around without circuit
protection. An ANL would probably be useful at the firewall junction
if we take a 10AWG to the main bus fuseblock, but the main feed from
the back to the starter must be installed with good physical
protection. Turning off the master switch before a forced landing is
extremely important.
The battery bus as near as practical to the battery. This has a
connection for the single Lightspeed ignition (other is a mag) and an
always hot cigarette lighter socket on the instrument panel, fed with
a 5A fuse.
An endurance bus relay also fed from the battery bus and located there.
I expect that running starter currents through the aircraft frame is
not a good idea. So run a second heavy feed wire up to the firewall
where a forest of tabs will be, including a strap to the engine for
the starter current.
This works out to two fat wires, one 12AWG, two 18AWG and two 22AWG
wires running from the back to the front. Those should all run inside
the same conduit. Should anything be done to prevent them chaffing
against each other, or the conduit ?
It is probably about 7 feet from the firewall to the battery, which
needs to start an Aero Sport Power O-375. Will 4AWG be sufficient ?
Or too much resistance, and he should upgrade to 2AWG ?
Anything else I should have thought of ?
Jeff Page
Dream Aircraft Tundra #10
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nuckolls.bob(at)aeroelect
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| Posted: Sun Feb 05, 2012 10:15 am Post subject: Z11 with rear battery in a Cub |
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At 11:15 AM 2/5/2012, you wrote:
A friend of mine is building a Smith Cub and has just decided he wants
to put the battery in the rear due to weight and balance considerations.
Wiring architecture is Z11. This is what I think he has to do. Please
correct any misunderstandings I have !
Main contactor at the rear as close as possible to the battery.
This feeds a heavy conductor to the firewall.
Yes . . .
This conductor cannot be practically circuit protected, so should be
contained in a conduit.
No . . . fat wires require no special protection.
Support the wire to avoid abrasion due to motion
and keep away from moving parts.
This cable is connected to the main bus at the firewall, and continues
from there to the starter contactor.
I'd take it to the starter contactor. Bring a smaller,
probably 6AWG wire from contactor to the main bus.
The main bus has the usual stuff, plus an amphib gear hydralic pump.
A 4AWG wire does not easily connect to the Bussman fuse panel.
Why not? First it can probably be a 6AWG wire. 4AWG terminals
for #10 stud are easy. Use welding cable to get soft, flexible
and user-friendly routing and installation.
Perhaps a junction at the firewall where the starter cable goes
through and a 10AWG wire to the main bus which can now be more
conveniently located under the panel ?
Use hot side of starter contatctor for junction point.
Concern: this is a lot of wire running around without circuit
protection. An ANL would probably be useful at the firewall junction
if we take a 10AWG to the main bus fuseblock, but the main feed from
the back to the starter must be installed with good physical
protection. Turning off the master switch before a forced landing is
extremely important.
Wires that carry cranking and/or major power distribution
don't need 'protection' beyond making them cold by turning
the master switch off. This is consistent with the FARs which
guide the design and assembly of all part 23 and 25 airplanes.
The battery bus as near as practical to the battery. This has a
connection for the single Lightspeed ignition (other is a mag) and an
always hot cigarette lighter socket on the instrument panel, fed with
a 5A fuse.
This can be a 10A fuse. As we've discussed earlier, the
I(squared)x t constant for a fuse is still much smaller
than the exemplar 5A breaker. Hence, it is no greater
risk for post crash fire ignition than the 5A breaker.
An endurance bus relay also fed from the battery bus and located there.
Yes, since you disconnect adjacent to the battery bus,
that fuse may be any practical size needed.
I expect that running starter currents through the aircraft frame is
not a good idea. So run a second heavy feed wire up to the firewall
where a forest of tabs will be, including a strap to the engine for
the starter current.
That would be ideal.
This works out to two fat wires, one 12AWG, two 18AWG and two 22AWG
wires running from the back to the front. Those should all run inside
the same conduit. Should anything be done to prevent them chaffing
against each other, or the conduit ?
No conduit. Just bundle them up and support on chafe-free
clamps.
It is probably about 7 feet from the firewall to the battery, which
needs to start an Aero Sport Power O-375. Will 4AWG be sufficient ?
Or too much resistance, and he should upgrade to 2AWG ?
Lots of variables here. The delta-weight for 2 versus 4
on 14 feet seems a reasonable 'sacrifice'. But depending
on starter, engine cranking requirements, battery condition,
local ambient temperatures, etc. . . . 4AWG MIGHT be lighter
than the owner would like . . . but he'll have to discover
that for himself.
Bob . . .
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Tundra10
Joined: 14 Jun 2010
Posts: 102
Location: Scarborough, Ontario
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| Posted: Sun Feb 05, 2012 12:36 pm Post subject: Z11 with rear battery in a Cub |
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Thanks for your answers Bob.
It will be easier to install the wires without a conduit.
6AWG direct from the starter contactor to the main bus is a good approach.
Yes, the cigarette lighter could have up to a 10A fuse, but not needed
for probably a 2A load. Wiring will be sufficient to upsize later if
needed.
It is important to able to start a float plane. Changing 4AWG to 2AWG
after the build would be a challenge, so I expect we will install 2AWG
to begin with if there is any possibility of 4AWG being too small.
Thanks !
Jeff Page
Dream Aircraft Tundra #10
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nuckolls.bob(at)aeroelect
Guest
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| Posted: Sun Feb 05, 2012 12:59 pm Post subject: Z11 with rear battery in a Cub |
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| Quote: |
It is important to able to start a float plane. Changing 4AWG to 2AWG
after the build would be a challenge, so I expect we will install 2AWG
to begin with if there is any possibility of 4AWG being too small.
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Sounds like a Plan!
Bob . . .
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