Of all the tasks and skills
required in building an airplane, wiring is one of the
things I like to do most — I've been looking forward to it the entire
I've also been researching materials and techniques for some time
now. In preparation I read Bob Nuckoll's book, the AeroElectric
Connection, and attended his weekend seminar in Eugene, Oregon. I highly
recommend this looseleaf book. Bob has a way of taking the fundamentals
and relaying them in ways that are easily understandable. You can find out
more about Bob as well as learn much about aircraft electrical systems
from his web site at www.aeroelectric.com. Along with the pictures below
I'll try to incorporate comments on some of the methods and techniques I've
After I took my fuselage out of the jig and flipped it over I began
to think about my battery installation since it was only a few steps down the
road. I began hearing from builders who were
flying their RV-8s with the same engine/prop combos I will be installing (O-360
A1A, Hartzell constant speed). All of them had gone with the standard firewall
battery location. They were reporting that they were
operating at the front edge of the CG envelope once they weighed everything. This was resulting in almost
running out of elevator in the landing flare. Hmm.
Now a forward CG loading is not altogether bad. It allows you to put a
significant "bubba" in the back seat, and/or to load up the rear
baggage compartment and still stay with the CG envelope. However when doing solo
aerobatics you would be doing so at the fwd edge of the CG envelope and would
experience higher stick pressures.
Van's specifies two possible battery locations: the firewall, and behind the
rear baggage bulkhead. They specifically recommend the rear location when
installing the heavier IO-360, but provide no recommendations otherwise. So
which battery location should I go with?
Many months earlier I had decided that I didn't like that vertical portion at
the right end of the forward baggage compartment. I was going to install a flat
floor all the way across as I had seen a couple of other builder's do on the web
sites. Then an idea struck... why not put the battery in the bottom of that
compartment? The benefits:
- Keeps battery out of hot firewall-forward environment —
improves battery life.
- Reduces clutter on firewall —
allows better organization of components.
- Requires no additional firewall wire penetrations —
less chance of leaks, shorts, or weakness.
- Moves it rearward for a small aft shift in CG
— helps with the normally fwd CG bias.
I have not calculated the actual CG shift this will cause, but I know
it will be in the right direction thus mitigating some of the characteristics
mentioned above. I have been told that it will be a SMALL change, but I get all
of the other benefits listed above so I have opted for this installation method.
Then I began thinking about a way to mount the battery and make the change
possible. See below for the results.
(click on a pic to see a larger version)
It didn't seem like good engineering to just plop the battery on the
F-822 floor skin. After all, a 22 lb battery becomes an 88 lb battery in a
4g maneuver! I wanted to find a way to have the load carried by the
firewall angles in the front and the F-802G gear box angle in the rear. I
ended up starting with the standard battery tray and then designing an
undercarriage that both spanned the two structural angles, but also tied
into the floor skin.
This is how it looks with the battery installed. The positive terminal
is to the left (aft) and will connect to the main contactor relay which
will be mounted on the gear tower. The negative cable will penetrate the
firewall and go directly to the starter. A braided cable will then go from
the engine case back to a thru-firewall stud which connects to a ground
block on the back of the firewall.
Here is my bracket shown upside down so you can see how it also ties
into the floor. Those 1/8" shims are riveted on and allow an AN3 bolt
to bolt to platenuts that were installed in the F-822 skin BEFORE the
bottom skins were installed. Sorry quickbuilders, this wouldn't be
possible for you. You could use Rivnuts as an alternative though.
The battery contactor has been
mounted mere inches away from the + terminal of the battery on the fwd
side of the right gear tower. Note that this is just a few inches away
from the main buses on the panel and in the lighting console... short
what the aft end of the battery compartment looks like most finished...
...and from above when looking down.
Most builders dread the prospect of
wiring up everything in their projects. For some reason I'm the opposite,
I was actually looking forward to it. Just like designing your panel,
laying out your FWF components, or designing your paint scheme, wiring
your plane is a reflection of your own individuality.
First off, my overall wiring
schematic is a combination of the concepts explained in Bob Nuckoll's book
and traditional 40 year old aircraft wiring concepts as manifest in the
Van's supplied diagram. Literally, my system is a hybrid of the two taking
what I think is the best of both. Specifically, The way Bob recommends
routing the alternator output, and using single point ground the system
make complete sense. I've had a bit of experience with car stereo years
ago and learned the hard way that ground loops are something to be
avoided, and single point ground is the way to do it.
If time allows, and I can get
familiar with an appropriate CAD package, I'd like to diagram out my
system. Until then it exists in my mind quite clearly. In the mean time, I
have listed below some of my personal notes from the Nuckoll's seminar
that I've incorporated into my system:
- Use single
point ground throughout, use Nuckolls tab system on firewall
- Solder all
battery & bus wire terminals, all others crimped only
wire: p-leads and headset/mic cables only
- Use RG400
instead of RG58 for all antennas
output (B lead) to starter contactor rather than bus
- Use 6 awg for
alternator output to starter contactor
- Use 8 awg
battery contactor to volt/amp meter
- Use 10 awg
from volt/amp meter to main bus input & lighting buses (both
from output post of meter)
- Use 14 awg
for aux outlets
- Use 14 awg
for landing lights
- Wire Hobbs
from hot side of Master switch with inline fuse
Oh, regarding what coax cable to
use, here's a quote from Bob Nuckolls: "I would
recommend RG-400 as the modern coaxial cable of choice. It uses
silver-plated double shielding and modern insulations like tefzel. RG-58
is an obsolete specification and no longer controlled for quality. It uses
polyethylene and PVC for insulation . . . no longer allowed in
(click on a pic to see a larger
Yea, I know, it sure looks
like a rats nest at this point. But there is a method to all this.
If you think this looks bad just wait, it gets worse.
I'm a bit fanatic for
keeping all wiring runs nice and neat. Basically I start at the
of each line, make the connections, do the labeling,
and then pull the wire toward the source until it has exactly the
slack, or "service loop", that I want and the wire runs
are all tight with no loops. I do that with all
wires on a branch and then put 3" sections of appropriately
sized spiral wrap on to keep things tidy. Major wire bundles are
then secured with adel clamps and #8 screws. Most of my interior
panels will be secured with stainless steel #8 hex head screws and
washers - a bit better looking than the standard steel variety.
Strictly cosmetic, but hey, I have to look at them all the time.
Work in progress! this is the fwd
side of the battery compartment showing the single point ground system
from AeroElectric Connection. Note the short ground cable, maybe 4",
from the battery.
The main ground stud penetrates the firewall, the other side is connected
to the ground strap which is connected to the engine case.
here's the end result almost finished.
(wing connector pic & text here)
(click on a pic to see a larger
Here's a rather poor
picture of my lighting console. It has the breaker/switches tied
together with a copper bus bar the same way I've done on the bottom
edge of my panel. You can see that on the Panel
page. BTW, the reason there's
a gap in the switches is an ergonomic consideration. If anyone read
Ed Wischmeyer's excellent article on panel design in Kitplanes,
October '99 issue, he points out that switches should be put in
groups of no more than four so that you can remember what they are
without seeing them. Also note the post light at the top of the
console, and another one on the gear tower for the headset and music
tied things back a little tidier than depicted here before I mounted it, but it's essentially
done. The bundle continuing back included: ELT wires, pax phones/mic/ent.,
right rear cockpit lights, and rear aux. power jack.
The finished lighting switch panel