Cozy MKIV - Chapter 22

Installing the Electrical System

Start Date: August 16, 1997

The first thing anyone doing an aircraft electrical system should do is buy the book "The AeroElectric Connection" written by Bob Nuckolls. It runs about $35 and has excellent descriptions of all the aspects of aircraft electrical systems.

After reading this, I spent about 15 hours over the course of a couple of weeks drawing circuit diagrams of the Master Bus, Essential Bus, gear/canopy/throttle alarm system, and battery/engine/starter/ground wiring. I tried to combine the electrical system as described in the plans with the general info in "The AeroElectric Connection". I made a list of all the parts I would need from Radio Shack, Digikey, Nuckolls himself, Wicks Aircraft and B&C Industries (information provided in "The AeroElectric Connection").

First, I made a shielded wire harness for the 4-place stereo intercom system. I chose to mount the jacks for the headsets and microphones in the shoulder support between the headrests (for the front) and in the vertical pillar between the rear seats for the rear passengers. This meant that the harness would have a length of approximately ten feet. I routed it inside the shoulder support and down the front seatback to the right side electrical conduit. After fixing one short in a microphone cable and using a motorcycle battery to power the electrical system rather than the battery charger I had used, I got the intercom to work perfectly.

transponder antenna I next made a ground plane from 0.032" aluminum cut into an octagon (easier than making a circle) approximately 12 inches across. I bent up four of the edges to increase the stiffness, drilled a hole in the center for the antennae I bought from Aircraft Spruce, and then created some tabs to hold the antennae in place in the nose of the fuselage in front of the pilot side rudder pedals. I created a tower and a tab on the center nose gear cover, as well as one on the fuselage side and on the fuselage top just under the large door. The tabs on the fuselage sides contain the antenna ground plane on the top and bottom, while a short sheet metal screw holds the antenna onto the tower tab. As it turned out, I SHOULD have made the antennae 5.63 inches in diameter, so I cut it down to this size and then remounted it in the one tab visible here.

Instrument Panel Layout I layed out my instrument panel in my CAD system at work, and then plotted it out. After a couple of iterations of this, ensuring that all the existing holes (cabin air vents) and features lined up with the plotted template, I taped the template to the instrument panel and began cutting holes with 3 1/8" and 2 1/4" hole saws. You can see the holes for the eight main instruments, as well as a couple of auxiliary ones in the side panel just above the left armrest as well as in the center console above the roll trim lever.

The template shows the layout for the center radio stack, as well as for the analog engine instruments on the right side.

switch plate
switch holes in Instrument Panel
I made a switch/light mounting plate for the top center of the instrument panel from 1/16" aluminum. I laid out the necessary switches, lights and breakers on this panel and drilled or cut out holes as necessary for mounting them, as well as holes for mounting the plate to the instrument panel. I then cut holes in the top of the instrument panel to clear all the components sticking through the back side of the switch panel.

I've also mounted the key/mag/start switch in the center of this switch panel, as well as an ON-OFF switch for the Navaid autopilot.

back of switch panel Here you can see the back side of the switch plate with all switches installed and most of the wiring run.

master, essential, and ground busses under the copilot seat This picture shows the master, essential, and ground busses under the copilot seat. I glassed a 1/4" plywood sheet in place to which to screw the busses. I bought the 14 slot master and essential fuse busses from J.C. Whitney, and the ground buss (all Fast-On tab busses) from B&C.

accessories I mounted the warning relay on a small sheet metal bracket I bent up from 1/16" aluminum just under the longeron behind the instrument panel on the co-pilot's side. I then floxed and glassed small 1/4" thick plywood strips to the fuselage side to which to screw the relay bracket, the dimmer circuit and master/essential buss diode bridge heatsink, as well as the warning circuit piezo buzzer (dual tone, like the backup horn on trucks). To orient yourself, you can see the cutout for the elevator torque tube and F-22 near the top left of the picture.

I ran wires back through the conduits for the strobe and navigation lights, as well as for the magnetos, fuel pump, and primer.

ch22_back.jpg (13376 bytes)Here you can see the wiring over the main spar in the rear of the aircraft, as well as the strobe power supply in the spar cavity.  I have positioned a Concorde RG25 battery pretty much dead center, and built a glass surround for it.  A 1/16" aluminum strap bolted to the firewall ensures that the battery cannot move.  Two rubber feet on the firewall and on the rear of the strap front ensure that the battery will not vibrate against anything solid.  The rear ground bus attaches to the firewall to the left of the battery, while the battery solenoid and ammeter shunt attach to the firewall to the right of the battery.  I used #4 cable from the battery to the ground and master busses in the front and rear, and #2 cable to the solenoid and continuing on to the starter connections. Two holes through the firewall allow power and ground cables to connect to any engine connection without having to route down through the electrical conduits.  I will seal these holes (as with all firewall holes) with high temperature RTV Silicone.

Due to weight and balance considerations, I had to move the battery to the nose.   I left the tray in the rear so that it could be mounted in either place, and ran #2 cable up to the nose for the positive line, and connected the battery ground to the front ground bus.

End Date: July 28, 2002


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Copyright 1997 - 2002, All Rights Reserved, Marc J. Zeitlin
e-mail: marc_zeitlin@alum.mit.edu