Chapter 22 - Electrical System

Start:  09/08/08
Finish:
Total Time: 154 Hours

When we initially laid out our timeline for our Cozy project, the electrical system was one we figured would be least difficult and time consuming...

...Then we actually started working on it.  Having plenty of experience in automotive and home wiring we were fairly certain we'd design a fool-proof electrical system, very simply, over a cup of coffee.  Not so.  After 6 hours of planning and debate, we've only completed the first draft of our power distribution system.  We have managed to nail down a number of specifications and design goals for the electrical system though.  First, the basics:

We'll post our wiring diagrams below, these have been created using ExpressSCH from expresspcb.com.  We will continue to post additional schematics as we get them finished.

This would also be a good place to spell out the avionics package we've decided on.  The center piece of the package is the Grand Rapids Technologies Horizon HX EFIS.  Our system will have dual displays, redundant ADAHRS, WAAS GPS w/RAIM, XM weather, and full engine monitoring.

Panel space is pretty limited in a Cozy, so to back up the EFIS, we've selected space saving components.  A 2-1/4" airspeed indicator, and 2-1/4" altimeter provide basic information in an EFIS out situation.  The backup attitude indicator actually does double duty by serving as the autopilot.  The TruTrak ADI Pilot II, not only functions as a 2-axis autopilot, but the display includes an electric operated attitude indicator, as well as digital directional indicator.

At the 36 Hour mark, we have finally completed what we're calling the first draft of our wiring book.  At this stage we're confident enough in the design to begin ordering components to actually began installing.  The major components like the EFIS itself, radios, intercoms, and AP head end we'll still be holding off on until closer to first flight, but we're going to present the wiring diagrams to the vendors involved and confirm our design.

Next steps before actually pulling wires was to go though and number each circuit, as well as determine proper wire size for each.  We also completed a load analysis to determine proper size alternator, leading us to increase our design requirements to 60 amps.  The increase comes from the addition of a DVD player with secondary screen for rear seat passengers, and a 12VDC power outlet with sufficient capacity to power a laptop.

For batteries we selected two Odyssey PC680 AGM batteries.  They are 680CA (about 300CCA) and 17Ahr each.  They will both power the starter, but after engine start, only the primary will be used.  In the event of a primary battery or main bus failure, essential electronics can be powered from the essential bus tied to the secondary battery.  Both batteries are normally charged through a Surepower Battery Isolator.  Our first thought for location of the batteries was atop the main spar, but that location proved too crowded as we have also planned to mount our electronic ignition modules and strobe driver there.  Our second choice was to mount the batteries under the rear seat bottoms.  We had to cut a channel into the seat bottom to allow the top rear edge of the battery sufficient space, but they fit ok.  We then fabricated a couple of battery trays under the seats to prevent the batteries from moving around.

Next we began mounting the master battery and starter solenoids to the firewall, as well as the power and ground distribution blocks.  We then began installing the starting and charging system wires, along with the battery isolator.

We're utilizing 3 fuse blocks, each with a 14 circuit capacity.  One will be the main bus.  One will be the avionics bus, fed from the main bus through a separate switch.  The last will be the essential bus, which will be fed through a separate switch off each battery using diodes.  We chose to mount the fuse blocks under the co-pilot seat.

Then we began pulling wires for the main bus, the essential bus, and the avionics bus.  We haven't decided yet on how to face the instrument panel, so for the time being we made a switch panel out of 3/16" MDF just to mount the switches on while we're getting all the wiring cut to length.  It's a bit messy in these early stages (as can be seen in the pics), but we'll get it all in order once we're ready to bundle the wires into harnesses.

First to be wired were lights.  We started with the landing and taxi lights, which were already mounted, just needed to be connected.  Next were the position and stobe lights.  This required a little prep work.  We selected position and strobe lights from GS-Air.  They are LED position lights with standard strobe bulbs and a 40/60 watt selectable strobe driver.  Test video of the Position/Strobe lights as well as the HID landing and taxi lights can be seen here.

We were initially going to use some sort of flush mount system with lenses, but these are best mounted on the surface.  To do so we made fairing blocks between the wing tip and winglet root to create a flat mounting surface.  We also needed to find a location for the strobe driver inside the cabin.  The strobe driver is fairly large, so the only suitable location we found was behind the rear starboard passenger seat, next to the battery isolator.

With the external lighting circuits wired, we next turned our attention to the inside the cabin.  The landing gear circuit was first.  We decided to make some changes from our initial plans on this circuit as well.  Most notably we've decided against using the auto-extension module that is available.  We also decided against using the gear actuation switch that was included, which we found to be too small.  We've chosen the 2TL1-1A switch offered by Allied Signal.  It is considerably larger and uses a spring loaded lockout to prevent inadvertent actuation.  It is very similar to the Ray Allen Gear Switch, except that it is 3 position (center off), and is DPDT (which is needed for our landing gear actuator).

Next we needed to begin wiring the avionics.  This meant we needed to finalize and purchase the radio stack.  The NAV/COM and transponder we had decided on a while ago.  We chose the Garmin SL30 and Garmin GTX327.  For the audio panel we considered the PS Engineering PMA9000EX and the Garmin GMA240.  We ultimately chose the Garmin GMA340 however, as it was less expensive than the PMA9000, and had a built in marker receiver that the GMA240 lacked.

We then had to determine the layout.  We used scale cutouts and arranged them on the panel.  We had been trying to determine what type of overlay we wanted on our panel, and in searching for an easy CAD program to layout the panel, we found FrontPanel Express.  With FPExpress we could draw our panel layout, upload it, and have an anodized aluminum panel made.  We used wire frame prints from the software to begin positioning and mounting the avionics while we wait for the panel to be cut.

Mounting the radio stack was one task the plans were completely unhelpful with.  There is absolutely no mention of how to mount any radios.  Obviously mounting will vary with equipment, but there aren't any suggestions at all.  So we improvised a bit.  We glassed some brackets onto the back of the instrument panel, then drilled mounting holes into the side of the brackets to bolt the avionics trays to.

Wiring the avionics is quite a task.  We labeled each wire as we cut it to length, and began crimping the pins and powering up the electronics.  First we checked out the intercom/audio panel.  All functioned well, and we held the first conversations inside the cabin with our headsets.

Next we connected the NAV/COM (including one COM antenna temporarily) and listened to traffic calls at Maple Lake airport (approximately 20 miles south of us).

Our instrument panel overlay arrived, and looked great.  We immediately began fitting it.  When we started drilling holes in the fiberglass IP bulkhead for the switches, we realized that the total depth of the instrument panel, with the overlay, was too deep for the switches.  To accommodate the switches we cut the top part of the IP bulkhead off (just above the stiffener rib), leaving just the overlay at the top.

Next we wired the interior lighting, starting with the fuel sight gauges, the instrument panel lighting, and lastly the map/cabin lighting.  We also installed cable tie mounts and nylon grommets though the firewall bulkhead.

We installed the Aveo air vents instead of the plans Whisperflo vents.  They are smaller, allowing a little more panel space for addition engraving we want (V speeds, etc.).  We also changed out the micro toggle switch provided with the nose lift with an Allied Signal 2TL1-1A.  It is substantially bigger (making it easier to feel for), and locks into each position, helping to avert accidental position changes.

Next we installed and wired the stick grips and associated relay deck.  The stick grips we chose are from Infinity Aerospace and are equipped to provide pitch trim, roll trim, speed brake, autopilot, fuel pump, engine start, and PTT functions.  Control of all but PTT functionality is split through a panel mounted toggle switch.  This prevents errant operation of controls, by a non-flying passenger.