Avionics Racking and Stacking & Power Distribution

Avionics Racking and Stacking

Over the last week I've been focused on getting all my avionics mounted either in the panel or behind the panel. Avionics and electrical for that matter dive into an entirely different set of skills and for me, uncharted waters per se. I can honestly say I've been planning my Avionics installation since before I ever began building as it's one of the things I am most excited about. I have several build log entries on my thoughts behind my layout. The most recent being specific to the panel design and electrical schematic.

The panel has changed very little since then although I did end up taking the TO/GA button off the panel and that will be placed on the stick. I found out I only needed a single button for AP disconnect and Control Wheel Steering (CWS).  The electrical on the other hand has changed quite a bit in terms of backup electrical and wire gauge, ect.  This document I'm changing as I move through the process, keeping it updated so I'll have an accurate representation of my system.  This will I assume be invaluable post build for maintenance purposes.

Jumping into it the first thing you really have to figure out is how and where you're going to place things behind the panel and attached to the sub-panel. I decided to start with the radio stack section because of the rack system I know where everything will go.  However the GTN650 is deeper than the 10 inches or so between the panel and the sub-panel so you have to cut a hole in the sub-panel to accommodate the extra depth.

To mark the hole to cut I removed the harness from the rack and put it on the panel insert which then I aligned it to the panel and tightened as much as I could.  Without the harness it almost fit all the way in there.  It allowed me to use a straight edge to mark out the area that will need to be removed.  Because of the wiring harness I went an extra inch to accommodate it on the left side.  Per the fuselage plans you also need to reinforce the cut out area and they give several options. One of which is to make a doubler out of .32 and the other uses angle pieces, but I had issues with that.  Using hand tools to make CNC level cuts is an skill I don't have, so instead of having a janky looking doubler I decided to instead go with the aluminium angle method.

I went with 1/8th angle over 1/16th angle.  It's a bit heavier but wanted to ensure this subpanel was reinforced and I don't know the stresses this area takes so better safe than sorry I assume.  I also had an piece of my old longeron that was removed from a previous mistake. This already had properly spaced holes drilled in it so using that saved me some time. I chose not to paint or prime it, but I did put a piece of rubber on the bottom to keep things in place.

With the sub-panel cut I decided to go ahead and mount in the panel piece and the G5.  Fun part is the G5 has a backup battery which allowed me to turn it on!  That makes it the first powered up avionics piece of equipment in N10JW.

I then moved the harness from my table to the inside of the plane.  Using the harness and spacing I was able to get a good visual representation of where I can locate equipment on the sub-panel. There's a lot of pieces that need to go back there and limited real estate available. I wanted to avoid placing anything behind the sub-panel for ease of maintenance.  So used some tape, snapped a pic and then posted on VAF to see what others had done.

Getting some solid feed back I decided to get building some mounts/rails for the GTR 20 (Comm#2), VPX and GTX 45R(Transponder/ADS-B).  I used a combination of various 1/16 aluminum angles picked up from Lowes and some scrap I had to fabricate these three items.  I also went ahead and screwed in the racks and placed the audio panel and 650 in.

The main issue with the GTX 45R is the depth, not only for placement but to be capable of removing it if need be later for service.  I decided to build a cradle out of angle and pull riveted it up to the panel and sub-panel.

GTX 45R cradle

After that I moved onto the GTR 20.  The idea is somewhat similiar to the cradle but instead of the uprights looking pieces I just put a few holes in the angle to mount the unit on top.  I tried different orientations of the piece but had to put it where it's at to avoid interference with my rudder trim and to still allow me to slide the GTX 45R out if need be.  Then just matched drilled some holes and pull riveted it on.

This is where I encountered my biggest challenge.  I could not for the life of me figure out a method to install the VPX and still allow access to the connectors while also not interfering with other connectors or the rudder trim cables.  I spent hours holding the VPX behind the panel trying to figure out what would work.  I then started thinking about way to mount it behind the sub-panel or putting large holes in the sub-panel to get it to work.

I really wanted to make it accessible and after fiddling around I determined I could put the VPX on a hinge, secure it to the sub-panel and then put another Lowe's special aluminum angle and attach it to the panel braces.  I originally tried to use my left over thumb quarter turn parts.  However the hole you have to drill for the receptacle really broke the diameter rule on the flange.  So I instead opted to use some additional angle to secure up the flange area and use a 1/4 bolt to attach.  Might be a tad overkill but better safe than sorry, don't want this thing falling off ever!  I have some 1/4" nutplates on order which I will install to make it a bit easier to unscrew later down the line. Here's a picture in the 'down' position.

Here is a picture of it in the 'up' position.  I chose this orientation because the Ethernet port is on the pilot side so it's easy to get to under the panel.  I was happy that it's secure, doesn't interfere with any of the panel pieces and allows me to get the rest of my panel pieces mounted in there without anymore spacing issues... or so I thought. (Edit: I thought the GSU (ADAHRS) mounted on the bottom of the panel, it's actually at the top and interferes with the way I mounted the VPX.  The solution is I'm mounting it on the brace to the left)

I ran into issues with both the GD40 (CO2 detector) and the GEA 24 (Engine monitoring).  Between the harness lead restrictions and the awkward shape of these I was having a hard time finding a place to put them that I was happy with.  After some thought I decided the best place for the GD40 is going to be on the backside of the center pedestal cover.  You have to remove these periodically for inspections so this is as easy as any place to get to it.  Not wanting to drill additional holes through the pieces however I decided to try and source a few Click Bond fasteners. These will allow me to attach the unit, and because it's not particularly heavy, it should work. (Edit: I realized later I didn't remove the carbon fiber under the insert.  Therefore I just used #6 countersunk screws and nylon lock washers)

That left me with the GEA which given the harness length really would only work somewhere to the right of the VPX, left of the radio stack.  I tried to make mount that I could attach to the subpanel and have it more or less hang out.  This is what I came up with...

It looked great but wasn't all that stable.  I pictured this thing rattling around causing all sorts of issues later on down the road.  I may have been able to extend the rails out and attach them between the panel and sub-panel for stability, but that would have required me to remove the fiberglass panel and probably mess up something doing so.

After removing it I messed around a bit with the harness and found I could put it under the GTR 20 and run it far enough on the other side of the sub-panel to make it a feasible mounting spot.  It's now nice and secure on the back side of the sub-panel essentially over the tunnel.  The four bolt pattern is visible in the below picture.  I put threaded side out on one of the bolts for a future adel mounting spot.  I also left the bracket end secured to the panel which will also function as a nice adel mounting spot if need be.

Power Distribution

In between working various equipment mounting tasks I tackled some of the main power distribution and grounding activities.  For power you run a #2 cable from the battery to the master contactor, to the starter contactor and then ultimately to the starter.  It's debatable on what is the best method for grounding an aluminium plane. The planes essentially locally ground everything to the frame, then you ground the engine to the engine mount which is grounded to the frame through the giant attachment bolts. My method is a more centralized approach and I'm grounding the starter to the engine mount, and then from there to the cabin where I have a massive tree of tabs for grounds (See picture above), and then a #2 back to the negative terminal of the battery.  This is probably over kill on grounding but I've flown several planes which have bad grounds somewhere resulting in audio feedback, intermittent lights or periodic failing antennas.

I previous ran conduits prior to securing the floor boards down.  My hope was that this would make running wires fairly painless.  This was not the case.... I don't know if it's the size of the conduit, the correlation or too many bends, but I tried the top two conduits to pull these large #2 gauge cables with no joy.  I decided to use the bottom one knowing it's a more straight shot and it finally worked out.  So in typical airplane build fashion the first one took an hour to do, the second about 4 mins.  I also went ahead and drilled the mid-fuse sections to accommodate the 3rd path for wires.  This allows me to still use the other two conduits as well as route wires from the wings to the panel area.

I have the battery side of things all terminated as well.  Leaving the power off obviously until I'm ready to start testing things.  Don't want to run into any issues or attempt a 'big bang' method of powering things up.

Wiring is reversed... had to remake cable.

Firewall forward I have the starter contactor and ANLs (giant fuses) installed.  These are for the alternator B-leads which are connected on the battery side of the starter contactor.  Additionally I'm route some #6 back into the cabin, into a separate ANL for the VPX.  The contactor side of that ANL will also feed my AC and Airhorn!

With equipment mounted and the possibility of everything having power I can push on to starting to connect things up.