Ch 22 - Electrical/Avionics - Part 5

Back plate (aka Junction Box)

With the business end of the instrument panel somewhat functional, it is now time to focus on the logistics of this upgrade, how to tie this panel into the existing electrical system, and how to connect all parts to each other.

Did I mention this is still a work in progress? Any or all of this might change in the near future, and at this time I am considering what I am doing as a proof of concept, knowing it will surely need some refining later on.

“Boy, do I smell another rabbit hole!”

So, let me first introduce you to the last component in the upgrade… the servos.

GRT servo

Ok, technically there's another item missing, the ARINC module, but since I haven’t received it yet I will kick that can down the road a little further.

So, what do the servos do, and how many do I need? 

Well, in the simplest of terms the servos are meant to replace the pilot's right arm (usually) so that he no longer has to hand-fly the plane. 

Personally, I love to hand-fly airplanes, and I do it as much as possible, as often as I can, but there are good reasons for letting “George” (autopilot’s stereotypical name) handle it sometimes... for example... very long flights.

The Long EZ has a standard fuel capacity of 52 gallons. With a reasonable 6.5 gal/hr fuel consumption, we are talking about an 8 hour endurance. I think I’d let George get his fair share of flying on a day like that.

Another perhaps less obvious reason is task saturation. Hand-flying an airplane can be very demanding and use up a lot of one’s attention, that leaves less brain power to plan ahead, especially in a single pilot scenario. 

While planning ahead is crucial when flying in general, it is of vital importance flying single pilot IFR (i.e. in the clouds), which I might end up doing from time to time in order to get back home. In such situations I would consider an autopilot essential, if not mandatory.

Back to the servos, what they do is take orders from the autopilot controller inside the Mini-AP (the AP stands for AutoPilot), which in turn takes orders from the pilot, and move one or more of the control surfaces to achieve the pilot’s directional objective. A little like the CNC mini-mill when you think about it.

The plane that Terry built has only one servo controlling lateral movements, allowing it to fly a course-line to a destination automatically, but still requiring the pilot to manually handle the vertical profile of the flight. 

While this is not a perfect solution, it does spare enough brain power to more easily stay mentally ahead of the airplane, and handle menial tasks like checking the weather, managing power plant and fuel quantity, looking for other airplanes, talking to Air Traffic Control or the passenger, etc. etc.

I chose to upgrade the system to two servos, one for roll (as per current setup) and an additional one for pitch. This will allow me to set, seek, and maintain any chosen altitude, and fly approaches to destination hands-off in crummy weather.

All of this electronic wizardry has to be installed somewhere, and be able to communicate with each other in order to work, and today I’ll be taking the first few steps into putting together the real backbone of the upgrade. 

Let’s talk about communication first, then I’ll show you the physical details of the upgrade. 

One very popular electrical connection is the D-subminiatures type, aka “dsub” connectors. If you were to look at the back of an older computer you might find a few  there (the VGA monitor's plug comes to mind). As you shall see in the photos below these plugs are indeed widespread.

D-subs on the back of the Minis

D-sub on the GRT servo

Yep! D-sub on the magnetometer.

Oh my! D-sub bonanza on the Garmin GNS480.

Even the still missing ARINC module has two such connectors on it.

Buying already made harnesses might be possible for some fairly standard installations, but not in my situation. Besides, I alway thought a better way would be to figure out what connects to what, and learn how to make the harnesses myself.

Believe it or not, this is not too difficult a task, and there is a lot of help out there for those wanting to venture into this arena. I’d like to mention SteinAir's YouTube channel on wiring, as they are very informative, plus one can buy all that is needed from them as well, or EAA's own videos on this subject.

D-sub connector making supplies

Since the connector-type decision was pretty much made up for me by Garmin and GRT, let's look at the D-subs a little more closely.

There are two types of D-subs, the ones you solder the wires to, and the ones you have to insert the pins into after having crimped the wire onto the pin. I chose the latter, and purchased the crimping tool with standard and high density pin dies. Once you get used to crimping, this method is so much faster, and your fingertips are safe from burns.

Life's too short for cheap wire strippers. This tool make stripping wires a pleasure.

22 AWG wire stripped and ready for crimping

Wire needs to show through the witness hole before crimping

Ratcheting crimping tool in action

Pin #9 getting inserted into the D-sub shell

"CLICK!"   The pins easily snap in place.

A fully configured EFIS D-sub connector

Back-shells are optional, but such a good idea for longevity.

Working on the opposite D-sub

In a previous post I might have mentioned that the electrical schematics of Terry's plane have been lost by one of the former owners, a real shame if you ask me. So, how is one supposed to tie this new panel into the existing electrical system?

Hmmmm... Because I didn’t want to have to undo any of the current wire bundles, I needed to design a completely self-contained system that connected only to power and ground. I also wanted to make it easily removable in order to take it home to diagnose, rewire, or reconfigure as necessary.

That's how I came up with the "junction box" idea. I figured I could bring the wires from all devices to the box, then do the "smart wiring" inside it. This means that all the harnesses outside the box would basically be straight-through harnesses (for the most part), so that a 37 pin D-sub from the radio would be brought to an identical 37 pins D-sub at the junction box. 

What this setup gives up in efficiency, it more than makes up  for in ease of installation and maintenance, since any harness could be easily removed and tested, and any remaining issue confined to within the removable box.

As I started putting pen to paper, the junction box morphed into a plate, but I still refer to it as the junction box. 

Wanna see how I made it?

The junction box/plate is held by 5" (12.7 cm) standoffs

A front view of the removable instrument panel

Reverse engineering the D-sub geometry. Many tries were needed to dial in the CAM G-code.

Successful 37, 15 and 9 pin D-sub G-code at last.

Making one of the six standoffs

Junction box/plate

Actually making the plate

Testing the D-sub fit

Front and rear structures

A view from the nose backward

Checking for fit of the Mini-AP

D-sub connectors mounted to the junction box/plate

Trying on the male 37 pin D-sub

Here are both Minis running on internal battery mounted in the new structure

Starting to string the MFD wires

As always, there were a few issues that needed to be solved along the way, but overall the junction box idea proved to be a great concept for centralizing all connections.

PFD harness completed

PFD, MFD and GNS P7 harnesses finished and installed.

Two of the four GNS 480 connectors completed and installed.

Next time I'll try to finish the rest of the wiring, then flip the proverbial switch, and hope that the "magic smoke" remains inside the expensive electronic equipment.