To Shunt or not to Shunt

As I was wiring my GEA24 today I realized there was a decision to be made about where and if to install a shunt for measuring current. I already actually installed a shunt on the firewall to measure the current coming in from the alternator B-Leads to the Main Bus. (again, I will go over my bus decisions later, but this is a Z-14 inspired design where there are two alternators and two batteries). Thinking more about that though, I am not sure I want/need a shunt up there.

The decision impacts what information I will on my EFIS display. I am already pretty accustomed to monitoring Bus1 and Bus2 Amps and Volts from flying a G1000 and G500 system with dual busses. I like having that information available to me as a pilot.

The thing about my design is that the VPX-Pro already speaks to the G3X over RS-232 to relay the main bus voltage and current draw to the EFIS. So the EFIS can be configured to use that VPX information about the Main bus. The downfall of my setup is that the VPX current draw is not the full story for my entire electrical system. Some of the current is being sourced from the AUX bus which the VPX does not have visibility of.

So my original thinking was that I wanted to be able to see how hard the alternator was working. Only one of my alternators will be active at a time because they both feed the same bus. In thinking more about it though, I think it would make more sense to know how much current my AUX bus is using.

Sources of data:

• Main Bus Volts: VPX
• Main Bus Current: VPX
• AUX Bus Volts: GEA24 J244-Pin28 (attached to the aux fuse block)
• AUX Bus current: Rear mounted shunt (or maybe I am going to co-located this up front with the aux fuse block)

The thing that I find a little confusing about this is that the "Main Bus Current" is not the current that is going over the 2GA wire going to the battery box, it is what the VPX is outputting. So if I was flying and wanted to know how stressed my electrical system is in total, there has to be some math involved. That said, there was no way around that. Ideally I would talk to Garmin and they would provide a touch screen option to the EFIS page that when you tapped current you could flip flop between battery current and alternator current with both busses being depicted. I think for now though I am happy. (although I have to go remount my shunt to a new location)

Here are the relevant sections from both the G3X manual and the Vertical Power manual regarding sensing current.

G3X installation Manual

Current
The GEA 24 and GSU 73 have provisions to monitor bus current from two different sources. Current can be measured either using a shunt resistor such as the UMA 1C4 (50mV/100A type) or a Hall effect sensor such as the Amploc KEY100 series.

Shunt sensors: Shunt sensors are connected to the SHUNT 1 and SHUNT 2 inputs on the GEA 24 and GSU 73 (see Figure 30-2.2 and Figure 30-3.1). The SHUNT 1 input can be configured to display either "Bus 1 Amps" or "Main Bus Amps". The SHUNT 2 input can be configured to display either "Bus 2 Amps" or "Essential Bus Amps". 

Hall effect sensors: Hall effect current sensors are connected to the GEA 24 or GSU 73 general purpose (GP) inputs (see Figure 30-2.2 and Figure 30-3.1). Hall effect sensors can optionally be calibrated to adjust for installation differences (see Section 34.4.19.4). The supported configurations for Hall effect current sensors on GP inputs are similar to those supported for shunt current sensors:

• Bus 1 Amps (Hall)
• Bus 2 Amps (Hall)
• Main Bus Amps (Hall)
• Essential Bus Amps (Hall)

Vertical Power: When using a Vertical Power unit, configure SHUNT 1 to "Vertical Power Main Bus
Amps" or "Vertical Power Bus 1 Amps" to use primary bus current data from the Vertical Power unit.
Configure Shunt 2 to "Vertical Power Bus 2 Amps" to use secondary bus current data from the Vertical Power unit.

Vertical Power Installation Manual

3.7 Alternator Current Sensing (Shunt) When planning your aircraft electrical wiring you must consider whether to wire an ammeter (usually a shunt or hall effect sensor provided with the engine monitor) on the wire connecting the alternator(s) to the main bus. The ammeters indicate the amount of current the alternator is providing. 

A shunt is not required to tell if the alternator is working. It is very easy to tell if the alternator is working correctly by simply looking at voltage. If you see 14 (or so) volts with the engine running then it is working. If you see 12 (or so) volts it is not working or not turned on or the devices are drawing more current than the alternator can provide (note, engine must be running). If you set your low voltage alarm on the EFIS at 13 volts, then you will get a low voltage alarm if the alternator fails. 

Since the VP-X provides basically the same information as a shunt installed on the alternator b-lead, our position is that adding a shunt gives you no additional meaningful information. 

A shunt on the alternator b-lead shows the amount of current the alternator is providing to power the devices and charge the battery. The VP-X total current reading shows the total amount of current the devices attached to the VP-X are using. The delta between the two is the battery charging current, which goes to (basically) zero after re-charging any loss from starting the engine or charging a run-down battery. 

If the battery charging current is important to you, then you should install a shunt. If not, then simplify your wiring and don’t install it. Your call. And of course each builder’s needs are different so there is no absolutely right answer. 

The EFIS displays a VP-X page which shows individual device current as well as total current through the VP-X. The EFIS also has an ‘Amps’ gauge that is used to show the readings from the shunt. In some cases the EFIS ‘Amps’ gauge can be used to display total system current from the VP-X. Please check with your EFIS manufacturer for details. 

If you don’t install the shunt then the shunt wires on the engine monitor/ EFIS are not used

Dual Battery setup (ETX-900s)

Hidden deep in the schematic is the heart of the system, two EarthX ETX900-VNT Lithium batteries. The VNT versions of the batteries have vent ports that allow one to externally route the off gas with the supplied tubing. I have installed these in a Z-14 like configuration with one Main Battery and one AUX Battery with a cross-feed contactor between them. I am using my AUX bus as the Aircraft Power 2 feeder in lieu of running something like an IBSS system. So the start-up procedure will be like this:

1. Move the AVIONICS toggle from Off to AUX in order to power the backup power to the essential components. (I will make another post with my AUX power configuration). This will give me a Radio, PFD, Navigator, and a couple of other essential items.
2. Move the MASTER toggle to the "Batt" position. This will enable the main 2GA wire going forward to the starter.
3. Start the Engine
4. Move the MASTER toggle to the "Alt" position. This will enable the alternator.
5. Move the AVIONICS toggle to the "ON" position. This will let the VPX-PRO power up the rest of the avionics.
6. Move the X-FEED contactor to "ON". This will connect the MAIN BUS to the AUX BUS and allow charging of the AUX system.

The first thing to do to enable this system was to build a new battery box to accept 2 batteries. I kept the original location, but by using Lithium batteries, I was able to put to batteries in the place of the 1 original. I welded some 1"x2" 6160 Aluminum Angle to form a box with some .063 2024 to form the bottom of the box that rested on the 1" leg of the sides.

I then welded two lengths of 1"x2" facing forward and aft in order to give the box a couple of mounting points to the original 4 AN3 nutplates. This was in perfect alignment of the original two tie down points as well so I could use the standard hold down bracket.

The new batteries are a little shorter and a little wider, but they are really stable. Shown here is the right side without the electronics tray. See the other post about the avionics tray that I tied into the battery box.

Panel Layout

One of the most rewarding aspects about building your own plane is configuring the electrical systems and avionics just the way you want. While I am not Instrument rated right now, I fully intend to get my IFR ticket as soon as I free up some time by not building, but flying. Ideally, I would also perform my IFR training in my own 10. We will see how that works out. 

I will get into the 2 battery 2 alternator system I have installed in later posts, but I figured I would start with my actual avionics. I selected the following: Vertical Power VPX-Pro, x2 GDU460, 3 axis autopilot with a GMC307, GTN650, x2 GSU25, GMU22, GTR20, GTX45, GMA245, GEA24, ACK ELT and all of the components to have these packages work together. I am leaving my right panel open for a RAM ball mount for an iPad and I did install 3 mechanical backup gauges front and center.

These components are installed into a Aerosport 310 Panel with a throttle quadrant in the center console.

It is a bit of a mess right now, but the heart of this system is physically installed into N50412K.

Wiring

I have been a bit busy lately not posting, but I have been building! For the past couple of weeks I have been really straightening out my electrical system. I spend a lot of time in CAD software at work and for me, it made a lot of sense to build out my electrical system in an actual schematic editor. While Eagle is more geared around building PCBs, it's schematic editor is pretty straight forward. It is also free to use, so I figured it would be a good platform to build out a GARMIN parts library so other people can benefit for the hours spent building out the parts. If you are looking for Eagle it is now part of Autodesk. You can get it here:

https://www.autodesk.com/products/eagle/overview

Over the next couple of weeks I plan to get the rest of my wiring system into Eagle. I have most of it in a drawing right now, but I am working on cleaning it up. I am at the point where I just made the connections, as I build the wiring harnesses themselves, I have been putting in the wire colors into the schematic and marking them off as done.

If you want access to my EAA Avionics library I am keeping it here: https://github.com/karrelsj/eaa

If you want access to my actual schematic: https://github.com/karrelsj/N5412K

FM-300R on a L1B5 sump

This is a post that is completely out of order as I forgot to publish it when I was working through this fit up. I have a Lycoming Thunderbolt IO-540 with a cold air sump off of a L1B5. when trying to put the FM-300R onto this sump I ran into a bit of a snag. i.e. the sump is in my way!

As you can see here I have about an inch and a half up or out that I need to get in order to get things to line up. I think the BPE cold air sump has a very angled forward face to it that allows this fit up to work.



No real big deal here. This is a standard problem which many places sell spacers for just this problem. In my case I called Don at airflow and ordered a 4 degree spacer. I figured this would do the trick. So close!!! I am about 1/8" from fitting.If you look at the top corner of the spacer you will see it pulled away from the sump because of the top of the servo contacting the sump.



I have the servo on upside down here just to take a photo. This brass hex (the adjustable jet) is what is contacting the sump. I did contemplate just taking the 1/8" off of this hex. Also, Don had mentioned that we could remove the adjustability out of this...


I went back and forth with Don though, and Jeff Schans shipped him out a sump to work on some fit ups. Don agreed to help prototype a new bracket that would push the servo out the 1/8" while also giving me a place to mount up the linkage cables.

Presumably the straight spacer could be pulled back closer to the sump, but I am not sure that buys you any cowl clearance because the straight spacer will be pushing the servo inlet closer to the cowling.

Intake Decisions

I have a FM-300R, the Show Planes cowling, and a Lycoming Thunderbolt IO-540 with a cold air sump off of a L1B5 ordered the intake from Bryan and Showplanes, but things are pretty tight.

BLUF, here are the paths I am debating right now:

1. Build a custom snorkel that is going to have some pretty sharp angles and a pretty funky setup around the FM-300R inlet
2. Cut a hole in the Show Planes lower cowl and install a Rod Bower ram air setup and fairing
3. Cut a hole in the Show Planes lower cowl and build a new air box/inlet

I have been swapping some components back and forth to see what option was going to be the best. To start, the FM-300R needs about 1/8" of additional clearance when using the 4 degree Airflow performance adapter. Right now I have been working with Don at Airflow and he has a cable bracket in prototype for this that not only holds the cables, but also gives me the additional clearance needed to fit the FM-300R with the L1B5 sump.

Airflow Performance straight spacer.

Airflow performance prototype linkage adapter.

This gets me pretty well setup without the cowl and intake. Getting the Showplanes snorkel intake in place with this setup is pretty tight. Well I guess it is a little more than tight as it doesn't fit. Here is the lower cowl installed in its final location with the FM-300 on a straight (NOT 4 degree) spacer.  There is about 1" of clearance between the servo and the cowl.

I ordered the straight spacer because with the 4 degree spacer, the intake transition had some clearance issues with the PlanePower 70A externally regulated alternator. As a note, the larger tube is not even on the "T" yet, this is just the T without the step up to the larger diameter tube that goes over the air filters. Also, this is with the alternator in the center of adjustment. Obviously, nothing is cut here either. I still need to transition the T to the FM-300R which has a lip on it for clamping purposes. I could bring this version of the T aft a bit in order to gain a little more clearance.

With the 4 degree spacer, there is much more space to work with.

With the 4 degree spacer, the 1/8 control linkage adapter, and the lower cowl put in place, there is roughly 2.25 inches of clearance. Which is not that bad.

With 2.25 inches of clearance and the angle of the cowl, this is what a 4" tube for a T would look like.

Here is what a non-deformed 4-5/8"s looks like. Mind you I have not run the engine yet, and you would have to expect that there would be settling that is going to occur as well as movement while running. This is the final dimension of the stock Showplanes intake tubing. 

For the left side of the intake, there is quite a bit of room to make the run directly up from where the notion T exists in the previous photos. The right side however there is not a lot of room between my vetterman (custom) exhaust and the PlanePower alternator. I was contemplating getting a smaller alternator, but I might as well not add those constraints to the setup if I am at this stage of the build.

This is pretty hard to see, but this photo is taken from the left side of the engine looking at the right with the T and the snorkel resting where ever I could get them to fit. The angles are possible to make, it is just a shame that this is the intake and there are going to be many >90 degree transitions.

Which really has me thinking why not just punch the intake straight out. I have been thinking about the Rod Bower design and I have an email out to him.

There are just sooo many bugs in Virginia that I feel like a large percentage of my flying is going to be with the ram air door closed sucking the warm air out of my lower cowling. I have been thinking about leveraging the C172 design.

I have also been looking at making that a little sexier and mimicking what the Sam James cowl looked like at one point.

Fly EFII ignition mount

I went with the FlyEFII System32 ignition (dual). They have a nice bracket for the coil pack, but this makes a pretty hefty assembly that needs to be mounted. There are various things people have done to mount this pack and most of them include standard adel clamps. I wanted something a little heavier duty, because this is a pretty heavy assembly. I ended up finding a light bar mounting bracket off of House Tuning that worked perfectly. This is the standard 7/8 tube rubber isolated light mount.

The coil pack holes accept AN-4s but the light bar mounts are 3/8s (or maybe even 5/16s). I placed a mcmaster order for some bushings (under bearings at mcmaster) to take the 3/8s holes and adapt them. The bushings didn't fit, so I reamed out the holes.

And then shortened up the bushings a bit.

This allowed me to install the front two mount points of the coil pack right behind the aft baffle on the main cross bar.

This looks like it is going to be perfect for the plug wire spacing (at least I hope so because anything other than this is going to be kind of a pain.

The last thing to do is make one hard mount point for the aft of the coil pack going down to the angled bar. I will do that another day. I am either going to come down the the angle bar, or I am going to install one of the bar mounts facing aft and flush mount a screw through the coil pack plate. For now though, it is pretty dang solid.