Pondering My Electrical System

Let me start with this. I’m not an electrical engineer so if anyone can bring some clarity to this I’m glad to hear it. As I get closer to actually having to wiring the charge system using both the external and internal alternators I’m trying to pin down exactly what I need to implement as far as the charge circuit and interconnection of the internal and external alternators. As I look around for ideas and wiring examples I’m facing some conflicting information. My original drawings (available in the Resources menu) were based on what TAF provided in their electrical system manual and what Rotax shows in their 914 installation manual. I’ve found conflicting (well different) wiring diagrams from Rotax regarding the wiring of the internal alternator. I also was thinking that passing the field trigger through the VPX for the internal alternator might be a bad idea. If the VPX dies then the engine should keep running, but restarts would not be possible since the VPX would be controlling the relay that ties the Engine Circuit to the battery. I’m also unclear why some Rotax manuals and TAF manual show a relay being used to connect the “B” terminal of the integrated alternator to the battery and also a relay to control the field terminal of the external alternator to the battery (to active the alternator). I believe the field connect of the external alternator can just be done with a normal switch between the IG (field terminal) and aircraft power since it’s fairly low current. The “B” terminal of the integrated alternator could just be connected directly to the battery (or via a diode) and then the “C” (field terminal) could be switched to enable or disable the alternator/generator. So why use a relay? If I don’t need to install a relay then I’d rather not since that’s one less part to break.

Also all the diagrams I’ve seen show no concern that the external and internal alternators all tie to the main aircraft power bus. From some research I found this is a big No No. You want to keep the alternators completely independent if both are on at the same time. This is different then a primary and backup alternator situation. The internal alternator should only be used to drive the main fuel pump, the turbo servo and other engine components. While the external alternator is used to drive all the avionics and the aux fuel pump. Of course the catch is that the battery needs to connect to both the Aircraft Power Bus and the Engine Bus so I would think you’d need to use a diode to connect the battery and internal alternator so the alternator doesn’t feed into the main aircraft power bus, but no diagrams show this.

NOTE (12/10/2017): Found this little nugget in the Rotax 914 Installation Manual which explains a bit about the reason to use a relay to switch the connection between the internal alternator and the battery. From the Rotax 914 Installation Manual:

“Never sever connection between terminal C and +B of regulator (e.g. by removal of a fuse) while the engine is running. Overvoltage and regulator damage can occur. During engine stop break circuit between battery and terminal C to avoid discharge of battery!”


This diagram (from Rotax) makes sense to me. When the master solenoid (19) is closed then the internal alternator (5,6) is enabled via terminal “C” connecting to the battery. Plus the engine components have battery power for starting. The external alternator (10,11,12) is enabled by the double pole master switch (16) which directly connects the battery to the IG (field) terminal. The only thing missing is isolation of the integrated alternator form the main aircraft power bus.

And here’s a similar digram from Rotax. The difference is the use of a relay (58) that is closed when the master solenoid (38,39) is closed and connects the B and Field terminals of the internal alternator to the battery. I also just notice that the main fuel pump was added to that part of the circuit as well. But still that doesn’t explain why you use a relay instead of just allowing the master solenoid to supply battery power to these devices directly. I always thought that the reason to use a relay is to switch a high current load with a load current load. In this case you’re not really doing that and it’s not adding any isolation between the integrated alternator and the rest of the aircraft power.




TOGA Button… Work in Progress

I thought I’d share this idea to see if I can get some feedback from other builders. I found a  DPDT (couldn’t find a DPST) switch small enough to fit nicely in the end of the throttle handle. I just had to remove some of the threads on the switch and will probably use some RTV to hold it in place. I think this would be a very convenient place for the Go Around button so I’m hoping to get this to work. The other idea was to use a button on the control stick, but since a DPST switch is needed (As stated by Garmin) and all the switches in the stick grip are SPST, then I’d have to use a relay and I’d like to avoid that if possible. So with the switch able to fit in the throttle handle I just need to figure out a nice way to route the wiring. I need to run 3 #22 wires so it’s not too large of a bundle. I know I can drill out the center of the throttle handle a bit and get the wire to the throttle arm pretty easy. The hard part is running the wiring down the throttle arm and into the center console. I’m thinking maybe down the back of the arm using a snap on plastic U shaped cover or maybe through a small aluminum tube that’s epoxied onto the arm. Anyways if anyone has any ideas I’d be glad to hear.

The switch fits nice into the bolt hole in the throttle handle and is short enough to allow for the wires to get soldered as well. I purchased the switch from DigiKey, it’s an NKK LP0125CCKW01F (Digikey #360-2524-ND). I believe there’s also red and grey. Now I just need to figure out how to run the 3 #22 wires to it. Fortunately I have an extra throttle handle and arm for hacking…. I mean testing.

More Connector Wiring

Time 2.5 hrs

Well the heat shrink labels came today so I was able to finish the connectors that I had stated to wire up the day before. 

Control stick wires labeled and pinned. 

Control stick pins in the connector. 

And finally the TE mini CPC connector mounted under the control stick. The gribs will have the female side of the connector on the pigtail. If I ever have to remove a control stick then I just de-pin the connector and pull the pigtail wires back through the control stick. 

I also wired up the connectors and VOR coax on the tail cone. 

Connector Wiring

Time: 2 hrs

While the epoxy was drying on the vertical stabilizer I started wiring up a few connectors. I decided to use TE mini CPC connectors. The connectors are sealed and lock together. They come in a few pin arrangements but so far I’m just using some 4 pin ones in the VS, 5 pin for the pitch trim and flaps, and 9 pin for the control sticks. I bought these from online-components.com. Their site looks pretty minimal and I was a bit hesitant to order from them, but the items arrived on time and it was exactly what I ordered. Their prices were also much better then Digikey or Mouser (with the exception of the pins… Mouser had the best price for those). This stuff adds up quick so it’s always good to shop around. I was hoping I could finish up most of these connectors, but I ran out of heat shrink labels so I need to wait on putting on the connectors until I can label the wires. 

Some pins done on the tail cone. These are for the tail strobe.

I decided to put in a connector between the VS and the rudder so I can take the rudder off if ever needed. 

All connectors done for the vertical stabilizer/rudder. I put in a female coax for the VOR. 

Put pins on the control stick wires. I’ll be using a 9 pin panel mount connector that will mount under the control stick. The other side will connect to the pigtails of the control stick grips. 

GMU11 Part 2

Time: 3 hrs

Things done today:

  • Removed old cabling for GMU22
  • Ran new #22-3 between GMU11 location and instrument panel
  • Ran CANbus cable between GMU11 and pitch servo
  • De-pinned CANbus pins from pitch servo connector
  • Pinned and installed 9 pin connector for GMU11

Today I did some prep work for the GMU11 even though I don’t have the new mount yet. The factory said they should be shipping them by the end of the week and sent me a drawing of where it will be mounted.

TAF photo of the new GMU11 mount and location in the rear fuselage. It should be doable from the access hole in the bottom.

I ran the power and CANbus wire for the GMU up the left center longeron. I probably would have picked a different route if I had more access to run the wire. I think this was the  best way for my situation since I can get to most of the longeron to drill and install cable ties. The cable splits here near the pitch servo. The #22-3 (power1&2 and ground) go to the instrument panel and the CANbus wire runs to the pitch servo connector to tie into the CANbus there. I ran a single bundle back from here to the GMU. I used a multicore wire wire for the power and ground rather then single wires just because the Garmin manual said to do this to cut down on any EMF.

Note (9-3-2017): Just realized that I should have used #20-3 wire since I’m tying this into a 5A breaker. Well the device isn’t going to pull nearly enough to heat up the wires (it only pulls 0.1A), but the wire would technically fry before the breaker pops. So I think I may move it to a smaller breaker (it was on a breaker shared by other Garmin stuff) and then it should be fine. Also it’s really hard to find #20-3 cabling so I probably would have had to use #18 which is really overkill.

The one problem with this route is the wire gets a bit close to the pitch servo. The tie in the upper right of the photo helps keep it away from the servo. I didn’t shrink the heat shrink here yet because I may have to adjust the split point a bit once I get the GMu installed. I wanted to leave enough cable on the GMU side so I could put the connector on.

Cables on the GMU side with pins and labels.

Connector almost done. I put silicon tape where the cable strain relief is to protect the wire and add some resistance. One important note is that the metal bar part of the restraining clamp has the raised part facing away from the wire. If you flip it over then it can eventually damage the wire.

All done, just need to get a stainless steel 8-32 screw and lock washer to attach the shield of the CANbus. I didn’t put a drain wire on the other multicore wire because you really should only connect the shield at one end of a shielded cable to minimize ground loop issues. The CANbus is one except to that rule. It would probably be OK to ground the shield for the other wire, but just keeping consistent. Also the terminator will be installed here as well since this will be the end of the CANbus. The GMU11 uses an external adapter for termination rather then a jumper.

NOTE: The ring terminal is a Raychem B-106-1401, I believe I ordered it from Mouser Electronics. It’s nice because it’s a crimp connector, but also is heatshinkable. This saves a step of crimping the connector and then adding heatshrink over the end of the connector.

Now just need to wait for the new mount and get that riveted in.


Start Circuit Test

Time: 0.5hrs

I put together a test circuit of how I’m thinking of doing the start circuit wiring for my Sling. The issue is that I’m not totally sure if I’ll be using the Rotax 915 (which isn’t available yet) or the standard 914. If I use the 914 then the typical master switch (keyed or not) and an aviation start switch could be used in place of all this. It would take a little less panel space and simply things a bit. However, if I use the 915 then a standard aviation switch can’t be used since it will use electronic ignition. The problem comes down to this. If I end up getting the panel cut and installed and then I choose to use the 915 then I need to rework the panel and probably get a new one and cut with different holes and layout. This circuit though would work for both engines so I’m thinking it may be the safest route, plus you get the cool glowing start switch. I also like the idea of the keyed master. If I have the key then I know I turned off the master.


I made a quick panel mock up to hold the switches. I mentioned in the video that the master solenoid is incorrect. I purchased one that needs power supplied to the coil to operate rather then just simply grounding the coil. Not a big deal, but typically you use the later as your master solenoid. The actual circuit can be found on the Resources page. It’s actually very simple, it just looks a lot worse with all the clip leads being used.

Here’s a short video that shows the circuit working. Sorry I suck at making videos and this is actually the first time I’ve every posted anything on line.

It’s been a while

Well I still don’t have my parts so I’ve been working to clean up the electrical diagrams and panel layout. I created a bunch of new objects for the panel so it’s looking pretty close to what it will look like in reality. I’ve also been contemplating whether I will wait for the 915is or use the 914UL. In either case I’m trying to design the electronics and panel so that it can easily accommodate both. For example I added separate mag switches rather then use the aviation type start switch. This layout will work with the mags on the 914UL or the A/B ignition units (ECU’s) on the 915is. I also like the idea of having a keyed master switch rather then a simple switch.. if I have the key then I know the master is off. This idea is used on the Sling 2’s that use the 912is and I didn’t like it as first, but now that I think about it more I kind of like it. The one thing I don’t like about the Sling electronics design is that they put the full load of the system through the master switch. I’d rather do the more traditional Battery Contactor and use a low current switch. For this reason I couldn’t find a keyed switch that was both the master and the start switch so I opted to do a separate start switch, but the cool part is I can use one that’s illuminated and it’s pretty easy to make the light come on when everything is switched on correctly (mags, master, etc) for the engine to start. Also I’m thinking of installing capacitive fuel level sensors in addition to the resistive floats. If I do I’ll need another way to read that level so I added in the separate 2.5″ gauge for that. Once I figure out which one is more accurate then I can wiring that one into the Garmin GEA24. I’m thinking maybe to put the separate gauge in the throttle quadrant so that if I don’t want to keep it I can just replace that part and not have a hole in the instrument panel.

Here’s the new panel layout.


Here’s the PDF of the electrical diagrams. Just email me if you want the Visio since WordPress won’t let me upload that file type