More Heater Box Work

Time: 3hrs

Did some work on the heater box today. I’m making a few parts from 304 stainless steel because of the higher melting point then aluminum. The idea is to make enough of the parts to be able to close the heater door flap and keep any flames (in the event of an engine fire) from entering the cabin through the large hole for the heater. All the parts should be made now. I did the bottom piece and incorporated the cable bracket into it, the flap, the flap bracket, and the hinge. I also had to make a spacer to go under the hinge because the old cable bracket that was there and now isn’t so there was a gap that needed to be filled. I’m not going to drill the holes to mount the flap bracket to the flap yet. I want to wait until the cable comes in so I can make sure it lines up OK.

Here’s the box almost all assembled. I’ll take more photos when it’s apart again in a few days. All the parts line up pretty well, even the parts I made. I’m still not sure how I’m going to make the large hole in the main plate. I’ll have to see if I can get a large step drill or a hole saw for metal. Also you can see the black silicone sheet (in the background) I got for making the gasket to use between the heater box and firewall for a better seal.

No more work on the plane for a few days due to a tip up to Yosemite.


Ground Terminal and Heater Box

Time: 3.25 hrs

Finally got to do some work on the plane today. It’s been a combination of being busy with the Holidays and lack of parts. The parts are slowly coming in so now I can get a few things done. Yesterday the ground terminal from Aircraft Spruce arrived so I put that it. I also added a few rivets to the heater box channel and started work on the Stainless Steel heater box parts.

The ground terminal was pretty easy to install the worst part was trying to remove the soundproofing material from inside the firewall. I decided to use an existing hole that was in the firewall for the main ground bolt and then just added a hole to the M5 rivnut for the smaller secondary mounting screw.

I reused the existing hole for the GPS antenna that I’m not using (drilling a new one). I only had to drill it out a little to fit the AN5 bolt. There will be a large sized ground wire that runs back to the battery as well as a few other items that will need local grounds.

Photo of the ground terminal mounted inside the firewall. It’s a 48 Fast-on connection block so I hope that’s enough. I didn’t could all my grounds, but I think it will be OK.

Here’s a far shot just to give some perspective of where it is. It should be pretty close to all the equipment so the ground runs won’t need to be very long.

Added in 2 3.2mm rivets to hold in the heater channel piece. I didn’t trust the rivnuts holding it in so this should help. When I mount the heater box I’ll use a silicone gasket so the slight height of the rivets and rivnuts won’t matter.

The other thing I worked on was making some of the parts of the heater box out of stainless steel. Stainless steel melts at a much higher temperature then aluminum so in the event of an engine fire I should be able to close the heat flap (I purchase a control cable that locks closed) and help reduce the chance of flames coming into the cabin.The SS piece I’m making is on the left and will replace the two pieces on the right. Working with the SS is a bit tough and since I’m incorporating the cable bracket into the piece it’s a thick 0.06 inch (16 gauge) piece so it’s not easy to bend or cut. It’s coming out OK so far. I messed up a bit and drill the hole for the cable a little off center. I don’t think it will matter and isn’t really noticeable. I’m still wonder how I’m going to cut the large hole in the middle I don’t think the step drills I have go that big. I will also need to make a new SS flap, cable bracket (that mounts on the flap) and have also bought SS piano hinge.

Battery Box

Time: 0.5hrs

Installed the EarthX battery box on the firewall. Now that I have the correct box which is a bit smaller and only mounts with four M6 screws (rather then six) I was able to go ahead and mount the battery box. I marked and drilled out holes 4 M6 rivnuts. I reused one of the holes that existed right under the 2 holes for the overflow bottle bracket so I only really had to drill 3 more holes and enlarge the other. I’m thinking maybe to us ether socket head type screws breathe then the button head screws because you can drill out the socket head screws and safety wire them. I don’t know if it’s necessary to safety wire everything since TAF uses button head screws on quite a few of the firewall mounted components. I suppose if I have issues with the screws coming loose (even after using locktite) then I could switch them out… not too hard to do later.

4 M6 rivnuts indstalled

Battery mount temporarily mounted with 4 M6x16mm stainless steel screws and washers. The overflow bottle should fit fine along side of the battery box. I think I will purchase the overflow bottle and make a bracket myself. I’d like to do a U-shaped bracket that is screwed on rather then using the TAF supplied one that uses two small brackets and a zip tie. The blue outline above the battery is for one of the GPS antennas. I will need to make the bracket removable so I can take the battery out. I made some foam mock-ups of the Garmin GA35 WAAS antennas that I’ll be using to figure out where to place them. I believe it’s recommend to keep them at least 9 inches away from each other, really just so they don’t shadow each other since they are only receiving and not transmitting anything.

NOTE: I had read a post by someone that found it necessary to cover the large square cut-out in the battery box with some aluminum. The reason was that the battery had disfigured a bit in the area that was exposed through the opening due to engine heat. I may contact EarthX to check if this is necessary or not. It could have been just a freak occurrence.

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.



Compact Rivnut Installer Saves the Day

Time: 0.5 hr

It’s not what was accomplished today it’s the tool that helped get it done that counted. Today I only installed 2 M3 rivnuts for the cabin heater box in the firewall. There are a total of 6 rivnuts 4 or which were straight forward to install. There were 2 that were close enough to the nose wheel bracket that my rivnut puller couldn’t fit to install them. The thought of removing the whole engine mount wasn’t something I really wanted to consider, but it seemed like the only solution until I found this simple compact rivnut installer. For a grand total of $7.58 plus shipping the problem was solved without having to tear apart the front of the airplane. The tool is considerable harder to use then the puller. You need to keep pressure against the rivnut as you use the allen wrench to compress the rivnut. Also it’s a bit difficult to sense if you’ve tightened it enough. I suppose you probably would get to a point where you couldn’t tighten it anymore and at that point the rivnut would be fully compressed, though I guess you could over tighten it. Fortunately I was able to see the back of the rivet enough to see if it had compressed enough. But for these few times where space is an issue I think it was well worth the money spent 🙂

With this done I’m hoping the the correct EarthX battery box will get here by tomorrow so I can get that installed along with the 2 mounts for the GPS antennas. I’m also checking with Torrance TAF if I can get the brackets needed for the coolant overflow bottle and oil tank so I can get those riveted on. The only remaining parts will be the fuel pumps which I’m told come with the engine and Vertical Power PPS which isn’t available yet. The PPS is nice because I was looking at trying to use a solid state master solenoid and their solution is just that. It also takes the place of the starter solenoid and main fuses. It supposedly will be shipping this January for around $1500.

The simple tool that saved the day. I probably could have made one from an M3 screw and a drilled out nut. The operation of it is simple: hold the body with a crescent wrench while turning the allen wrench which compresses the rivnut.

Here’s the problem. Now why didn’t the manual say to install these before installing the engine mount 🙂

Here goes nothing.

Easy Peasy. They’re installed. Happy Me.

Firewall Stuff

Time: 1.0hr

Did a few things on the firewall today:

  • Installed M3 rivnuts for the heater box
  • Installed M6 rivnuts and rubber mounts for the generator regulator
  • Looked at how to install the EarthX battery box

So it turns out there are two types of EarthX battery boxes. One is called custom fit and the other is multi-use. I ordered the custom fit box from Aircraft Spruce and it was back ordered. It eventually showed up a few days ago and it looked a bit different then I remembered, but I thought maybe they just changed the design. After looking at the EarthX site for battery dimensions I notice din fact that there were two different boxes and Aircraft Spruce sent me the wrong one. I ended up getting the other box directly from EarthX since it was a bit cheaper. The custom fit box is a little smaller and mounts with two M6 screws rather then 3 so it will make the mount a bit easier and the large holes for the M6 rivnuts don’t need to be drilled in the channel brace that runs cross the firewall.

I also installed some M3 for the heater box. Most went fine, but the last two in the center by the nose wheel can’t be installed with the rivnut puller I have unless I take the hole engine mount and hose wheel off so I can get to them with the tool. I found an M3 puller that looks to be very low profile so hopefully that will fit in there so that the rivnut can be installed otherwise I’ll have to remove the engine mount to install the two rivnuts…ahhhhh!

Installed 2 M6 rivnuts and rubber mounts for the generator regulator.

The mount screws stick through a bit on the front of the firewall. I can cut them back later.

Here’s the M6 rubber mount I found on Amazon. The screws are M6 x1mm so they screw into the M6 rivnuts fine.