EGT/CHT wiring

May 25th, 2015

The last major firewall-forward wiring job is to connect the thermocouples that measure exhaust gas temperature (EGT) and cylinder head temperature (CHT). There's two probes per cylinder, so it ends up being a fair number of wires.

Rather than using the old-style ring terminals to connect the thermocouples, I decided to use these cool little connectors from Omega Engineering. A lot of aircraft manufacturers are switching to this method of connecting thermocouples since they're easy to work with and there aren't any concerns about insulating the connections.

I cut off the ring terminals and installed the connectors on all eight probes. By the way, you have to be careful when wiring Type K thermocouples… the red wire is the negative side!

I'll skip over the couple days I spent working out the wiring routes and strain relief arrangements, and just walk you through the final product. Starting on the starboard side of the engine, the thermocouple leads and connectors are all bundled up and secured with high-temp tie wraps:

I made a little standoff out of a strip of 4130 steel, painted for corrosion resistance. It ties into the lower valve cover screw on the #3 cylinder and supports an adel clamp that secures the wire bundle. In this photo it's held on by a cleco since I intend to replace it with a high-temp silicone clamp when I get my next parts order in the mail.

The wire bundle goes down the #3 intake tube, where it's secured by another pair of adel clamps near the bottom:

It's sort of hard to see in this photo, which is looking up at the bottom of the right side of the engine, but the bundle of thermocouple wires loops around a diagonal engine mount tube and attaches to the transverse tube with more adel clamps. Although it doesn't look like it in this photo, there is plenty of space between the wires and the diagonal tube.

Now we're on the left side of the engine, looking across to the right. The wire bundle crosses the aircraft, following the transverse engine mount tube via three pairs of adel clamps:

At the port-side landing gear socket, the wires turn the corner and head up one of the engine mount tubes towards the top corner of the firewall. Yet another pair of adel clamps routes them up the tube, and the wires for the #2 and #4 cylinders split off here as well:

The main wire bundle runs up the back of the tube, where it's attached with… you guessed it, another pair of adel clamps:

At the top of the engine mount, one more pair of adel clamps helps the wire bundle turn the corner, and then it disappears through the port-side firewall penetration:

Meanwhile, the wiring on the left side of the engine is more straightforward. The wires and connectors are all bundled together and tied off – you can also see how I ran a tie wrap through the middle hole in each pair of connectors, to help prevent them from pulling apart:

I used the cable for the alternate air door as a convenient way to route the wires on this side of the engine. I cut some 1/2" lengths of rubber fuel hose, and used more high-temp tie wraps to hang the wire bundle from the cable sheath. The result is a neat way of running the wires that looks nice and keeps them away from the ignition leads:

Inside the fuselage, the thermocouple wires are connected and more or less bundled, although at some point in the future I'm going to have to schedule an epic tie-wrapping session for the behind-panel wiring:

All eight of the probes are reading room temperature, which is a good sign. The fact that the #2 EGT is reading a little higher is just a rounding fluke – the EGT temperatures are rounded to the nearest 5 degrees.

I lost track of the number of adel clamps I installed as part of this little project. It sounds easy now, but each pair of clamps was a mini-nightmare all by itself. These clamps are difficult enough under benign circumstances, but in the FWF area where you can barely get your fingers on them, let alone a wrench, it can seem nearly impossible at times. I hope I don't have to install any more adel clamps for a good long while.

ELT wiring

May 3rd, 2015

The ELT uses a fairly rinky-dink DIN connector for its power and GPS position inputs. Rather than trying to solder these tiny pins while standing on my head inside the airplane, I decided to solder them on the bench instead:

Then I fabricated a short DIN-to-Molex adapter. Inside the airplane I used a mating Molex plug with crimp pins.

What this enabled me to do was to also build an in-situ test adapter, complete with diagnostic LED. Then instead of having to leave the wire for the LED in the airplane all the time, I can just plug this little thing into the harness and watch the LED blink when the ELT gets data. Which is what I did – sorry, it didn't photograph well, so I can't show you a picture.

The ELT also comes with a separate little box, containing a buzzer that sounds when the ELT is activated. I suppose that would help the CAP find a crunched airplane in a forest, for example, but I expect its main purpose is to alert bystanders to an accidental ELT deployment after a hard landing or whatnot. I mounted the audio box on the aft side of the F-707 bulkhead:

The audio box uses its own battery, an unusual 3V lithium number. I'll need to crawl back there and replace it in 2024:

I secured all the wires that go to the ELT, and there are actually quite a few. I used plastic tie wrap anchors to route the wires to and from the audio box, whose mounting screws are visible here on the forward side of the bulkhead:

I fabbed a length of RG400 coax for the ELT antenna, and used more plastic anchors to secure it loosely to the bulkheads and floor.

Everything is shipshape in ELT land now, I believe. Per the manufacturer's guidance, the antenna cable does not penetrate any bulkheads or aircraft structure, and is only loosely restrained elsewhere, so it can still move around and hopefully won't get cut during a… shall we say, "a deceleration that rearranges the configuration of the airframe".

Transponder antenna cable

May 3rd, 2015

One last job to finish up the transponder installation – crimping the BNC connector and securing the short run of coax that goes from the transponder to the antenna. Done and done:

Transponder cooling fan

April 19th, 2015

One of the unfortunate side effects of mounting the transponder underneath the baggage floor, as I did, is that the hot transponder is trapped inside a box with no air movement. The transponder puts out 250 watts, and I didn't want it cooking itself under there, so I engineered a little cooling fan setup to help keep its temperature under control. I started with a cheap 12V squirrel cage blower meant for desktop computers:

I made this little mounting bracket out of scrap aluminum. The big hole in the center is sized to fit the opening in the bottom of the fan.

The cooling fan will exhaust through a hole in the F-706 baggage bulkhead, which means I need a doubler plate for the hole. Since the fan has a rectangular outlet, I used the mill to cut a matching hole in a piece of 0.050" scrap:

Here's the finished doubler – nice round corners on that hole, thanks to the milling machine. I love the milling machine, even though I'm a complete novice at using it.

Inside the airplane, I used a unibit and files to make a matching square hole in the bulkhead, below the level of the floor. The hole isn't nearly as pretty as the machined one in the doubler plate (nor is it as weirdly trapezoidal as it appears in this photo!) but it will serve its purpose. I did make sure not to leave any square corners, lest they provide a place for cracks to develop.

Riveted the doubler plate and fan bracket in place, using a combination of solid and pulled rivets:

Here's a shot looking forward from inside the tailcone, so you can see the doubler on the aft side of the bulkhead. For reference, the hole dimensions are approximately 1/2" by 11/16", so not all that big – about the same size as the hole for a wiring grommet.

Here's the fan installed on its bracket – warm air comes up through the hole in the bottom of the bracket and exhausts through the bulkhead into the tailcone. I think the tailcone should be a low-pressure area in flight, which should also help move air towards the tail.

Testing to see if it all works – you can't see it in this photo, but the fan is whirring away and blowing air through the bulkhead.

Everything all cleaned up and reinstalled:

Here's a close-up of the fan, showing how I used a tiny plastic clamp to secure the wires from flopping around:

The fan blows lots of air when hanging in free space, but just for a test I temporarily installed the starboard baggage floor and ran the fan with the floor in place. I was pleased to see that the fan didn't seem bothered by this at all – the pitch didn't change and it still moves plenty of air with the floor installed. I was worried that I'd also have to engineer a cooling air inlet to provide enough input air for the fan to work with, but I guess enough air gets into the transponder compartment through gaps in the corners of ribs and so forth.

To run the fan, I used the switched power output pin on the transponder, which is normally used to power a blind-mount altitude encoder. Since I don't have one of these, I can use that pin for my cooling fan. I measured the fan's current draw at about one-sixth of the maximum current capability of the switched power output pin, so no problems there. The nice thing is that the fan will turn anytime the transponder is on, and I don't have to fish another set of wires through the whole airplane to run it.


February 1st, 2015

The magnetometer is a little gadget that measures the strength and direction of the local magnetic field, which allows the AHRS units to figure out their current heading, among other things. If you read the installation requirements literally you might think that there's nowhere in the airplane you can mount it to meet every one of the requirements, but in the RV-7 a good spot is the F-714 aft deck, just ahead of the horizontal stabilizer. Garmin makes a mounting plate you can use for this, although anyone who has progressed this far ought to be able to knock one together from scrap aluminum in no time at all.

I happened to have one laying around, so after some careful alignment I drilled the fastener holes through the aft deck:

The magnetometer mounts into this combination nut ring and alignment fixture, which I riveted to the plate with AN426AD5 rivets. A rivet that big is way overkill for such a lightweight piece, but that's the size of the countersink you're given so that's what you've got to use.

I bought some brass screws and locknuts from McMaster-Carr, and used them to attach the plate to the aft deck. Brass because you want to avoid putting ferrous material (e.g. steel screws) next to your magnetometer. I could have used rivets here, but I wanted this plate to be removable in case I ever need to reach my hand down in there to do maintenance work. Also I really didn't want to crawl back into the tail again to set the rivets.

Test-fitting the magnetometer to determine where to terminate the wires:

I put a tie wrap anchor on the aft face of the F-709 bulkhead and used it to secure the the connector for the magnetometer wiring:

Wider shot of the aft fuselage with magnetometer installed:

Happily, the magnetometer came up and started talking on the first try: