2021 Done [300.0 hours]

I spent most of the second half of 2021 slowly working on the aircraft wiring. It seemed like a never ending job but eventually it all came together and by December I was able to re-install the panel + avionics and make it all work. Here’s what I completed, among other things:

  • All tail wiring, including both batteries, avionics shelf fuse blocks, ADSB receiver, antennas
  • Pitch and trim servos
  • A/C evaporator wiring, sensors and controller board
  • All overhead wiring and overhead panels, lighting etc.
  • Flap motor, sensor, joysticks,
  • Door annunciation switches
  • 3 screen AFS system, IFD GPS, and all associated avionics/audio/radio wiring
  • Panel switches, and wiring for both B&C regulators
  • SDSEFI dual ECU system, MAP sensors and FWF wiring
  • Custom dual battery/dual alternator power board for redundant EFI power
  • Custom TFT display (front panel) and controller CPU (under pilot’s seat)
  • Pitch and aileron trim wiring
  • Wing, heated pitot, and emergency fuel transfer wiring
  • Engine monitor wiring
  • USB power for various panels
  • Headset connections x4 and USB power at the rear of the centre console

I used a dual 600W bench power supply to bring the system to life, it can provide up to 35 Amps of current to each of the right and left battery buses, but more importantly I was able to set lower current limits during the initial bring-up stages, and only increase those limits as more systems are brought online. More importantly, there continued to be no smoke. Using this bench supply also allowed me to run the entire system indefinitely, without worrying about having to charge the batteries (which are in place but disconnected).

I rebuilt my “engine simulator” with a bit more design thought this time, and included a second coilpack and set of drivers so that I have both the “left” and “right” ignition systems. I replaced the cheap ch*nese injectors, some of which previously melted, with another set of cheap ch*nese injectors, but with a new injector driver board design which is running much more reliably. I also put a slider pot on the throttle cable so that I can adjust rpm using the real throttle, which allows me to examine operation of the power system across the usual engine operating range without climbing out of the aircraft to make the adjustment. It’s also kind of cool to push the throttle forward and have something actually respond! My old test fuel pump seized up (since I used it in water), but that’s OK, I simply used a blower motor to emulate the actual fuel pump. This had the advantage of being able to blow air across the various load resistors which I use in place of the injectors most of the time, since the injectors are so noisy.

When it comes to electronics, there’s always a lot of effort required to get from a “working prototype” to a stable, production worthy design, and this is true in the case of the EFI redundant power board I previously described. Four months ago my wife’s latest 50kg puppy smashed me in the knee, and after hobbling around for a few months I got a scan and went in for knee surgery in late December. During the latter part of this time, and since, I haven’t been in a position to climb into and around the fuselage, so I took the opportunity to do a lot of sit down work to fill out the missing/broken pieces of firmware/software and refine the hardware to the point where it is approaching what I would call a production worthy design. The latest prototype is installed in the aircraft, and since completing the wiring I have been able to operate it in-place along with all the rest of the avionics, simply by replacing the connections to the injectors, ECU’s, coilpacks etc. with flying connections to the engine simulator sitting on an adjacent bench. I also made the “screensaver” functionality work, which allows me to write the rest of this post with actual images saved from the little custom display, rather than horrible camera photos.

The startup, door checks, checklist items etc. are more or less as I previously described them, so I’m going to show what the engine monitoring displays look like during normal as well as abnormal operations, where all “abnormal” operations are brought about by physically messing with the engine emulator, i.e. every abnormal condition is physically *real*. First, here’s the monitoring display for coilpacks and fuelpumps during what represents “normal” operation, with both ignition systems running and one fuelpump operating:

The Coilpack display show the current rise as each coil is driven. The linear current rise is controlled by the coilpack drivers, with a nominal 3.5 msec dwell time used in the emulator. The actual dwell time measured is 3.3 msec, and there is also an indication of the peak coil current – 5.2 Amps and 5.5 Amps respectively. Although the power board locates and measures every current pulse, the display is set up to update at a rate of 10 times per second.

The fuelpump display shows the run time of the pump (5.5 minutes), and the average pump current (5.5 Amps in this case, a bit higher than the actual fuel pumps). The graphical sample has a ripple because of the pump commutator action. This was a good quality blower pump and the commutator ripple is fairly consistent. To illustrate what a bad pump looks like, I connected a cheap auto-store 12V plug-in fan to the right pump position. The current draw is much lower than a fuel pump, the system self scales so ignore this, but look at how bad the commutator switching is:

If you saw this sort of ugly switching on an actual fuel pump, you would want to replace it. A more usual problem would be a commutator segment that started dropping out, this would be an early indication of trouble even though the fuel pump may still be achieving proper fuel pressure. Displaying this sort of information is just another tool to monitor equipment performance and provide early indication that there may be trouble on the way, just as we rely on engine monitors these days to display comprehensive EGT/CHT etc. information.

Switching to the fuel injector display, here’s what I was presented with when I brought up the new emulator system with my six new cheap Ch*nese injectors (idling at 500 rpm):

I have several scaling options to fit pulses into the display, in this and all cases described in this post the injector scaling is set up so that one, fixed scaling is used across all six injectors. This allows relative comparisons to be made between injectors without having the underlying software skew the results. All six injectors are opening, the little squiggle on the rising edge occurs when the pintle opens, but injectors 3 and 6 open later and reach a lower peak current for this short (3+ msec) firing interval. At low RPM (short injector opening duration), one could expect that cylinders 3 and 6 would run quite a bit leaner than the other cylinders. This might be otherwise observed as rough idling, but probably doesn’t matter in the overall scheme of things. At higher RPM, these injectors are open for slightly less duration than the others, but this might be compensated for with the usual individual cylinder AFR adjustment available in the SDS system. I’m hoping that, when I start up my real engine, the injectors supplied by SDS will show more uniformity than these injectors (bought on EBay).

I introduced various types of injector failures, to get the display system responding properly. Here’s a case where cylinder #3 injector has shorted turns in the solenoid, it is still managing to open but then goes into over-current fault during the latter part of the opening cycle (500 rpm):

The over-current fault level is set to 3.3 Amps, there is no way to determine how much in excess of this the actual fault current is, so I simply represent it with a red bar. Since the fault is in the injector itself, the fault only occurs during injector pulses. If you touch the display on the faulty item, it will bring up the detailed injector display as follows:

There isn’t really much more information here. The system defaults to “automatic” fault detection and retry. I have included the ability to latch faults and reset them, but I doubt this is the sort of activity you want to undertake in flight. Note, however, that since the fault is only occurring as the injector fires, the fault is clearly in the injector itself, and not in the wiring. Is the injector staying open? Since it did in fact open, the injector will likely stay open despite the fault condition during automatic retries, where the current is pulsed back on every few msec; however, there is no way to really know. There is a brute force means to live with this sort of fault in the air – disable the left supply, causing the injectors to switch over to the right supply only, with each injector having a 5 Amp slow blow fuse in place. Maybe the fuse would blow, maybe not. You would be a cavalier pilot to try this sort of thing mid flight, since it would also mean that the left ignition system would be down.

A wiring fault is detected by sensing continuous fault conditions for any duration in excess of 100 msec. Here’s the display presented in the case of a wiring fault (still 500 rpm):

The display updates in more-or-less real time to such events. To bring about this fault, I simply shorted (to ground) the 12V supply to cylinder #3 in the engine emulator, and by the time I looked up at the panel the above display was present. If you saw this in flight the engine would be running rough and you would want to throttle back and land ASAP.

Another type of fault is an open-circuit in the injector, or wiring to/from the injector. In this case, there simply won’t be any current at all. Here’s an open circuit fault in cylinder #6:

Yet another type of injector fault is one where either a poor electrical connection outside or within the injector, and/or a deteriorating injector coil, is causing the resistance of the overall injector circuit to increase, effectively decreasing the ability of the injector to open. The fault may be intermittent. The injector is mechanically OK, it just opens late and draws less current:

This engine would be idling a bit rough, but at higher RPM’s the problem would manifest itself as cylinder 6 running a bit lean. Notice how the injector opens late, but it still does open, and is open for perhaps 14 of the nominally 16 msec opening time:

Notice how the display self scales to fit the wider injector pulses into place. The purpose of the display is to present a comparative, qualitative, impression of how the injectors are behaving, not to present any sort of precise absolute measurement. I have toyed with some other options to keep showing the rising edge even when the opening time (pulse width) increases, but haven’t really settled on whether this is a useful mode or not. In any case, if you saw the above display during flight, you would ground the aircraft and replace the injector. Hopefully you would note this during run-ups and never get airborne in the first place. Here’s a more extreme case, of an injector/wiring situation where the injector is failing to open at all (500 rpm):

If you saw this after start, you wouldn’t bother going any further.

One type of failure I couldn’t emulate is a partially sticking pintle. I don’t have such a bad injector on hand, but I plan to try and acquire one by calling around some repair shops in the area. Based on what I’ve seen though, I think the display will show such a condition quite easily by noting the small current reversal coming and going, or moving around as the pintle sticks. This might be a use for the type of display that focuses only on the rising edge regardless of opening duration.

A few other displays. Here’s what happens if there is no communications with the EFI Power Board:

A few statistics displays, not very interesting inflight but useful for ground diagnostics:

This latter display is actual junction temperatures on the power board for each of the ten channels, on a day when ambient temperature was around 25 degrees C. Shorting any channel’s output to ground only raised that channel’s junction temperature by 7 degrees or less (with auto fault sensing/retry in place). I’ve operated the system with the power board enclosed and heated to around 60 degrees ambient, with no junction temp exceeding 75C. Since the TPS* devices used on the power board are rated up to 125 deg C junction temperature, there’s clearly plenty of margin here, and if the ambient temperature behind the panel is much beyond 60 degrees, I don’t think I’d be sitting in the aircraft anyway. I do have (defrost) fans on the top of the panel which weren’t operating during these tests, although no forced air cooling is required for the EFI power board. I also note there are no power diodes in any main current path soaking up energy or heating up the surrounds. The board runs cool.

Where to from here?

Now I can walk again, I can get back to the job of finishing the aircraft and entering phase 1 flight test. In terms of the EFI power board, when I started the project, the end game was simple – once the hardware/software design was complete and tested, I was going to do a small production run in a commercial facility that specialize in such things, so I can get a set of boards that have the required level of manufacturing quality built in. The supply chain crisis railroaded this idea for 2021, and I’ve already had to make a few substitutes for components on the BOM that have been end-of-life’d. The biggest problem is the main power switch semiconductors and the STM32 microcontroller used on the board. Everywhere is out of stock, and likely to be for the remainder of 2022. I have enough parts here to hand build a few more boards, but not enough to provide for (say) a 10 unit proto/production run. It takes me around 20 hours under the microscope to hand assemble a board, and I’m coming around to the notion that I may have to fly off phase 1, or at least part of it, with a hand assembled board that I’ve thoroughly tested, including temperature testing in an oven. The inherent redundancy built into the board and system design helps get my head over this hurdle.

There is only one other software item for this system remaining on my to-do list before first flight, and that is to implement logging. The system has ample bandwidth and storage to log all data, sampled at up to (say) 5 times per second, for the duration of a flight. Any anomalies noticed during flight can be re-examined by playing back the log data, perhaps in tandem with log data from the Dynon engine monitor. The log data can simply be copied onto a USB stick, plugged into one of the ports accessible with the pilot side seat slid back.

Here’s a short video, with the panel alive and the engine emulator going, pushing the throttle from idle up to 2700 rpm:

A note on A/C ducting

A long time ago I designed and 3D printed some bits to perform the A/C ducting on the evaporator. Along with the wiring behind the baggage bulkhead, and overhead (where the ventilation controls are), I sat all these ducting pieces in place since, with the overhead completed, I could see how much air I can blow through pressurization of the overhead. The evaporator outlet duct has extra 2″ outlets, for scat tubes down to the baggage bulkhead, if I needed more air outlet volume. The A/C evaporator has three scroll fan settings, call them low, medium and high. In addition, I added a 4 inch bilge blower to the outletpath, driven by a variable speed drive. The latter item turns out to be the most efficient way of moving air. With the evaporator ducting just sitting in place and leaking like a sieve, and the extra 2″ outlets taped off, I measured the various blower combinations available with all four overhead vents wide open. I measured vent outlet velocities from 0 through to 16 metres-per-second, fairly consistent across all outlets, depending on the various blower settings. Across the four vents, this corresponds to moving air at a maximum rate of 1,277 CFM. This is enough to completely move the air volume of the cabin several times per minute, so I don’t see the need for the extra ducts on the baggage bulkhead. I’ll either re-print the evaporator outlet part, or more likely print and glue on some caps to block the extra 2″ outlets.

I have to lower the evaporator shelf down to rivet the final skin on. It’s a bit of a jigsaw puzzle, but I can raise the shelf back into place and secure all the wiring/ducting with the skin on. Has to be that way, for future servicing. Final securing of the ducting in place will also prevent the leaks that occurred with it all just sitting there, which can only increase the measured vent air flow.

I’ve got enough sensors in place to have a future “AUTO” mode for the A/C and external air ducting, controlled by a small micro-controller that sits on the evaporator shelf, accessible with the baggage bulkhead removed.

Finally, here are some additional photos of items discussed in this post.

  • g1a
    Sorry about the knee....
  • g1b
    EFI power board, with temporary wiring out to engine emulator
  • g1c
    Centre console
  • g1d
    Rear seat headset connections and USB power
  • g1e
    Sneaky CPU board under pilot's seat
  • g1f
    Front seat headsets and USB power
  • g1n
    Dual bench supply powering the system
  • g1g
    Live panel. Note TFT display above pilot PFD.
  • g1h
    Overhead wiring in progress
  • g1i
    Overhead panels in place
  • g1j
    A/C evaporator ducting
  • g1k
    A/C evaporator, rear NACA vent ducting
  • g1l
    A/C evaporator ducting
  • g1m
    Measuring overhead air vent outlet velocity
  • g1_bridge
    RIP Bridget

Inlet plenum success! [1.0 hours]

It took some time to get back to my “Showplanes Cowl with A/C left hand inlet plenum problem”, but this week I finally unleashed my new 3D printer on the problem, and the result was great.

This is a complex part, and I evaluated several different slicing applications to figure out how to do the necessary support structures. I wound up using Cura, because of its “Tree” support capabilities. It generates all sorts of weird tree trunk/branch constructs to support the part while it is being printed. This results in less interference between the support and the part.

It took 4 days 16 hours to print the part in ASA, using a bed temperature of 100 deg C and a 0.4mm nozzle at 250 deg C. I did gear up to use a dissolve-able filament (HIPS) between the support and the part, but decided for the first trial to simply use the one extruder. As it turned out, the support was easy to rip away with a pair of pliers, so I’ll stick with the single material process to save time and complexity.

There were a few areas where the support came slightly adrift, causing rough regions on the part. I need to fix this by manipulating the support to have better adhesion to the bed. It takes about 5 hours to render the model, another hour or so to “repair” the STL, and about an hour to slice the result and generate gcode for the printer. Although the part as printed is certainly usable, there are areas I can improve on. Since each printed part is a 5 day exercise from start to finish, and comes with a filament cost, I won’t be spinning revisions too often.

There are some new materials around, Polyamide with Carbon Fiber filler, which are stronger and have higher operating temperatures, up to 180 degrees C. I can print these materials if I install a hardened nozzle on the printer, but I’ll hold off on this until late in the build because the filament is expensive, and there are new products hitting the market all the time.

For now, I have the entire process under my own control and I’ve been able to print a perfectly acceptable part – mission accomplished for the inlet plenum, finally. Now I can finish the front baffles in this area.

How many hours have I spent on this? A lot, between assembling the printer from parts, calibration, printing test parts, evaluating slicer software and monitoring the printing of the plenum. None of this is really direct work on the air frame, so I’m going to simply log 1 hour for this activity, knowing full well that it was many times this.

  • f58a
    Starting the inlet plenum print, layer 1
  • f58b
    A day in, and a long way to go
  • f58c
    A bit over half way
  • f58d
    Nearing the end
  • f58e
    Print complete!
  • f58f
    Support rips away quite easily
  • f58g
    Complete part less support
  • f58h
    Installed - and a perfect fit!
  • f58i
    2 inch connection is for heat muff SCAT tube
  • f58j
    Room for A/C connections
  • f58k
    A/C line connections will work OK
  • f58l
    Air filter fits inside here, fits into standard Showplanes system
  • f58m
    Clearance around A/C compressor
  • f58n
    Side view

Where did winter 2020 go? [100.0 hours]

I got an E-mail from someone wondering why I stopped posting.

I’ve been steadily working the project, but having reached the “90% done, 90% to go” stage, it’s been hard to progress many tasks through to completion. Winter down in Tassie was long and cold as usual, and the short days tend to slow down workshop hours. Thankfully that’s now behind me for the year.

The inlet plenum 3D printing work reached a stalemate. After many Covid delays, I received the second prototype and it was damaged in transit due to poor packing. I worked with the vendor and insurance to have it replaced, and after yet more Covid delays the replacement arrived, also smashed in shipment. I was able to tape together enough pieces from the two parts to decide that the shape was going to be correct; however, I lost faith in the high temperature epoxy material – it was clearly too brittle. The correct material is ASA, but the quality of the first prototype was not good enough. In order to compete in online 3D printing services you need the lowest price, and there’s a big difference between how this first prototype was printed and how I would want it done. I talked to a few vendors in Australia, and it was clear that the cost would rapidly escalate into the thousands of dollars, for each printing, and I might want to do more adjustments yet. I decided to shelve the work, and ordered a commercial grade large form 3D printer. After months of delays, some again due to Covid-19, this thing will arrive here next week. The only problem is it arrives in 8 boxes and I have to assemble it. Once I get it up and running, I’ll be able to print the plenum as many times as I care to, with the quality I want, before calling it right.

I designed all the overhead panels, some auxiliary panels for headset connections etc, and settled on the replacement lower panel arrangement for the Aerosport 310, which was previously incorrect. I worked with AFS to get these designs finalized and the panels are being cut and printed at this time. In the meantime, I made some cardboard replicas and used these to do most of the overhead wiring. I’m not willing to post a photo of that.

I’ve been doing the wiring, which is a big job for an RV-10 made bigger by my choice of 2 batteries, 2 alternators, a 3 screen panel, A/C and SDS EFI. The wiring is about half done overall, and currently looks like a disaster. I started out wanting to simply do the wiring behind the baggage bulkhead and through the overhead, so that I could finish up the tail and put the final skin on. This plan rapidly devolved into an acceptance that I had to do all of the wiring, there are too many interactions to lock up one part before all parts of the puzzle are solved.

Associated with the wiring are some custom electronics. These pieces are:

  • Backup EFIS power. The backup EFIS power nominally comes from the second battery/alternator supply. However, what is the action in the case of an electrical fire and smoke starting to fill the cabin? It has to be “both masters off”, there’s no time to figure out which system is the problem. The engine will keep running because the EFI system is separately powered, but in order to keep minimal EFIS functionality, the regular backup battery must be used. So, I designed a board which will supply backup power to the AFS system, which will come from the secondary battery/alternator system if it is running, otherwise will come from a backup battery. Changeover is automatic, and the functions are ground tested as part of the runups.
  • SDS EFI power. Although the ignition systems, the two fuel pumps and the two ECU’s are on physically redundant battery+alternator systems, there is only one set of injectors so I needed what some would call an “essential bus” to run the fuel injectors. Any place physically redundant systems have to come together is a problem so I’ve applied some effort to this which I’ll post more about in a few months time.
  • Monitoring and logging. Between the extra TFT display on the panel, the EFI power system, the battery backup system and the A/C there are various sensors and data monitoring that I wanted brought back to a central non-essential point. A small embedded computer system sits under the pilot’s seat and has various communication methods to collect data and present information on the extra TFT display. This may eventually be a last resort backup EFIS, but not initially. I needed a set of interfaces for this embedded system that went beyond what is commercially available, so I’ve done a board for that as well. The prototype worked OK with a couple of jumpers and I’m re-spinning the board now.

In between all these activities, I found I was missing various miscellaneous/low-cost parts, which I’ve procured on a slow track with a few consolidated shipments from the US, again affected somewhat by Covid delays.

Reading through all of the above, it’s clear that I’ve done quite a bit over the past few months, but haven’t managed to actually finish anything. When I finally do manage to finish something, I’ll put up a celebratory post that will include some photos.

I have no idea how many hours I’ve spent on the above over winter – quite a lot – but I’m just going to log 100 hours because I really haven’t kept track of it all.

A/C Evaporator

I bought my A/C kit quite a few years ago from Airflow-Systems. I was shipped a so-called “Australian” evaporator, which is actually a product called a Monster Trunk System, part #685000-VUY from https://www.vintageair.com. There was a collection of metal parts and adapters in the kit, with no obvious way to set up the air flow and no instructions for this evaporator unit. The evaporator contains a 3 speed high volume scroll blower, which is ill suited to pressurizing the overhead console. Several other builders have supplemented this evaporator with an inline blower  which is more capable of pressurizing the overhead console. Yet another technique has been to forget about the overhead, turn the unit around and fit enough ducting to blow air straight into the cabin – see here.

I was already committed to a conventional mounting position, having done the inlet ducts and cutout for the overhead several years ago. What I needed to do was complete the evaporator outlet ducting, including an inline blower to suitably pressurize the overhead console, cabin flood air ducting, and a means to use the rear NACA vent air, via the Aerosport products NACA vent valve. This exercise is complicated by the fact that there isn’t a single right angle anywhere in the system, and it all rapidly turned into a 3D modelling exercise. First though, here’s a description of the evaporator inlet system I put together a few years ago:

The F-1006 bulkhead attachment for air to pressurize the overhead is a difficult area. In order to get enough air volume, a significant cutout is required. This mandates a doubler plate, and there is not much room to fit one. Edge distances, strength, clearances, Aerosport overhead console flange dimensions and screw positions for the rear baggage bulkhead all come into play. I wound up using both a shim plate and a doubler, in order to tie the doubler in with the rivets which secure the top of the F-1028 baggage bulkhead channel. Rivets for the doubler are flush on the rear side (where the manufactured heads are inside the overhead console air space) and flush on the front side where the baggage bulkhead overlaps the F-1006 flanges. I lowered and moved the normal position of the baggage bulkhead top screws, in theory they should have landed right in the middle of the Aerosport overhead lower flanges, in practice they are a little above this point but still easy enough to install.

This whole area is so busy, it is difficult to find a good way to “attach” the required air duct(s) to the bulkhead. It’s also not reasonable to have hard attachment points between the bulkhead and the evaporator/shelf, due to vibration and cracking. The idea of this design is to use a 3D printed bulkhead attachment block to achieve the following:

  • It can be fitted to the F-1006 bulkhead after the top skin is riveted on, sealed with some form of gasket material, and brings the air duct attachment plane clear of the bulkhead.
  • Two long #6 screws act as locating pins for the flange of the main attach duct.
  • A thick/soft gasket or manifold can be used between the rear of this bulkhead attachment point and the main duct, to seal airflow and provide vibration isolation.
  • If (when) the evaporator arrangement changes, a new 3D duct can be printed to mate with the existing bulkhead attachment point.

I use only one hole for airflow into the overhead, the side with more area (the F-1028 is offset from the center). Manifolding air into both sides complicates things and is pointless – what matters for overhead air is pressure, not volume. I wanted electrical connections into the overhead as well, so these are on the right hand side of the bulkhead, and will be sealed off in the overhead.

  • f20a
    Cutout for overhead air feed, with shim and doubler plates
  • f20b
    Holes for electrical conduits into overhead, with doubler plate
  • f20c
    3D printed template for bulkhead attach block, to verify it clears all obstacles.
  • f20d
    3D printed ABS bulkhead attach block, bulkhead side.
  • f20e
    Bulkhead attach block trial fit. Installation can only happen after top skin riveted on.
  • f20f
    A/C evaporator on shelf, with 3D printed inlets.
  • f20g
    3D printed inlets are contoured to match the curved front surfaces of the evaporator.
  • f54a
    Production inlet parts, made from tough epoxy, to replace the ABS prototypes
  • f54b
    Production inlet parts in place
  • f54c
    Cutouts done, nutplates in place
  • f54d
    I added a stiffener to the front face
  • f54e
    Fitting a rubber seal over the original evaporator inlet
  • f54f
    Cover plate in place, screwed on. Rubber pads on front of inlets, riveted on and sealant applied
  • f54g
    Cutout in evaporator shelf for receiver/dryer
  • f54h
    Clamping shroud for receiver/dryer

For the evaporator outlet, I designed a manifold which caters for the following requirements:

  • Fits onto the two irregular shaped outlets on the evaporator, with a simple rubber seal and some screws.
  • Provides an outlet for the inline blower. I used a 4″ blower, because it fits. A 3″ blower would probably also be adequate.
  • Provides a pair of outlets for cabin flood air. These should probably be 2.5 inches each, I used 2 inches because that’s the attachment size I have room for on the front (top) bulkhead.
  • Provides a pair of inlets for the Aerosport NACA vent valve, to feed vent air from outside into the system
  • Provides a place to mount a temperature probe
  • Can be assembled in-place, or if necessary by lowering the rear edge of the evaporator shelf (after removing the support).

The following pictures show what I came up with. I had the prototype fabricated in tough epoxy, and it fitted fine except for an indentation on the top that I made to clear the top stiffener. For some reason my measurements were off, and the indentation missed the stiffener by 20mm. I fixed this up and made some other improvements, and just ordered the final version which should arrive here in another week or so.

The 4″ blower just fits in the required space. I’m mounting it to a metal bracket that will be riveted to the cover plate I made up for the evaporator inlet. Also mounted on this cover plate are three relays (for the scroll fan) and a pwm controller for the inline blower. A wiring harness for this can be seen in the pictures, not properly laced up or secured yet. A high side pressure sensor, and evaporator air outlet temperature sensor, are included. The system controls will be on the overhead, except for the master “A/C on” switch which is on the front panel, pilot’s side. Turning the A/C off (before rolling) will be on the pre takeoff checklist, if necessary it can be re-engaged at some point during climb out. Part of the wiring includes a connector that could be used for a micro-controller that would be capable of climate control, if the rotary switch in the overhead is set to the “auto” position.

The final duct is to go from the outlet of the axial blower to the overhead. I printed some prototypes for this on my own consumer grade 3D printer using a flexible material. Once the shape was correct, I decided to order the production part in SLS Nylon. This will be very strong, but will still have enough flex to effectively detach the evaporator/blower assembly from the airframe. Although I will be assembling all of the final components with the top skin still partly open, everything is designed to be removable and reassemble-able after the skin is in place. It won’t necessarily be pleasant working back there in the hell hole, but it can be done. For assembly, I’m going to take advantage of the skin being off and will cheat as follows:

  • With everything in the tailcone finished, and with the evaporator/shelf removed, cleco the top skin on in its entirety. With Rosie outside on the rivet gun, and me inside, we’ll rivet the holes across the front and towards the rear on each of the three stiffeners.
  • Remove all the remaining clecos, allowing access from each side.
  • Fit the bulkhead adapter and the (flexible) duct from the inline blower outlet to the bulkhead. This can’t be done until after the (above) rivets are set.
  • Install the evaporator, shelf, outlet manifold, inline blower, NACA vent valve etc, using the access from each side to make the job easier this first time.
  • Fit the remaining refrigerant hoses etc. and charge the system. I plan to use an electric motor with a grooved pulley and a long serpentine belt as a means to run the compressor for this step.
  • Check for leaks and proper operation.
  • Cleco the skin back up, climb inside and finish riveting on the skin.
  • evap_out-1
    Evaporator outlet manifold
  • evap_out-2
    Top view
  • evap_out-3
    Bottom view
  • evap_out-4
    Right side view
  • evap_out-5
    Left side view, small hole is for temperature sensor
  • evap_out-6
    Front pump, vent attachments
  • evap_out-7
    Rear vent attachments, slot for top stiffener
  • evap_out-8
    Evaporator and vent attachment points
  • f56a
    Back from the printer
  • f56e
    Left side, temperature sensor
  • f56f
    Temperature sensor
  • f56g
    Aerosport NACA vent valve in place
  • f56h
    Left side cabin vent scat tube attachment point
  • f56i
    Right side clearances OK
  • f56j
    Top view
  • f56k
    There's just one problem....
  • f56b
    Bulkhead adapter 3D printed in tough epoxy
  • f56c
    Bulk attachment point
  • f56d
    Bulkhead adapter in place
  • ba2
    Bulkhead adapter design
  • ba1
    Bulkhead adapter, left side view
  • ba3
    Bulkhead adapter design
  • f56l
    Printing the bulkhead adapter prototype
  • f56m
    Finished bulkhead adaptor prototype
  • f56n
    TPU is very flexible
  • f56o
    Top view with bulkhead adaptor prototype in place
  • f56p
    Checking bulkhead adaptor prototype fit
  • f56q
    Side view of bulkhead adaptor prototype



Inlet plenum progress [2.0 hours]

Construction work has been spotty for the past few months due to some work commitments. Time to catch up on a few posts.

I received the prototype 3D printed inlet plenum, after a very long delay caused by Covid-19, a shipment lost in customs, reprinting a replacement, more shipping delays etc. This part was printed in ASA material by a vendor in Sweden. Quality is good apart from some areas where the wall thickness should have been greater.

The part fitted perfectly around / through the compressor, the air filter shroud mount etc. The inlet ramp was also good, perfectly horizontal and lined up with the opposite side (standard Showplanes fiberglass plenum) within 1mm, which is good enough for me. The front edge of the inlet ramp is too far forward, requiring me to trim too much of the cowling. While it would work, it doesn’t leave as much of the cowling inlet hole as I’d like, to get nutplates and some sort of overlapping seal in there. This is one of the areas where the wall thickness is also a bit low. Clearance on the bottom side to the cowling is good, a bit over 1/8″ at the closest point. The rear scat tube connection for a heat muff is also good.

Overall, close to a hole in one which I’m relieved about given how complex the part is. I can now proceed to finish the front of the baffles and around the governor. I’m going to need to address the wall thickness issue and trim back the front edge, which means re-printing the part. These are simple adjustments but I’m going to also look at whether any better alternatives exist than the ASA material I’ve used.

It’s really hard to see much from the pictures, because the 3D printed part is black, but they show the general idea.

  • f55a
    Prototype inlet plenum in place
  • f55b
    A/C fitting clearance OK
  • f55c
    A/C fitting clearance OK
  • f55d
    Prototype inlet plenum in place
  • f55e
    Good fit around A/C and into air filter shroud
  • f55f
    Good fit around A/C and into air filter shroud
  • f55g
    Horizontal, and about 1mm below right hand side plenum - good enough


Tunnel heater hose [6.5 hours]

Some time ago I added some brackets to the front of the tunnel, so I could secure the rear heater hose. With Control Approach rudder pedals, the hose needs to be secured in the center of the tunnel, clear of the control arms off to each side of the tunnel. I was going to use a 2″ Adel clamp around the scat tube, based on what another RV-10 builder had done.

After assembling this, I didn’t like it because:

  • It was difficult to install the Adel clamp, while lying on my stomach with the seats removed and reaching in under the panel. The rear heater hose has to be sort-of scrunched against the short front heater hose in order to get it positioned in the middle of the tunnel.
  • The large Adel clamp, held by a single bolt, was not very secure and could have a tendency to rotate over time
  • If anything came undone over time, the compacted rear heater hose would push loose items towards the rear, straight into the rudder pedal arms.
  • I still had to come up with a solution to replace the standard F-1051J Scat tube support, since this support interferes with the internally run rudder cables when the Control Approach rudder pedals are used.

After a few minutes pondering these problems, the solution hit me – design and 3D print a pair of Nylon brackets to retain the scat tube. The brackets then simply slide onto the scat tube from the rear. For the front bracket, I bolted the Nylon piece to the Aluminium angle retainer on the bench, slid it onto the scat tube, lifted the rudder pedal arms, positioned the bracket assembly and screwed it into position. I also drilled a pair of small holes into the Aluminium angle in order to add a safety wire each side, that way if the brackets ever came loose for any reason, the assembly could not fall aft and interfere with the rudder pedal arms.

For the aft bracket, I had already a long time ago drilled and dimpled the holes on the right hand side of the tunnel for the standard F-1051J scat tube bracket. The lower of these two holes is close to the right hand rudder cable. It would have been better to raise this hole by about 1/2″, but that is ancient history. I resolved this by using a low profile (AN364) lock nut and embedding the nut into a hexagonal cutout in the bracket, as shown in the pictures. I used a pair of 0.063″ shims on each side, with the holes countersunk, to complete the assembly.

It all worked great, both brackets can be easily removed and reinstalled, so any future maintenance that requires removing the rear heater hose for better tunnel access will be easy.


A couple of other RV-10 builders have asked me for the models, and one questioned why I elected to use the metal shims on the aft bracket. I used the shims simply because I didn’t think my consumer grade 3D printer could do a good enough job of the countersinks, when printing them in Nylon, vertically. In any case, I added an option to the model to have no shims, which widens the aft bracket to compensate for the missing shims, and adds countersinks to the sides to allow for the #8 dimples in the tunnel walls. I’ve added pictures of this version. The three STL files can be downloaded using the following link:



  • front_heater_bracket
    Front scat tube bracket
  • aft_heater_bracket
    Aft scat tube bracket, replaces F-1051J
  • f47a
    Front Scat tube bracket assembly
  • f47b
    Aft scat tube bracket, replaces F-1051J
  • no_shim1
    Aft bracket "no shim" version, with included countersinks
  • no_shim2
    Aft bracket "no shim" version, with included countersinks
  • no_shim3
    Side slopes to match tunnel

3D printed system brackets [3.0 hours]

Under the front seats, there are four “systems brackets”, F-1084A/B, which have slots for a fuel line, brake line, and electrical wiring. In my case, there are two fuel lines, so one issue is how to deal with the return line. Another issue is the fact that s/s braided teflon lines are different in diameter than the Aluminium tubing lines that the system brackets were designed for. There is apparently enough scope to squash them in with a sliced apart grommet.

None of this sat well with me, and it was a simple matter to design a replacement upper bracket section and 3D print it in Nylon. There are two right-hand and two left-hand brackets, which have snap-in rings for two fuel lines, one brake line, and electrical wiring. Each ring has a slot for anchoring a cable tie, or waxed string tie, if needed. Each part takes about two hours to print, and bolts straight onto the standard lower systems bracket F-1084A. If in the future I need to make a change, I can simply print up new brackets and bolt them in place.

The brackets weigh 7 grams each. This replaces the metal upper section (F-1084B) and three snap bushings, which weigh a total of 9 grams, so there is no weight penalty in this change.

  • f15a
    System bracket design
  • f15b
    System bracket design
  • f15f
    Part being printed
  • f15g
    Printing complete
  • f15c
    Parts replaced weigh 9 grams
  • f15d
    Nylon system bracket weighs 7 grams
  • f15e
    Plenty of stretch in Nylon support rings
  • f15h
    Four system brackets
  • f15i
    Bolts to standard metal parts

3D printing an aircraft

The conventional method for wing wiring in an RV is to run a corrugated conduit from wing root to wing tip. In accordance with Van’s recommendations, I’ve enlarged the tooling hole in each rib, and will run 16mm conduit through this hole, secured in place with RTV. Many find that an extra conduit is required, and run it through rib lightening holes. The issue is how to secure this second conduit. The lightening holes in the wing ribs have an arc shaped recess next to them, so I can’t use the Panduit fittings I used in the empennage. I didn’t like Van’s suggestion of drilling a #30 hole and using a cable tie around the conduit. I looked around for a commercial fitting, but couldn’t find anything suitable.

This problem led me to design and 3d print a suitable fitting, custom made for RV-10 wing ribs. The pictured design holds a 20mm conduit, and has two smaller holes for either air lines (e.g. Angle of attack from pitot) or RG-400 cable (for antennae mounted in the fibreglass wing tip). There is a slot for a cable tie, and the fitting nests into the arc around the lightening hole. It is held in place with two pop rivets, and these are positioned so as to cause no interference with a bucking bar held blind inside the wing, since this is how the bottom skin must be riveted. There is a left and right (mirror image) version of the fitting for the respective -L and -R wing ribs.

Common materials used on consumer grade 3d printers are not suitable for this application. PLA has a low glass transition temperature (Tg) of 66 degrees C, so the fittings would easily deform inside a wing parked in central Australia. ABS has a much higher Tg, and is strong enough, but has poor chemical resistance. An AvGas leak working around inside the wing would cause the part to degrade.

The material I’ve used is Nylon – specifically this product. It is very strong but still flexes under load, has a suitable high Tg of 82 degrees C, and very good chemical resistance. It’s harder to use on a consumer grade printer, but once suitable settings are established, the results are excellent and very repeatable. You wouldn’t use this process for anything structural on an aircraft, but I’m sure there will be plenty of other applications for a 3d printed Nylon part before I’m done with this project.

It takes about an hour to print each part, and I need 30 of them. They weigh 4 grams each.

Postscript: The conduit clips work great, I’ve added some pictures of the assembled wing with conduit in place.

  • wing_clip1
    Upper side of conduit clip. One rivet hole is recessed so that an LP4-3 rivet has enough depth to set properly
  • wing_clip2
    Rib side. The "bump" fits into the recess around the rib lightening hole. Slot on right hand tab is for an optional ...
  • w19c
    The Nylon filament
  • w19a
    3d printing in Nylon
  • w19b
    Part nearly finished in printer
  • w19d
    Part complete, ready to pry off the bed
  • w19e
    Position of part on bottom side of W-1011L wing rib
  • w19f
    Position of part, showing 20mm conduit, AOA air line, and optional cable tie
  • w38a
    Right wing gap fairings complete, wiring conduit fitted
  • w38b
    Wiring conduit in right wing
  • w40e
    Conduit, pitot, AoA lines in left wing
  • w40f
    Checking pitot mast arrangement in left wing