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.
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.
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.
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.
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.
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:
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.
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.