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.

PostScript:

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:

Download

 

  • front_heater_bracket
    front_heater_bracket
    Front scat tube bracket
  • aft_heater_bracket
    aft_heater_bracket
    Aft scat tube bracket, replaces F-1051J
  • f47a
    f47a
    Front Scat tube bracket assembly
  • f47b
    f47b
    Aft scat tube bracket, replaces F-1051J
  • no_shim1
    no_shim1
    Aft bracket "no shim" version, with included countersinks
  • no_shim2
    no_shim2
    Aft bracket "no shim" version, with included countersinks
  • no_shim3
    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
    f15a
    System bracket design
  • f15b
    f15b
    System bracket design
  • f15f
    f15f
    Part being printed
  • f15g
    f15g
    Printing complete
  • f15c
    f15c
    Parts replaced weigh 9 grams
  • f15d
    f15d
    Nylon system bracket weighs 7 grams
  • f15e
    f15e
    Plenty of stretch in Nylon support rings
  • f15h
    f15h
    Four system brackets
  • f15i
    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
    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
    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
    w19c
    The Nylon filament
  • w19a
    w19a
    3d printing in Nylon
  • w19b
    w19b
    Part nearly finished in printer
  • w19d
    w19d
    Part complete, ready to pry off the bed
  • w19e
    w19e
    Position of part on bottom side of W-1011L wing rib
  • w19f
    w19f
    Position of part, showing 20mm conduit, AOA air line, and optional cable tie
  • w38a
    w38a
    Right wing gap fairings complete, wiring conduit fitted
  • w38b
    w38b
    Wiring conduit in right wing
  • w40e
    w40e
    Conduit, pitot, AoA lines in left wing
  • w40f
    w40f
    Checking pitot mast arrangement in left wing