Rear avionics shelf done

Yep, finished up this shelf today, for the most part working in half-hour shifts before retreating to the air conditioning. Houston summer has definitely arrived in full force.

After cutting another section off my wing skin to start with, I laid out the cuts. The nice thing about using a dedicated form block like I did in this case is that it can double as a template – just lay it on the sheet and trace around it, and since my flanges are intended to be 3/4”, the same as the block thickness, the flanges can be traced using the edges of the block. Much faster than doing a bunch of careful measuring. I also laid out my tooling holes in the block and pre-drilled them, so I could use the block to drill those in the pieces and ensure that both parts could be attached to the block.

Some time and a bunch go cutting and filing later, I had the sheet pieces ready for forming:

From there, it was just a matter of clecoing the sheet to the form block, clamping it in the vise, and applying some strategic physical violence with a hammer. The only hangup here is that thank to the material’s springback, I can’t quite get a 90° bend with this block. That would require relieving the edges of the block a bit, and then wouldn’t be symmetrical and thus wouldn’t work for these pieces.

One mostly-finished wing piece:

After squaring up the flanges, it was finally time to cleco the assembly together and see how it fit. Seems decent:

After that there was just laying out the drilling the rivet holes, and eventually squeezing rivets to assemble everything. After a test fit in the fuselage, I needed to work on the outside edges a bit to match up to the fuselage taper, but eventually it fit like a glove:

And here it is with the cardboard unit mockups sitting roughly in their planned homes:

I still haven’t determined if the remote com radio will be located here or behind the panel up front. I realized yesterday that when I was mocking up stuff for the forward avionics shelf, I completed forgot about the ECU unit for the SDS CPI2 ignition. Granted, I haven’t gotten anywhere close to having a real layout for that shelf, so it’s not a huge problem, but it is another thing that could make me decide to move the radio aft.

I’m not convinced that’s going to be necessary, though. I have the beginnings of an idea for basically a two-level setup behind the panel, but it’ll require some tinkering to see if it’ll work. The general idea is to have two low-mounted shelves on the right and left, which will be hinged at their outboard ends and can be swung down beneath the panel for access. The fuse blocks could go on the left side, and the CPI2 controller unit on the right. (I want this to be somewhat-easily accessible, since it has some fuses in it that might need service in a pinch)

Above this would be another flat shelf, high enough to clear the units on the hinged shelves below, and that could house all the other units, stuff that can be less easily accessible. Getting to those would require pulling the EFIS units and/or the entire center panel.

My fuse blocks should come in from Stein Air tomorrow, which will help me start to determine whether this idea is feasible or not. Coming up: lots more photos of cardboard and cheap plywood mockups, I suppose…

Posted in Avionics, Fuselage | Hours Logged: 2

Avionics shelf: the next generation

Alright, so here we are looking at v2.0 of this oddly-shaped shelf. Yesterday was kind of a bust overall, but it eventually got me looking at this problem in a different light. Most of the challenge with this shelf is the way the aft portion sort of wraps around the bellcrank area, and trying to work that with aluminum angle wasn’t doing too great. But last night I eventually concluded that using angle was overkill anyway.

So today I started on a new shelf, this time just using aluminum sheet with formed flanges to provide stiffness. This was also the beginning of a new approach to handling the odd shape. Now, I could have done this all as one piece, but it would have been a bit complicated, especially when it came to forming the aft edge. So instead I decided to take a piecemeal approach. To start with, I’d form a small shelf that went straight across the fuselage, and then form two separate “wings” for the angled portion, which I could then rivet onto the main shelf.

I started with the simply part today – the straight “main” shelf. Material was donated by a surplus wing skin, which was damaged years ago during our move from Atlanta. It’s been just sitting around waiting for the day it was given a new purpose. Basically, I cut off one bay of the wing skin to start with, the used my plywood shelf as a template to get the angles right where they’ll meet the longerons, and finally added allowances for a forward and aft flange. Then there was just lots of cutting and filing and fine-tuning and cleaning up edges, before it was finally time to do some forming:

The tooling holes being used to hold the piece on the forming block are in the locations where screws will attach the shelf to the longerons, but just drilled to #30 for now. (I didn’t want to go look up the right drill size for the screw, I’ll figure that out later)

Getting the whole thing cut out and formed was about an hour and a half of work, but the end result fits nicely in the fuselage, just ahead of the bellcrank:

After attending to other stuff in the afternoon, I found a bit of time after dinner to pick up where I left off and get started with the “wings.” The main shelf didn’t really need a dedicated forming block, since the edges were just straight, but the wings are a different matter. The good news is that since they’re mirror images of each other, I only need to make one block.

Here again, the plywood mockup served as a pattern, though I had to make some slight concessions due to the dimensions of scrap wood I had on hand. Basically, I’ll lose about 1/4” off the back of the wings – which should be no big deal. I decided to call it a night once the block was done, before getting into cutting up more sheet. Here’s a look at the block alongside the main shelf, showing where it fits. The main shelf and wings will simply be riveted together through the mating flanges:

Tomorrow: more cutting and hammering…

Posted in Avionics, Fuselage | Hours Logged: 2

Trying to make an avionics shelf

So when I got started nice and early this morning, I decided to go ahead and start fabricating my aft avionics shelf, and I figured I’d start with the most complex part of the thing – the carefully-bent piece of angle to support the sort of wraparound shape of the back of the shelf.

So I cut off a chunk of angle long enough to do the deed, plus some extra because I’m not a complete idiot. Then I used the plywood mockup I had to mark where my bends would be, and cut out notches to allow for those. Those were rough cut on the band saw and then refined with files, and then the bends could be made. It ended up nicely fitting with the template:

Next, I used the template to trim the ends to length, and all I had left to do was to remove one leg of the angle to leave mounting tabs that could bolt to the longerons. And that’s where this went sideways – I set up the trim wrong, and ended up mangling the ends. Welp…learning experiences abound. So I cut off another chunk of angle and started over again. This time I got the mounting tabs done right, and the piece ended up fitting nicely in the fuselage:

Well, except for one thing. I neglected to account for how, where this piece crosses the fuselage centerline, there’s the beginnings of the elevator bellcrank mount, which sticks up enough that the center of the angle contacts it:

I made an attempt to cut a small relief in the angle there for clearance, removing about 3/16” of the 1/4” angle, but it still contact lightly, though it at least sits level now. Still, I’d probably need to make that relief cut twice as deep to ensure no contact, especially under loads in the air. At that point, I’d have removed a lot of the rigidity provided by the angle here, and right at midspan, where I most need that rigidity.

So long story short, I think I’ve got to go back to the drawing board here. What I may end up doing is figuring a way to tie into the nutplates on the baggage ribs, which are there to support a rear battery box, if installed – but mine is going up front. That would at least provide some nice support to the center of the shelf, but making everything line up properly will likely be a challenge.

So yeah, time for some more thinking.

UPDATE: After mulling over this some more, one conclusion I’ve come to is that I am seriously overbuilding this avionics shelf. Basically, my planned structure would consist of aluminum angles to bridge across the fuselage, with sheet rivet to those angles to form the shelf. However, the upper baggage shelf – which will sit right above this location – has no sort of steparate stiffeners whatsoever. It’s just .025” sheet, with a little rigidity provided by way of bent flanges forward and aft.

Van’s doesn’t list a specific max weight for this shelf, but for the sake of comparison, everything I’ll be putting on my avionics shelf is just over three pounds. If I were to end up putting the com radio back here, it’d go up to about 3.75#. I’m sure that baggage shelf can support well over that!

So with this in mind, I’m going to start with rethinking my shelf and just using sheet aluminum with bent edge flanges for lateral support, instead of the complexity of bending angle in fancy ways. I’ve already got an idea how to use this approach to build my strangely-shaped shelf, too. Maybe I’ll give that a try tomorrow.

Posted in Avionics, Fuselage | Hours Logged: 3

Even more systems planning

Pretty much the same story as last time. Tonight I made up cardboard mockups of the remaining units I need to plan for, and which I at least sort of intend to put behind the panel. Then I got to looking at the area behind the panel with an eye towards making up an avionics shelf. What makes this interesting i that there are two braces that go across the cabin, tying together the tops of the gear towers. These seem like great starting places for a shelf, except one is higher than the other for some reason.

Still, it seems like people are using these for their shelves; one photo I found simply makes the shelf sort of L-shaped, placed at the level of the lower brace and then having a vertical element to tie into the higher one. I’m tentatively leaning towards just having a sloped shelf going between the braces. I kind of like this better than the L-shape thing, and I think it’ll actually make the stuff back here a little easier to work with for maintenance; I can either remove the EFIS units or the entire panel center section, and then have the shelf stuff presented nicely to me as I sit in the seat.

Anyway, for the time being I cut out a piece of cardboard roughly the size of the shelf and tried laying stuff out on it. Everything seems workable off the bat, though I haven’t carefully accounted for the space needed for connectors. I’ll likely end up with some things mounted against the baggage bulkhead as well, most notably the two voltage regulators (not shown in the following photo).

So none of this is final by any means, particularly since I need to account for those connectors. I guess maybe I should make up cardboard versions of those as well; it feels a little weird but this is definitely something I need to take into account. Mostly now I just get to move stuff around over and over again.

In other news, I’m continuing to accumulate parts. Got some tailwheel stuff coming in tomorrow, just ordered my fuse blocks, and it would appear that the prop governor I ordered via a VAF group buy is about to ship. Oh, and I might have a line on a slightly-used exhaust system for about half the price of a new one. Not super confident that one will work out, though – it’s the Vetterman system I want, but it’s for an IO-360 in an RV-4. Part of me wants to think that the packaging in the cowling can’t be that different, but since Vetterman has different part #s for the two exhausts, clearly there are some differences. But maybe the RV-4 exhaust can be made to work…the savings would certainly be nice.

Posted in Avionics, Fuselage | Hours Logged: 1

More systems planning

Another day spent figuring out where to put things. Today’s focus was figuring out what to do for an aft avionics shelf, something that was a lot more challenging than it seemed like it ought to be. For whatever reason, I had very little success finding photos of avionics bays in RV-8. I had the idea of putting a shelf just ahead of the elevator bellcrank, just behind the baggage bay, and I found a photo of someone who’d done that, but it just didn’t seem workable to me, at least taking the simple approach of running two pieces of angle laterally between the longerons to serve as the edges of the shelf.

The problem is that doing that only gives a few inches of depth on the shelf, and both the ELT and transponder are much longer. This seemed problematic enough that I tinkered around with several other ideas, most notably putting the shelf just below the ADAHRS shelf, bridging the middle longerons. This location would provide good access through the baggage bulkhead, but it would put those avionics far too close to the ADAHRS units. Since those include magnetometers (read: compasses), they need to be away from basically anything with a power connection.

So putting the avionics here would require either relocating the ADAHRS, or else additionally using a remote mount magnetometer. Any other ADAHRS location would have been hilariously inaccessible, and the remote magnetometer seemed like obnoxious extra work, though I still considered it as a backup plan.

Finally, I came up with an idea to get better shelf space in the original location. Instead of using two pieces of angle oriented perpendicular to the long axis, I could only have the forward angle like this, and at the rear, have two angle pieces…um, angle in…from the aft bulkhead to a point forward of the bellcrank. This allows for a lot more depth where needed for deep items, even if it does look a little weird.

The piece of plywood I used to mock this up actually has the cutout around the bellcrank a fair bit overdone. More realistically, either I’ll have the two angled lines simply meet at the centerline, or else add a short lateral piece at the place where the angles meet the cutout below:

This location provides plenty of depth for the transponder (left) and ELT (right), as long as they’re located outboard. Also shown here is a 2D representation of the ADS-B receiver. This location also provides ample space forward of the units to allow for wiring connections and harness routing. Originally I was going to mount the remote com radio back here as well, but since both the control head and the antenna will be located much further forward, I’m going to prefer putting that behind the panel if possible. If necessary I can still probably fit it back here, too…maybe over beside the transponder.

Next up will be figuring out some kind of avionics shelf behind the panel. I’ve got a couple ideas for that area, none of which I’m super enthused by so far, so that’ll probably end up being another bunch of hours spent puzzling over things. I think I’m also going to go ahead and order a few items that I’ll be finding homes for, since they’re relatively inexpensive – I’m specifically thinking of the fuse blocks I’ll be using behind the panel.

Posted in Avionics, Fuselage | Hours Logged: 4

Systems planning

For the time being, I’m stepping away from the pitot layout stuff, in favor of a more general look at systems. As I alluded to previously, I’m getting to the point where it’d be nice to have some electrical supplies on hand for stuff like extending the pitot heat harness. But if I’m going to order wire, I’d like to go ahead and do it in bulk, so I’m pivoting to working out my avionics layout so I can then at least rough out all the harness routing and then get a materials estimate so I can order a whole bunch of wire and stuff.

So I started actually a couple nights ago with the arts & crafts portion of this program – making mockups out of cardboard for assorted boxes. At first I was just going to sorta mass-produce mockups of everything, but then it became clear that in most cases, I don’t need a full three-dimensional mock; just a flat box with connection locations identified is probably good enough.

In any case, after making flat mocks of both voltage regulators, I moved on to the ADAHRS units. For these, a three-dimensional mock was called for, since vertical clearance will matter, especially for maintenance accessibility. Then, after making these, there was kind of a pause for some documentation reading and plans changing.

Both ADAHRS units connect to the Skyview system using the Skyview network connection. My original plan was to run a single network cable back to the tail, then add a splitter cable to accommodate both ADAHRS units. However, I’ll also have the pitch servo for the autopilot back in this same area, which also accepts a Skyview network connection, albeit with a couple of pins modified. So if I use the splitter, I’ll still have two cables running back here; I could change that by using a Skyview network hub, which allows tying together up to five devices. I got curious as to how much more expensive the hub was than the splitter cable, and discovered…it’s actually cheaper. Huh.

So yeah, I’m going to put a hub back here, and then I can essentially run a single network cable for everything in the tail, and then branch out from the hub. That, in turn, gave me something else to make a mock for (though just a flat piece in this case).

Finally, I wanted to mock up the shelf on which the ADAHRS and the hub will live. Cardboard didn’t seem like it’d cut it here, but I had some thin plywood scrap that would work well. I ended up having to cut it with a hand saw, since my bandsaw kept throwing the blade (seems the lower wheel needs a new tire). But with that done, I could try laying out the units here to see how it fit together.

Here’s the shelf in place. This is just behind the aft baggage bulkhead, which is convenient for maintenance. Accessing these will only require removing that bulkhead, which is just held on with a few screws. (This glosses over the part where I’ll have to be lying inside the fuselage. The word “convenient” is kinda relative here.)

Anyway, with the bulkhead out of the way, I’ll be able to easily get in with a screwdriver to remove a unit for service, and having the hub here will make it easy to add service loops on the network cables. Since the cables attach to the backs of the units, this will allow me to detach a unit, then remove the network cable after pulling it out of the bay a bit. The next photo shows what this looks like from the baggage area, just forward:

I figure the network feed cable from the forward fuselage will come back along the left side somewhere, then climb up the bulkhead to connect to the network hub. From there, another cable will descend back down the bulkhead to the pitch servo. The two cables to the ADAHRS will just have a short run.

On the front of the ADAHRS units, the pitot/static/AOA lines will connect. That’s the one thing I want to think about before I commit to this – what will that plumbing look like? Each of those lines will need to be teed somewhere in this area to connect to both units. These lines will also come from forward on the left side of the fuselage, since the pitot/AOA lines will be coming from the left wing. The static line will probably feed from aft – I need to double-check where the static ports will be located.

Next up will be planning the rest of the aft avionics. Somewhere back here will live the com radio, the remote transponder, the ADS-B receiver, and an ELT. I suppose it’s time to go look at other people’s builds to see what they did for mounting those items. A shelf bridging across the lower longerons seems like a decent approach, but it’s a long span and would need to be pretty well stiffened, especially to mount the ELT.

Decisions, decisions…

Posted in Avionics, Fuselage | Hours Logged: 2

Pitot fittings; heat control packaging

Nothing too exciting tonight. I decided to keep working on pitot stuff, possibly going as far as getting the soft lines routed in the wing and prepositioned. First order of business was to trim the hard lines on the pitot and get them flared for the AN fittings. This sounds fairly simple, but it’s been a long time since I did any flares, and these are kinda critical, since they’re attached to a rather expensive piece of equipment. Put simply, requiring several tries to get these right wasn’t going to fly here, and so I did several practice flares using the pieces of line I’d cut off, and only then went on to doing the real thing.

And here we go, a pitot tube with the fittings in place. They’re not final-torqued or anything, just kind of there for now:

From there, I started thinking about what I’m going to do with the heater control box. The original plan had been to affix this to the outboard side of the bellcrank bay, where it’d be easy to access if necessary. That’s not quite as simple now, since I put the pitot tube one bay further outboard than planned – the wires between the box and the tube aren’t long enough to stretch across two bays. So that leaves me with a couple of choices. First, I could position the box one bay further outboard as well, retaining the ability to use the wires as provided, but making access far more annoying if I ever have to remove or replace it. Second, I could position it in the original spot, and extend the wires somehow.

If I do extend the wires, there’s the question of how I do that. Simply cut them and butt splice in wires, so I can retain the existing terminals? That ends up making maintenance still problematic, since I’d need to route that extended harness in such a way that it’s properly secured, but still removable without a ton of problem. Not quite sure those two goals can be easily reconciled.

Well, maybe I just create an intermediate harness. That does introduce more connectors, and thus more potential sources of failure, but at leas it mitigates maintainability. But using the same terminals on the harness is a problem. The spade connectors for the high-current wires are easy enough, but the two temperature sensor wires use a micro Molex plug, for which I’d need a special crimping tool.

Anyway, I’ll refrain from going through my entire thought process here, but this is pretty much where I’ve landed: I want the control box accessible, which means extending the harness. Since I’m already extending the harness, and since the pitot is now effectively at the halfway point between the bellcrank bay and the wingtip, I’m going to put the box at the wingtip. This way, it’ll be much easier to access for maintenance – I can just remove the wingtip, and have much better access to the harness routing than if I had to work through the access cover.

To extend the harness, I think I’m going to make an intermediate harness, which will be semi-permanently installed in the wing. I’ll use spade terminals for the high-current wires, and for the small temp probe wires, I’ll use a trick from SteinAir, where you put D-sub pins on each wire, and wrap them in heat shrink after connecting, thus making a secure but still detachable connection. These pins will work well for the tiny wire used for the sensor.

Of course, since I don’t really have any wire on hand, I suppose this means I won’t be finishing this harness any time soon. I’m starting to think I should pivot from wing stuff to really getting going on systems, so I can plan out harnesses and put together a big order of wire and other electrical stuff. So I dunno, maybe tomorrow I’ll start making mockups of boxes. It’s a good time to do so, we’ve got a fair amount of cardboard stacked up and awaiting a trip to the recycling center…

Posted in Wings | Hours Logged: 1.5

Fitting the pitot mast…with extra fun

So…last night, I finally felt I’d put to bed the whole endless debate of where to put the pitot mast. Tonight, it was time to finally put an end to the thinking and get started on the doing. Amusingly, as we’ll see later on, I may have overdone the “put an end to the thinking” aspect.

The fitting of the mast is primarily done with the wing skin – locating and drilling the rivet holes between the skin and the base, and cutting the hole that the mast actually pokes through. For this, Dynon provides a nice little paper template. The instructions for positioning it, though, I wasn’t a huge fan of, at least for this application. They want you to draw reference lines on the skin and use those to position the template, but the matching lines on the template don’t go to the edge. I guess I could have trimmed the template, but…well, I didn’t.

Instead, since the existing holes between the skin and spar are kind of the starting point for the location, I used those to locate the template, holding a flashlight under the skin so I could spot the holes against the template and get it taped in place:

Next, I punched the rivet hole locations, along with the two holes that form the ends of the teardrop-shaped skin cutout, then used a razor to cut out the teardrop portion so I could trace it onto the skin with a sharpie:

At this point, the template can come off. First, the rivet holes are drilled in the skin:

Followed by the two holes to start the cutout:

Then it was time for lots of trial and error. I roughed out the cutout with a cutoff wheel in the dremel, then switched to a cutting bit to work out to the marked line, and then spent a lot of time slowly removing material with files until the mast would just slip through the cutout.

Next up was using the skin holes to drill into the mast base itself, and this meant clecoing the skin to the wing and slipping the mount in place. And this is where things got interesting. As soon as I had the skin in place, I saw the absurd mistake I’d made. The pitot was intended to be mounted at the outboard side of the bay just outboard of the bellcrank location. But I actually cut the hole one more bay outboard:

Now, you might think that I’d be pretty annoyed upon discovering this, but honestly, my initial reaction (and still my feeling right now) was amusement. All that time I spent trying to really carefully consider the location, to think through all the packaging, being really sure about it, and then cutting the hole in the wrong place. You can hardly write comedy that’s better than this.

Granted, one reason I can take this so lightly is because I decided to make the pitot attachments serviceable outside the wing. If I’d needed to service (ie connect/disconnect) those inside the wing, this would have been a pretty obnoxious mistake, as it would have been nearly impossible to get to those fittings. Here, though, my service loops for the wiring and lines will just live one bay further outboard. The only real concern is that the heat controller will still need to be mounted in the bellcrank bay, where it’s accessible through the cover. The included wiring between the two won’t be long enough to reach, so I’ll need to extend it. That isn’t a huge deal, though.

Anyway, after shaking my head ruefully a bit, I just kept on going with things. I drilled through the skin holes into the mount base, clecoing as I went, then removed the mount and skin again. The final piece of the puzzle here is a little piece of aluminum angle, which ties the mount base into the adjacent rib. This is needed to secure the mount properly against the air loads it’ll experience in flight. Simply having the mount riveted to the skin isn’t enough – the setup is pretty wobbly.

This part is pretty straightforward – the angle is first aligned with the aft most rivet hole in the base, and while held flush against the rib, that rivet attach hole is drilled. Then I just needed to drill a pair of rivet holes between the angle and the rib. To do this, I needed to ensure the mount was in place as if it were attached to the wing skin, but without the wing skin in place. No problem – I just secured a piece of angle across the ribs, then clamped the mount base to it. I drilled pilot holes in the small angle piece on the drill press, then used those as a guide to drill through the rib.

Here’s the final product, obviously seen without the wing skin:


This just leaves working out the heat controller location before I can pretty well call the pitot stuff done for now. Which I suppose means I’ll be moving on to performing the service bulletin on the aileron hinge brackets. And then maybe starting to make cardboard mockups of avionics and stuff so I can start planning wiring harnesses.

Posted in Wings | Hours Logged: 2

Pitot heat testing

As alluded to last night, tonight was the time to do some data-gathering about the pitot tube. The first step was to find a container to make a suitably large ice bath. I wanted to be able to keep the pitot tube running and heating for an extended period of time, basically to allow as much time as possible for heat to make its way up the hard lines. Eventually I settled on the removable liner from the crock pot.

Then there was the matter of a fixture to hold the pitot tube. Here inspiration struck; I have a fixture I made eons ago for holding tubing in place in the drill press. It’s just a 2×4 with a pair of 45° cut blocks attached, providing a nice V-shaped pocket for tubing to rest in – and this made it also handy for resting a pitot tube in. I just needed a way to secure the tube in place, and for that I added a piece of scrap lumber over the top, with a couple wood screws to make a clamp of sorts.

Here’s the finished setup, ice bath and all: 

After applying power to the pitot heater box, I left this setup to run for about an hour, which ought to be plenty of time to get good heat soak. Then I measured the temperature of the hard line at 1” intervals, using an instant-read kitchen thermometer.

Now, something to consider here is whether or not I was testing the worst-case scenario here. I believe it does, since keeping the tip of the tube in an ice bath should cause the heater to run at its maximum duty cycle, as opposed to running it in ambient air, where the heater won’t need to do as much work. There is, however, the slight question of whether the ice bath would help to cool stuff upstream from the mast. One could probably have a lengthy debate about how well either of these scenarios resemble real life, but honestly, I’m not looking for real life…I want the worst-case scenario, to poke at an upper limit for heating of those hard lines.

Anyway, the end result of this is that I ran two tests: first, the ice-bath test, then a second one with the tube in the warm ambient air. The results were about as I expected, with the ice-bath test heating the hard lines more. But even in that case, they didn’t get very hot at all, barely north of 100°F even just a few inches from the pitot unit:

I haven’t had any luck finding a heat rating for the nylon tubing, but seeing as how 105° isn’t even outside the realm of possibility for a really hot summer day here in Houston, I don’t think it’s worth worrying about. Also, 5” is about as short as I’d possibly cut these lines, since that’s the length of the mast.

Long story short, I think I’m perfectly fine to shorten the hard lines and position the fittings inside the mast. This means I don’t have to worry about a carefully-crafted bend in the hard lines, or needing access to the transition fittings inside the wing for service. I can just provide enough slack in the nylon lines to pull the pitot out of the mast and disconnect the fittings out in the open.

That just brings us back to picking a spot. I really wanted to make this work at the inboard spot, but I just don’t like the interaction between even the soft lines and the aileron push tube. I know it’s been done, and I’m sure even if there is light contact between the push tube and the lines, it’ll be fine, but…again, I just don’t like it. I went so far tonight as to feed one of the lines out to mid-span to get an idea of what the packaging would look like, which reinforced that I’d need some tight bends to avoid the push tube.

So it looks like it’ll be outboard. I intend to secure the mount one bay outboard of the bellcrank/tiedown location. This should be far enough away to minimize potential interference from tiedowns, and still reasonably accessible from the access cover at the bellcrank.

Here’s a look at the packaging in the bay with one of the two tubes. The tubing will be routed through existing holes in the ribs, albeit protected with snap bushings in the final product. Not shown here is extra material for a service loop in this bay – I’m not sure if I’ll put that here or elsewhere.

Then we move one bay inboard to where the aileron bellcrank lives. I’ll want some additional hardware here to ensure the tubes are carefully routed between the bellcrank and the spar, just to absolutely ensure they never interfere with each other.

So at this point, I think this is finally settled, and I can now move on to the somewhat scarier – but less ambivalent – task of fitting the mount and cutting the hole in the wing skin.

Posted in Wings | Hours Logged: 1.5


Yes, the pitot mount, again.

But first, a digression: I did have a good weekend. I’ve been “organizing” a local RV lunch for a couple years now. We used to rotate between two local airports with restaurants, until one of the airports was closed. So there hasn’t been a lot of variety. Josie and I have been talking about wanting to host a lunch at our hangar for some time now, and we were going to do it back in April, but then our good friend coronavirus came calling. But we finally did it this past Saturday.

Attendance wasn’t super great; I think the weather played a part, but I wonder if some folks are still hesitant about getting together. I missed some of my regulars. Still cool to have a few RVs parked on my ramp, and also to have an experienced builder look over my project and not mutter anything unflattering under (or over, for that matter) his breath.

Today was mostly a recovery day for me – I was up at 4 AM yesterday to get ribs on the smoker – but I got back to, well, staring at the wings and scratching my head. Last time, I’d pretty well settled on the inboard pitot location, but then wondered if the aileron push tube might be an issue. The short answer is “yes.” The tube runs pretty close to the spar, and is almost directly in line with the pitot mast, which makes it, at minimum, challenging to route the lines so as not to interfere.

Now, before I go on, I figure it might be helpful to show a bit of what I’m talking about with this setup. Below we have a look at the overall pitot system. The actual tube itself slips into the mast, and will be attached with screws. Protruding into the fuselage from the tube are two soft aluminum lines, one for pitot pressure and a second for AoA pressure. At the end of the aluminum lines are shown a couple of fittings, which provide the transition from the hard lines to the flexible nylon ones that will be plumbed into the fuselage. This particular pitot tube is also heated to prevent ice buildup and loss off airspeed; the wires are for the heat system, and connect to an external controller box:

Now, a lot of the packaging fun revolves around that transition from the hard lines to the soft ones. We want to secure the hard lines at some point to ensure they don’t rub against something (like the mouth of the mast) and get damaged over time. We also need to put some kind of a bend in those lines – at their current length, they contact the upper wing skin when the tube is inserted into the mast. This bend has to be pretty gradual, though, since it has to be able to feed through the mast. And that means I’m pretty limited in where I can place the ends of those hard lines – where, again, they need to be secured.

Now, if I can shorten those hard lines, I can gain some more flexibility here…I think. In fact, if I could make them really short, I could conceivably have the transition fittings actually live inside the mast. I’ve seen an install where the builder bundled them and wrapped them in self-fusing tape to hold them in place. With that approach, there’s nothing but nylon tube inside the wing, and a service loop can be added, so if the pitot tube needs to be removed, it’s simply removed to expose the fittings, and they’re disconnected outside the wing.

Remember, though, that the pitot tube is heated. And heat might be a problem for plastic fittings. Notably, the install with very short hard lines was not a heated install. I did a lot of poking around to see if there was any info on how long the hard lines needed to be to prevent damage to the nylon fittings. It seems that Garmin recommends a minimum of 8” for their similar heated pitot, but Dynon has nothing like this in their documentation – in fact, it says nothing at all about shortening the lines, either positive or negative.

Searching VAF yields some interesting discussion about just how hot the hard lines may get. At least one person has bench tested the tube and observed no real heating of the lines at all. Someone else pointed out, though, that this is a regulated heater – it only runs the heater as needed to reach a desired temperature, and in a warm shop, that might not be much at all. In the real world, in a serious icing situation, it could be running a lot more. Of course, there’s also the question of whether that cold ambient air will counter the heat buildup.

Anyway, what I’ve decided to to – but didn’t do tonight, because this idea came to me late – is to do some experimentation. I have a 10A 12V power supply, sufficient to power the tube. And if I make a decent-size ice bath to submerge the tip of the tube in, I should be able to simulate the worst-case scenario, where the heater runs at a high duty cycle. The ambient air will be much warmer than in a real scenario, which will make my measurements more conservative.

So that’s my next move…tomorrow. I connected the tube and controller to my power supply briefly tonight, just to confirm that the tube heats up. The main thing I’ll need to figure out is some way to hold the tube so it stays submerged in the ice bath. Stand by for some creativity…

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