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…

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.

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.

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…

Well, not a ton done today or over the past week, though I did get to do some flying today (and also eat out for about the second time since March). I was able to spend a little more time looking at pitot tube position planning.

Last time we “talked,” so to speak, I was strongly considering mounting the pitot tube out by the wingtip. I did ask for opinions on VAF, and while nobody told me my idea was completely stupid, there was a somewhat general consensus of “but why?” I was approaching the question more from a “why not?” perspective, so I didn’t find this convincing.

However, the one obstacle to my planned location was figuring out how to tie the mount into the rib. The base of the mount rivets to the skin and spar, but it also requires bracing to the rib, to resist what I assume is a torsional load on the mount base, caused by drag on the pitot tube way out in the wind. Dynon provides a small piece of angle aluminum for this, but this presumes that the mount is stalled adjacent to the non-flanged side of a rib. My planned location was on the flange side, which would require me to get creative with this reinforcement.

But then I hit on a simple solution – still keep the mount at the second-most-outboard rib, but on the inboard side – then I could reinforce it as per normal. The only concern would be whether I could bend the pitot lines in such a way as to feed them through the tube and then through the immediately-adjacent lightening hole. Figuring this out required a practical exercise, and my initial plan had been to just go ahead and bend the pitot lines and experiment in real-time, shall we say.

But here’s the problem with that: those lines can be bent only once. So what if I bent them and eventually found my chosen location was no good? And then what if that bend was problematic for any alternatives? Clearly committing to the bend was not smart. I needed some sort of analog with which to test.

I spent some time poking around. Maybe some leftover fuel vent line? Huh, apparently I didn’t have any leftover. I have plenty of extra fuel line, but that’s twice the diameter of the pitot lines, and I’ll probably need more of it for fuel-system stuff down the road. Then the perfect thing occurred to me: the pile of Romex wire on a random shelf. The entire bundle is just about the same size as the pair of pitot/AoA lines. So I chopped off a piece the same length as the lines, and went to work.

The good news was that it was easy to figure out a bend that allowed for threading the lines through the mount, into the rib lightening hole, and out towards the wing tip. The bad news was that the lines only reached about halfway through the bay between those ribs. This would give me a new problem of needing to put some sort of support piece in the middle of the bay to support the ends of the lines, but the only thing to attach to would be the spar. Drilling a couple holes in it might not be the end of the world, but maybe still not a great idea.

That was pretty much the end of the wingtip idea. At this point much of the potential access benefits weren’t going to materialize, and there’d be significant mental energy in making this setup work at all.

This left two basic choices: either just outboard or just inboard of the aileron bellcrank bay. As mentioned before, the outboard location poses the possibility of interference with tie downs, but it’s also more difficult to access through an inspection cover, and also makes the bellcrank bay more crowded. So after lots more clamping the mount in various places and futzing with that poor abused piece of Romex, I settled on one bay inboard of the bellcrank.

So hey, let’s see some pictures. First, here’s that Romex, bent as appropriate to go through the mount. That 90° bend has to be pretty gradual to make it through the fairly-long mount:

And here’s a look at what the intended mount location looks like with he fake-bent tubes in place. On the other side of that rib from the mount is an inspection cover, so there will be good access here. The tubes can make a nice 90° bend through the lightening hole, and a piece of angle can be mounted to the rib to support both the tubes and the wiring from the controller. The controller will likely be mounted on the inboard side of that same rib, again easily accessible through the cover.

Overall, I think this will be just as accessible as my wingtip location, and significantly less complicated to implement. The nylon lines into the fuselage will be shorter, and can just run along the spar instead of through the conduit as I’d originally envisioned, which will make the conduit less crowded. And since this is all inboard of the bellcrank, I don’t have to worry about routing around that.

Although, as I type this, it occurs to be that the aileron push tube from the fuselage will go through that same hole I’m considering here…creating a potential interference. So I guess tomorrow I need to temporarily connect the tube to the bellcrank and get an idea where it’ll sit in here. That might be a dealbreaker, we’ll see. Maybe I’ll end up outboard of the bellcrank bay after all.

Welcome to the world of systems planning. Everything seems to affect everything.

Well, today was one of those days spent almost entirely on chin-scratching. It didn’t start off that way, but that’s just because there was some more cleanup work to be done. I didn’t get around to vacuuming up all the mud dauber debris yesterday, so I did that this morning, then worked on removing some of the highly annoying artillery fungus reside on the inboard spar ends. From my research, this stuff isn’t corrosive, but at least I can remove the highly visible stuff.

From there, it was time to consider what to work on first, and after rolling this around for a bit, I decided to look at the pitot mount in the left wing. This started with a bunch of VAF reading. The factory-recommended location for the pitot is in the same bay as the aileron bell crank, but that’s also assuming you’re doing the basic pitot tube – literally just a piece of tubing that sticks out of the bottom of the wing. Installing a more complex pitot, especially a heated AoA tube like mine, is quite a bit more complex. Instead of just one line, I have two, not to mention the wiring for the pitot heat controller.

Plenty of people have adopted other locations, generally moving to an adjacent bay, either inboard or outboard. Which direction is kind of hotly debated; some thing going inboard might get too close to the prop wash, others think moving outboard makes it likely for tie downs on the ramp to interfere with the tube. There’s the additional consideration that moving the mount puts the pitot further away from any of the access covers, potentially making maintenance more annoying.

Now, all that stuff I’ve read before, but I ran across a different idea entirely today – that of putting the tube way out near the wingtip. At least one person has done this and reports no issues, and others seem to think that it should be a fine location. It’s worth noting that this is where the pitot is located on some Grumman light aircraft. I really like this idea, for a couple reasons. First, it makes maintenance access super easy. Instead of working through a small access cover, one could just remove the wingtip and have good access through the rib lightening holes. Second, there’s no concern about routing wires and tubing in the vicinity of critical control systems (the aforementioned aileron bellcrank bay).

I haven’t decided for sure to go with this, but I did spend a whole lot of time looking at the pitot mount clamped in various locations in the spar. Assuming I do go with the far outboard location, I think I’ll end up positioning the mount beside the next-to-last outboard rib, like this:

The pitot tube itself, which fits into that mount, includes two soft aluminum lines, which will connect to the nylon lines that will route the pitot and AoA signals into the cabin. There must be a minimum amount of the aluminum lines left; since this is a heated pitot, there’s the concern of that heat damaging the nylon lines. My loose plan, in this case, would be to bend those lines toward the wingtip, and install a piece of angle in the rib lightening hole near the spar. Here, adel clamps would provide security and strain relief for both the line and electrical connectors. The pitot heat controller could simply be mounted on the outboard side of that outboard rib.

Will I do this? Yeah, still not sure. I need to check in on VAF, where I posed this question and I’m sure I’ll hear some reasons why this is a horrible idea. It remains to be seen if anyone has a convincing argument. Normally “follow the plans” is pretty good on its own merits, as a way to avoid additional work, but since fitting the pitot tube is essentially a from-scratch operation in any case, there’s no additional work associated with picking this different location.

So we’ll see. If I do settle on this spot, I’ll probably go ahead and bend the aluminum lines on the tube so I can get a more practical (ie less imaginary) look at packaging, and work more towards nailing that down.