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…