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Heated pitot tube testing: how long do the hard lines really need to be?

N546RV

Well Known Member
As I continue with the saga of working out pitot tube packaging, one part that's consistently annoyed me is dealing with the hard lines on the pitot unit itself. Due to the need to be fed through the pitot mast, only a very large radius bend can be made on these, which in turn is limiting in figuring out where to secure/strain-relieve their ends inside the wing.

I've seen installs where the builder cut the hard lines short enough to position the transition fittings to nylon tubing inside the pitot mast itself - this eliminates the problem of dealing with that bend, inherently secures the fittings from vibration (if wrapped in, say, self-sealing tape), and with a service loop in the soft lines, allows the fittings to be disconnected outside the plane, instead of working through an access cover.

However, in all the cases I've seen with this approach, the pitot was not a heated unit. Adding heat into the equation changes things, since now there's the possibility of heat from the tube causing mischief with the nylon tubing that many (including me) use to plumb the aircraft.

Presumably the ~10" of hard-line on my Dynon unit is long enough to have no issues, and I'd think one could trim some of that if desired...but how much? The documentation with the Dynon pitot makes no comment, positive or negative, about trimming these lines.

So I decided to put the "experiment" in experimental, and gather some data. The basic idea was to operate the pitot heater on the bench, and measure the temperature of the hard lines at intervals from the unit. Naively, one might just hook up the heater, let it run for a while, and then get to measuring, but this is a regulated heater, and keeping it at the proper temp in warm Texas air would require a very different duty cycle (and by extension heat soak) compared to dealing with cold air and ice at altitude.

I decided to run two tests. For the first, the tip of the pitot was submerged in a sizable ice bath for the duration of the test, in an attempt to maximize the duty cycle of the heater. The test was then repeated with the tube in ambient air, mainly for the sake of thoroughness.

Temperature measurements were taken using a quality instant-read kitchen thermometer, and only after the system had run for about an hour to really stabilize.

To summarize the results: As expected, significantly higher temperatures were observed with the ice-bath test, but even in that case temperatures were not particularly high. The base of the pitot tube and the hard lines could be comfortably touched, and at 3" from the tube itself (the closest point at which temperature was taken), it only reached 105.5?F. At the end of the tube (approximately 10"), the temp was 86.5?F. These represent about 25? and 6? above the ambient air temperature at the time.

Full set of measurements from both tests can be seen here: https://i.imgur.com/5JfEv1B.jpg (if you can read my chicken scratch)

I haven't been able to find specific properties of the nylon tubing in my SafeAir1 plumbing kit, but some Googling suggests a maximum operating temp of at least 150?F. If accurate, this would provide a decent safety margin above the measured temperatures even with the hard lines cut absurdly short - not that I plan on doing that.

The conclusion I've reached is that the use case I've been considering should be perfectly acceptable. That use case would involve trimming the hard lines to about 5", at which point heating of the hard lines doesn't even match plain old ambient air temperatures at the height of a Houston summer. Therefore, I intend to go ahead with this setup.

Hopefully this information is of interest to other builders.
 
Useful, thanks

I've been trying to figure out how to plumb to my heated pitot for a while, this info will give me a few more options. I was trying to ensure that I didn't cut the fixed tubing, but it's **** hard finding ways to make everything fit nicely.
 
Thank you for doing and posting.
Would you expect much difference with an unregulated pitot heat?
 
It's simple and consistent

A couple of assumptions:
The tube is in free air (not trapped where it would heat up the air).
No flow through the tube (in this case, it would actually help).

In free air, a pipe/tube/etc. will reach ambient temperature in less than ten diameters. Cool thing is, it doesn't matter how hot the source is as the local heat transfer is a factor of the delta temp. You could do a detailed calculation using changing ambient conditions, temperature limit of the polymer you'll utilize, etc. but you'd still have to make assumptions so really not worth it. Ten outside diameters will be plenty of design margin. Best of luck
 
A couple of assumptions:
The tube is in free air (not trapped where it would heat up the air).
No flow through the tube (in this case, it would actually help).

In free air, a pipe/tube/etc. will reach ambient temperature in less than ten diameters. Cool thing is, it doesn't matter how hot the source is as the local heat transfer is a factor of the delta temp. You could do a detailed calculation using changing ambient conditions, temperature limit of the polymer you'll utilize, etc. but you'd still have to make assumptions so really not worth it. Ten outside diameters will be plenty of design margin. Best of luck

Does air actually flow through a pitot tube or is it stagnate once it hits the instrument?
 
Thank you for doing and posting.
Would you expect much difference with an unregulated pitot heat?

Possibly. I'm making the assumption that, in my testing, I was causing the heater to run full-bore, but I didn't confirm that. One thing I could have done, but didn't, would have been to measure the current to the heater to verify the output level.

Anyway, in my test cast, the assumed max output of the heater was being countered by the ice bath, thus keeping the temperature of the tip at a reasonable level. I stuck my temp probe down near the tip at one point just for curiosity's sake, and as I recall the water close to the tip was at about 60?F. I expect that keeping the tip cool definitively affected the temp change up the lines.

The worst-case scenario, I think, would be a heater running at full output in non-freezing air, which definitely falls in line with having an unregulated heater. The good news is that testing an unregulated unit would be a lot easier - no need for the ice bath, just put power to it and wait a bit.
 
There's no flow, per se'. I would assume some very small exchange of fluid at the business end of the pitot simply because the aero isn't perfect. Not going to entertain effects due to changes in dynamic pressure through speed. Not enough to change anything. Keep it simple. If you have to seek infinitesimal amounts of margin, it's probably not a great design in most cases.

I mentioned it because even a small amount of real flow changes everything and wanted the distinction made. I did a "root cause analysis" on some failed componentry decades ago (industry intentionally withheld). It was an easy one, didn't need any real analysis. An instrument sense line had a calibration tee left off after servicing. The normally dead leg now had a small flow of 800 DegF hot air. Fortunately, only equipment was damaged. Some people, not just here, can take a statement and run the wrong direction with it. A little caveat or two is usually justified. Keep it simple, as stated. Attention to detail is great just don't swat mosquitoes with a sledge hammer. Best of luck.
 
Here's how I did mine, on the Dynon heated pitot. What you see hanging free there is my service loop, secured through the inspection port. Works great and has seen a fair bit of IFR heated use in flight. Just over 4 years and almost 700 total hours.

oNG.jpg
 
Air density

Since you are relying on the surrounding air to cool the tube, dont forget the air is only half the density at 18000?. But it is also colder, so they probably offset one another.
Funny thing, you are probably the first person to actually test this, as opposed to the design engineers who probably just guessed.
 
Did you factor in the heatsink effects the pitot mast and airframe affect in temp dissipation?
Glad to see you are using Safeair fittings, best in my opinion!
 
Howdy everyone. This may be useful to someone; a link to my work so far on my heated pitot installation:

https://www.youtube.com/watch?v=w6d73WsSo2M

There is another video demonstrating how the pitot itself is mounted to the access cover.

All in all it seems to be very mechanic friendly and should the need arise, easy to disassemble and repair.
 
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