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.
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.