Airspeed sensor tube, does length matters?

Hello all.
I would like to ask you, dear members of the arduplane forum, if the lenght, inner diameter and type of material of the air tube of the airspeed sensor matter in terms of affecting the precision of the sensor. I have searched all over the internet about longer air tube cables and have found nothing.

I'm building my first rc plane with Pixhawk, and my problem is that the df13 cables I have are all too short, and so I don't have enough cable and rubber tube to place the airspeed sensor as much closer to the front as possible.

Since I'm in Angola, it's preety difficult to import new parts, and I would like to know if, instead of extending the df13 cables to the sensor I was thinking if I could use a much longer tube to place the pitot much ahead in the plane. Since I can be given me some unused hospital leftover tubes, like those types usually used in blood transfusions, transporting oxigen, etc., I would like to know if those can be the solution to my problem.
I tried a sample once and even though they are a bit tougher than the original rubber tube, and it has 4mm external diameter and about 2.5mm inner diameter, and they can fit in the sensor well tight. I hope the materials and the lenght aren't a problem and wont make wrong readings, causing the plane to crash.
I'm looking forward to read about your experienced oppinion.

Much obliged,

Roger Monongo

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  • Hi Roger,

    The airspeed sensor works by comparing the pressure of the ambient air - the side facing tube - with the pressure of "ram air" from the front facing tube. The difference is a function of airspeed. The system does not move much air through the tube, rather it just applies pressure to it. If the tube was very, very small in diameter or very long if might affect the rate of change able to be delivered by the sensor, but I don't think that will be an issue with the size tubing you are planning on using. The material does not matter either as long as it does not leak.

    I see no reason why your plan to use medical tubing will not work. Comments, anybody?

    Good luck with your project!

    Mark Lewus

    • you should be fine. No leaks is important as well as smooth bends. On manned aircraft they use very long lengths of tubing to get the air from the pitot tube to the instruments. 

  • I want to thank you all, you've been very helpful!

  • There is not air moving through the tube, it's just getting compressed with the ram air pressure. The length of the tube doesn't matter as long as there are no leaks.
  • 100KM

    Roger, I don't understand why you can't extend the DF13 wires? I just solder mine to the length required.

    • It's extremely difficult to get new materials or equipment here in Angola. All the materials I have I got them from when I go to Portugal 9000km away, once a year, and bring the parts in my luggage. Since those medical tubes are much easier available I guess it's an obvious improvised choice to consider.
      Anyway, even when I'm in Europe, buying ready made cables, crimped and with the df13, can only be possible if they are bought online, but the stores I've found them are not many and/or the few I find have ridiculous prices.
      In my oppinion it was a bad choice for the developers of pixhawk to have chosen plugs and cables that are not as common and cheap like servo cables and plugs. The dificulty of finding replacement plugs or even crimping a cable ir ridiculous (not even mentioning the aditional difficulty being in Africa, which is a special case and of course is nobody's fault).

  • Any surgical grade rubber tube will be fine, so long as it makes an airtight seal at both ends. The only issue you will need to take care with is the layout of the tube within the airframe. You need to ensure that there are no sharp bends or kinks that would affect airflow and hence create pressure variations within the tube.

  • should be fine! I've seen some tubes on the order of 1m long. Any small errors can be fixed while tuning your aircraft but I wouldn't worry about it.

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