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  • Static Port Location

    I see several have located their static ports behind the cargo area, I am wondering why? My build mentor has his static ports on the boot cowl of his Carbon Cub, it seems it would be a lot easier to plumb the ports close to the panel/avionics. Certainly locating the static ports so far aft requires a longer plumbing run.

    I am not trying to reinvent the wheel, just checking/wondering if the BH design dictates the aft static port location.

    All help and ideas are welcomed

    Jay
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  • #2
    Jay - this has been discussed for decades, including the old "Yahoo group" long before the forum existed.

    Many Bearhawk builders have had trouble with getting inaccurate readings from static ports located on the boot cowl. I know some other aircraft types have made it work, and some Bearhawks made it work too, but not all.

    IIRC, the Bearhawk community's collective experience has shown that static ports work reliably, in practically all cases, when located on the sides of the fuselage at the next station behind the aft bulkhead (station numbers not on hand sorry).

    Hope this helps, someone will correct me if I misremembered that.

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    • #3
      Perhaps that location has the least amount of airflow variation? From what I’ve seen from you tube videos and builder blogs, there are a number of different ways to do the boot cowl. Maybe each of those methods has a different effect on the airflow and causes the result to vary build to build. The section aft of the baggage area is pretty constant.

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      • #4
        Turbulence is a bad thing for static ports. Behind the baggage compartment is as smooth as it gets. Look at the location on a Citabria or Husky.
        Last edited by spinningwrench; 01-12-2022, 01:30 AM.

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        • #5
          I plan to install my static ports on the side of the fuselage behind the bagage area. This is very similar to where they are on my piper comanche.
          Model B quick build started 2021

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          • #6
            OK, it seems although a few have done otherwise, I will put my static ports behind the baggage area.

            Thanks everyone
            N678C reserved
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            • Nev
              Nev commented
              Editing a comment
              It's actually pretty straight forward to do it that way, then run one line forward. I queried it too at the time. It's one area that I've had no further issues with.

          • #7
            From an aeronautical engineer on the old Yahoo group:

            A pitot-static system works by measuring the difference in pressure created by the forward speed of the aircraft (pitot pressure at the inlet to the pitot tube) vs. ambient pressure at the static ports (a pressure as close to ambient as is possible). The pitot system is relatively insensitive as long as the pitot tube is pointing somewhat forward (+/- 15 degrees into the relative wind).

            The static system is very sensitive to the local environment and at best is a compromise. The shape of the fuselage has areas that have higher than ambient pressure and other areas that have lower than ambient pressure and the pressure in these areas changes with flight conditions, vent opening/closing, cabin leaks etc. The internal cabin pressure is almost always lower pressure than ambient due to the shape of the fuselage. If I remember correctly, the Cessna manual for the 172 says that a 200 ft error occurs when changing to the alternate static which opens inside of the cabin

            When an airplane manufacturer certifies a plane for IFR the static must be accurate (+/- 25 ft total error) over the entire range of speed and control positions. This requires a survey of the aircraft static port locations vs a reference static system. (usually done with reference to a trailing cone system that follows the airplane by several hundred feet). It can take several tries to get a static port located so that it meets requirements. Another way to obtain an approximate check of the static system is to fly over a known elevation (field, lake etc.) at several speeds and see what the altimeter shows. You have to set the local barometric pressure in the kolsman window of the altimeter. You will need a ground observer to observe your true height above the reference so you will have a comparison to the altimeter reading. This second method only gives approximate data but is better than nothing.

            For a homebuilt aircraft, usually the best static location is obtained by copying a production aircraft static system and location from an airplane that is as similar as possible to the homebuilt. This location still needs to be checked by one of the above methods.

            Static error can ruin your whole day. I have a professional test pilot friend that was testing a factory prototype. Tested stall speed on the aircraft was 58 IAS. After an avionics change was made which also affected the static system, the airplane stalled on final with an indicated air speed of 84 mph on the first flight after the changes. Luckily he survived but the airplane was scrap.

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