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Thread: VSI behaving badly

  1. #1
    Birdseyeview's Avatar
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    Default VSI behaving badly

    I had bought a used VSI a couple of years ago during my build phase but didn't get to test it until recently after my first flight. In recent flight testing it wasn't working and I thought it must be defective, so I bought a new one from ACS. The new one is a Falcon brand 2kft range in 2 1/8" dia that has one port in the rear for the static line only. During my first flight with the new VSI I didn't have it hooked up to the static line coming from the pitot tube on the wing, just open to the cockpit ambient. In that config it didn't seem like it was working so on the next flight I hooked it up to the static line and while I'm climbing it indicates down and when in level flight it reads down as well but to a lesser degree. During decent it goes back to around zero, where it reads while I'm on the ground. All my instruments are round steam gauges. The pitot tube that I have is pictured below and appears to be oriented straight into the airstream. I checked the line connections in the wing and they appear to be connected properly. At the back of the VSI gauge the static line to it is Tee'd off the static line for the adjacent ASI and altimeter, both of which appears to be reading and functioning fine. Has anyone out there ever had a similar issue and fixed it somehow?

    IMG_8527.jpg
    Larry Olson
    Kitfox Series 6 - 1st Flight Oct 2021
    Tri-gear, smooth cowl
    912 ULS

  2. #2
    Senior Member PapuaPilot's Avatar
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    Default Re: VSI behaving badly

    If I'm not mistaken when a VSI is backwards it has an internal leak or cracked/unsealed glass face.

    You don't need to hook a VSI to the static system test it, you can place it in the cockpit and flight test it for proper function. Try the new VSI in your plane or someone else's and see if it responds the same.
    Phil Nelson
    A&P-IA, Maintenance Instructor
    KF 5 Outback, Cont. IO-240
    Flying since 2016

  3. #3
    Senior Member 109JB's Avatar
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    Default Re: VSI behaving badly

    Does your static probe only have the one hole in the side. It should have at least 2, but many have 4. The holes should be directly opposite each other on the other side of the tube. 4 would not be practical on yours since the pitot is on top. If I were to build a pitot static tube like that, I would put them in an X pattern. However, I would separated the pitot from the static tube and do the multiple holes.

    Pitot static.png
    John Brannen
    Morris, IL
    Sonerai IIL (Single Seat)
    Kitfox 3/4 1050 - Rotax 582 (Back Flying and sold)
    Kitfox IV 1050 - Rotax 582 (sold)
    Kitfox IV 1200 Speedster - Rotax 912 UL (project)
    Piper Twin Comanche (Sold)
    Glasair 1 FT (Waiting to start)

  4. #4
    Birdseyeview's Avatar
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    Default Re: VSI behaving badly

    Phil - thanks for the tip. It appears to me as well that the VSI must be defective since it's indications are so odd and counterintuitive. It looks like I need to return the VSI and get a different unit. I was initially thrown off by the fact that the used VSI which I tried first also didn't function at all so I assumed that there must be something wrong with my static line. However, the VSI hookup is so simple and it should have functioned properly with no static line attached. It seems to me that with the static line not connected the VSI is subject to a potential cabin pressure difference compared to outside but it should still indicate in the right direction if the VSI is functioning properly.

    John - My static probe only has the one hole in the side of the tube but it is relatively easy for me to drill that hole through the other side of the tube if that will help its future accuracy. In looking at various diagrams of pitot tubes on line most do have a minimum of two holes directly opposite each other.
    Larry Olson
    Kitfox Series 6 - 1st Flight Oct 2021
    Tri-gear, smooth cowl
    912 ULS

  5. #5
    Senior Member 109JB's Avatar
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    Default Re: VSI behaving badly

    Depending on the airflow, that one hole could be facing into the wind, essentially making it a pitot tube, or it could be downstream causing suction. Having equal size holes opposite each other tend to even out the suction on one side and pressure on the other. That's why when you put static ports on the fuselage you generally put one on each side of the fuselage. I'd drill a hole in the other side and retest.
    John Brannen
    Morris, IL
    Sonerai IIL (Single Seat)
    Kitfox 3/4 1050 - Rotax 582 (Back Flying and sold)
    Kitfox IV 1050 - Rotax 582 (sold)
    Kitfox IV 1200 Speedster - Rotax 912 UL (project)
    Piper Twin Comanche (Sold)
    Glasair 1 FT (Waiting to start)

  6. #6
    Senior Member PapuaPilot's Avatar
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    Default Re: VSI behaving badly

    I totally agree. Check this first after drilling the hole through the static tube.

    If you have already removed the VSI from the panel I would take it on a flight with it sitting on the seat or on top of the instrument panel. If you live in an area with steep roads you could even test while driving your car.
    Phil Nelson
    A&P-IA, Maintenance Instructor
    KF 5 Outback, Cont. IO-240
    Flying since 2016

  7. #7

    Join Date
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    Default Re: VSI behaving badly

    If your altimeter is working correctly then your static source is "OK", not too far off. On my KF3 the instrument static line was just open behind the panel. The VSI and altimeter seemed "OK" but the airspeed would wander a bit on the low end as the pitch and yaw changed, mostly during a hard slip. Adding the single static source in the back left side of the fuselage per the Kitfox drawing fixed the problem.

    The point here is that if your airspeed and altimeter seem to be working correctly on your static source then it is "OK". My bet is that made in China VSI is the problem.



    Quote Originally Posted by Birdseyeview View Post
    I had bought a used VSI a couple of years ago during my build phase but didn't get to test it until recently after my first flight. In recent flight testing it wasn't working and I thought it must be defective, so I bought a new one from ACS. The new one is a Falcon brand 2kft range in 2 1/8" dia that has one port in the rear for the static line only. During my first flight with the new VSI I didn't have it hooked up to the static line coming from the pitot tube on the wing, just open to the cockpit ambient. In that config it didn't seem like it was working so on the next flight I hooked it up to the static line and while I'm climbing it indicates down and when in level flight it reads down as well but to a lesser degree. During decent it goes back to around zero, where it reads while I'm on the ground. All my instruments are round steam gauges. The pitot tube that I have is pictured below and appears to be oriented straight into the airstream. I checked the line connections in the wing and they appear to be connected properly. At the back of the VSI gauge the static line to it is Tee'd off the static line for the adjacent ASI and altimeter, both of which appears to be reading and functioning fine. Has anyone out there ever had a similar issue and fixed it somehow?
    Building a KF IV Classic

  8. #8
    Senior Member 109JB's Avatar
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    Default Re: VSI behaving badly

    Quote Originally Posted by napierm View Post
    If your altimeter is working correctly then your static source is "OK", not too far off. On my KF3 the instrument static line was just open behind the panel. The VSI and altimeter seemed "OK" but the airspeed would wander a bit on the low end as the pitch and yaw changed, mostly during a hard slip. Adding the single static source in the back left side of the fuselage per the Kitfox drawing fixed the problem.

    The point here is that if your airspeed and altimeter seem to be working correctly on your static source then it is "OK". My bet is that made in China VSI is the problem.
    The above is not true. Airspeed works on differential between static pressure and pitot pressure. Altimeter work on static pressure. The VSI unlike both of these works on the RATE OF CHANGE of static pressure. Just because an airspeed and altimeter "seem" to be working correctly does not necessarily mean the static pressure is fine.
    John Brannen
    Morris, IL
    Sonerai IIL (Single Seat)
    Kitfox 3/4 1050 - Rotax 582 (Back Flying and sold)
    Kitfox IV 1050 - Rotax 582 (sold)
    Kitfox IV 1200 Speedster - Rotax 912 UL (project)
    Piper Twin Comanche (Sold)
    Glasair 1 FT (Waiting to start)

  9. #9

    Join Date
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    Lawrenceville, GA
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    Default Re: VSI behaving badly

    Quote Originally Posted by 109JB View Post
    The above is not true. Airspeed works on differential between static pressure and pitot pressure. Altimeter work on static pressure. The VSI unlike both of these works on the RATE OF CHANGE of static pressure. Just because an airspeed and altimeter "seem" to be working correctly does not necessarily mean the static pressure is fine.
    I agree. But if you descend a thousand feet at a steady rate and the altimeter is steadily winding down then the VSI should be trending down. If you climb up at a steady rate and the altimeter is steadily winding up then the VSI should be trending up.

    So if the altimeter is smooth and the VSI is not then the darned thing is busted.

    Personally I would rather buy a used VSI from an instrument shop than a POS Falcon gauge.
    Building a KF IV Classic

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