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Thread: Garmin G3X IAS display

  1. #1

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    Garmin G3X IAS display

    Our Garmin G3X stops showing IAS at 20kts, this is a bit irritating when your plane can still fly at lower IAS.....
    does anyone know if we can change this in the setup?
    the funny thing is that the data log clearly shows the lower IAS down to 13kts -> why can't the display then show us these values.

    I do know that at 13kts IAS the Pitot tube is not working very well so it may NOT REALLY be exactly 13kts IAS, I can live with this, as long as I know at which IAS I will start dropping a wing then it doesn't really matter whether it is accurate or not.

    does anyone know how to change this?

    our aircraft is a BCSC REV2 with a IO360 engine and a CATTO 84x43 prop
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  2. #2
    skywagon8a's Avatar
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    13kts IAS? That number is extremely low for any airplane. Rather than altering your Garmin G3X settings, why don't you change the angle of your pitot tube more nearly in line with the air flow? Assuming that in order to be able to achieve these low numbers the nose on your plane must be extremely high. Point the pitot tube down more.
    N1PA
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  3. #3

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    yeah I understand that, but it STOPS reading <20, so from 20IAS below it is purely by feel whether the wing is flying or not, which doesn't really bother me but it would be nice if my display could show me some numbers.

    the BCSC does fly at VERY LOW airspeeds, this is the beauty of the airframe, recently I spent about 10 minutes with the IAS off the dial, windows open, rpm set at 1900, just dragging along enjoying watching the cars overtake me (and they were driving real slow to avoid potholes), we were 2 up and had about 80# of kit in the back and the aircraft just cruised along beautifully. I later checked on the data logger and we were flying at around 16-17IAS

    I dont really want to turn the pitot tube down as my mission profile is normally between 40-70Kts IAS, so the current position is fine for this, it would just be nice if my Instrument could give me data, even erroneous data, at below 20
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  4. #4
    skywagon8a's Avatar
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    That is truly spectacular! That must be a World's record for fully controllable slow flight. Are those speeds really IAS and not GPS, which would be ground speed? Change the design of your pitot tube. Mount it on a flexible joint with a vane which keeps it pointed into the air path. Pitot tubes are not at all accurate when the air flow deviates more than 10 degrees away from the alignment of the tube.
    The vanes behind this tube keep it pointed in the proper direction. Also the size and location of the static ports are important.

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    My Cub does very well. I have measured 28 mph using the GPS with no wind during slow flight just above the water when flying solo. This was an attempt to determine minimum controllable flying speed. It would not remain in the air below that speed. The IAS was about 10 mph higher. At that speed the controls are extremely sensitive in pitch.
    N1PA

  5. #5
    Eddie Foy's Avatar
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    Living proof that you need some ICE-T.
    https://www.skybrary.aero/index.php/Airspeed_Conversion
    "Put out my hand and touched the face of God!"

  6. #6

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    here is a little screenshot from a video clip I took on the above flight.

    like I have said before I understand the IAS is NOT correct and the wind calcs of the garmin are ALSO not 100% at these low speeds, but here it is...

    Click image for larger version. 

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    if I subtract the wind speed from the GS I get 15kts GS, it varied mostly between 16/17 GS.

  7. #7

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    That’s pretty slow. Could you see anything in front of you?

  8. #8

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    Quote Originally Posted by KevinJ View Post
    That’s pretty slow. Could you see anything in front of you?
    Haha sure, but who needs to see ahead when you can look out the sides

    Sent from my SM-G930F using SuperCub.Org mobile app

  9. #9
    skywagon8a's Avatar
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    That is a fancy all inclusive piece of electronics, nice. I'm no electronics expert but I do have an air data computer of a different manufacture in my 185. It requires the input source to be correct in order to give the proper information. If I don't ensure that the heading and barometric pressure are initialized correctly all of the output data is in error. What is the source of the information feeding the airspeed tape? If it is the pitot and static ports they must be providing accurate information in order for the computer in your G3X to properly function. If the wrong pitot pressure is being used, the G3X derived information will be in error. Without knowing anything else, I stand by my original assessment that your pitot/static air sensing device (pitot/static tube) needs some tuning and alignment attention.
    N1PA

  10. #10

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    Quote Originally Posted by skywagon8a View Post
    That is a fancy all inclusive piece of electronics, nice. I'm no electronics expert but I do have an air data computer of a different manufacture in my 185. It requires the input source to be correct in order to give the proper information. If I don't ensure that the heading and barometric pressure are initialized correctly all of the output data is in error. What is the source of the information feeding the airspeed tape? If it is the pitot and static ports they must be providing accurate information in order for the computer in your G3X to properly function. If the wrong pitot pressure is being used, the G3X derived information will be in error. Without knowing anything else, I stand by my original assessment that your pitot/static air sensing device (pitot/static tube) needs some tuning and alignment attention.
    at 40-70kts the TAS agrees with my GPS GS with the odd 1-2kt difference every now and again which I would put to sensors lagging a bit. so I am happy with my pitot installation.

    the question is whether anyone out there knows how to make the G3X speak to me at lower than 20kts IAS - specially since its data log tells me it is gathering the data....just not showing it to me.

  11. #11
    skywagon8a's Avatar
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    Quote Originally Posted by bodumatau View Post
    at 40-70kts the TAS agrees with my GPS GS with the odd 1-2kt difference every now and again which I would put to sensors lagging a bit. so I am happy with my pitot installation.
    Tie a piece of yarn to the jury strut near your pitot tube. Notice the angle in relation to the tube when you are flying at minimum speed. If it is more than a 10 degree difference the pitot data entering your instrument has a compounded error.

    My IAS has been tested and is 100% accurate from 150 mph IAS down to 40 mph IAS. Below 40 an error of 5+ mph appears due to the relative wind to the pitot tube difference.
    N1PA

  12. #12

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    It isn't a pitot problem. The GDU's minimum speed display is 20mph, or 20kts if you prefer knots. The lowest tape marker is 30mph but the numerical and tape indicators come alive at 20.
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  13. #13

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    Quote Originally Posted by stewartb View Post
    It isn't a pitot problem. The GDU's minimum speed display is 20mph, or 20kts if you prefer knots. The lowest tape marker is 30mph but the numerical and tape indicators come alive at 20.
    so if I change to mph it still comes alive at 20? = 17kts?

  14. #14

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    20 in either measure as I understand it. I've never used knots.
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  15. #15
    www.SkupTech.com mike mcs repair's Avatar
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    in the installation manual, it mentions the 20 as a low limit.... still reading.....


    sec 36.4 Air Data Troubleshooting
    Under normal operating conditions, the ADAHRS provides the following air data information:



    • Total Air Temperature is measured
    • Outside Air Temperature (OAT)
    • Indicated Airspeed (IAS)
    • True Airspeed (TAS)




    • Barometric Altitude
    • Density Altitude
    • Pressure Altitude
    • Static Pressure



    • Differential Pressure


    NOTE


    TAS information
    can only be displayed at speeds greater than 20 Knots (TAS is invalid when the aircraft is sitting still, or if no OAT probe is connected).


    The displayed OAT (Figure 36-1) will either be a number (when valid), or red-X'd. Valid OAT is required to calculate TAS, so if TAS is invalid because OAT is unavailable, both OAT and TAS will be red-X'd, which indicates the problem is OAT (not air data) related.


    A dashed-out value for TAS may simply mean that the true airspeed being measured is too low to be of value (e.g. when the aircraft is on the ground). If the TAS indication remains dashed out at speeds greater than 20 knots and/or the OAT indication is Red -X’d (as shown in Figure 36-1):



    1. Check the GTP 59 OAT probe wiring and connection for faults.
    2. Check configuration module wiring for damage, replace if any is found.
    3. Replace the GTP 59 OAT probe.
    4. If the problem persists, replace the ADAHRS with a known good unit.





  16. #16
    www.SkupTech.com mike mcs repair's Avatar
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    Quote Originally Posted by KevinJ View Post
    That’s pretty slow. Could you see anything in front of you?
    everything is mounted differently on these fuselages for better slow/view, wings etc.... stewart or someone can explain it..

  17. #17

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    Quote Originally Posted by KevinJ View Post
    That’s pretty slow. Could you see anything in front of you?
    can see fine, its all a question of what you are comparing it to. if you fly a J3 or a Bücker Jungmann then you learn that being able to see forward is overrated, only need it to avoid other planes, all the rest you can do fine out of your side windows

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