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Thread: Airspeed indicated to GPS

  1. #1
    Colorguns's Avatar
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    Airspeed indicated to GPS

    My airspeed instrument at about 50MPH and lower do not match ground speed even on a calm no wind day. At 35 MPH indicated the GPS ground speed is still 40-45 MPH. any Ideas what to do to get them to be more accurate to each other. I have tried adjusting the pitot tube angle and that does not help. I just fly it to what it feels and do use the AI as a reference.

    Doug

  2. #2
    mvivion's Avatar
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    Quote Originally Posted by Colorguns View Post
    I just fly it to what it feels and do use the AI as a reference.

    Doug
    Yep, that's the drill.

    MTV

  3. #3

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    try blowing the line clear from the gauge end of the pitot tube. Do not blow into the line with gauge attached takes little pressure to screw things up.
    DENNY

  4. #4
    cubdriver2's Avatar
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    Stop looking at GPS and ASI when landing

    Glenn
    "Optimism is going after Moby Dick in a rowboat and taking the tartar sauce with you!"

  5. #5
    BC12D-4-85's Avatar
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    If I can still see my intended landing spot (safe angle of attack) and my final rate of descent isn't excessive (as learned through experience) I ignore the ASI...in gusty condx add the gust difference to the initial approach airspeed. Practice seeing, hearing, and other senses over reading instruments near the ground.

    GAP

  6. #6

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    Every now and then (= not often) on a calm day my airspeed matches my ground speed and (I know you'll be amazed) my watch and my phone even have the same date and time on them so then in order to burst my zen bubble I just look at my compass and marvel at the sun setting in the East and say "Oh yea it's a Cub" and I understand.
    Last edited by OLDCROWE; 01-02-2017 at 08:43 AM.
    Remember, These are the Good old Days!

  7. #7
    Dave Calkins's Avatar
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    Pitot error at low airspeed/higher angle of attack to fly slow.

    Not pilot error

  8. #8
    skywagon8a's Avatar
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    Colorguns,
    First Indicated airspeed is a function of ram air pressure/ambient (static) pressure and GPS is ground speed.
    The pitot tube wants to be aligned +/- 10 degrees to the line of flight to avoid turbulence at the pitot opening.
    The relationship of the pitot tube and the static ports wants to be constant.
    In order for the pitot/static ports to be accurate at higher angles of attack they should be mounted on a gimbal which keeps them pointed at the proper relationship to the relative wind.

    The airspeed of my Cub has been calibrated over it's entire speed range and is accurate from VNE down to 40 mph. Between 40 and stall speed the error increases to as much as 5 mph.

    When you get down to the landing phase of flight you need to transition to the seat of the pants indicator system.
    N1PA

  9. #9

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    Whatever you decide, do not use GPS ground speed as an aid to aircraft control.

    Aispeed indicators are rarely accurate across their entire range, and a pitot compounds that problem. Part of a J-3 checkout is to cover the aispeed indicator for several turns around the pattern. We have one Cub that approaches at 50 mph indicated, and another that approaches at 40 knots. Same actual speed, different indicators.

  10. #10
    skywagon8a's Avatar
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    Quote Originally Posted by bob turner View Post
    We have one Cub that approaches at 50 mph indicated, and another that approaches at 40 knots. Same actual speed, different indicators.
    Bob,
    Once upon a time I did an STC project using the newest model Cessna A185F. It's airspeed indicator was calibrated in knots. The FAA refused to apply the STC to the previous years identical model. The ONLY difference between the two model years was that the airspeed indicator in the one year older A185F was calibrated in mph.
    N1PA

  11. #11
    Steve Pierce's Avatar
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    Look at where the pitot tube is and what happens to the air there when flaps are pulled.
    Steve Pierce

    Everybody is ignorant, only on different subjects.
    Will Rogers

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