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Thread: Erratic Airspped Indicator

  1. #1

    Join Date
    Oct 2013
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    Petal, Miss
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    Default Erratic Airspped Indicator

    After getting the airplane rebuilt and finally back in the air, I've been trying to improve my land skills. Trying to nail the airspeed at 55 on the approach is next to impossible. The airspeed is all over the place. I usually just wing and discover I'm much too fast.

    Should I expect an airspeed indicator in this airplane to be a little more behaved? Or am I'm just going to have to learn how to wing it.

    One thought I had in mind was that the static pressure is ported to the cabin, and there is no static port. There's not one on a Cub either, but I've never seen an airspeed indicator swing so wildly as this one does.

    Thanks,
    Chuck Gruby
    Petal, Kit Fox III Flying

  2. #2
    Russell320's Avatar
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    Oct 2009
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    Chandler, AZ (P19)
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    93

    Default Re: Erratic Airspped Indicator

    Your pitot tube may be partially blocked.At high speeds you may not notice it, but at low speeds plugged or partially plugged tubes will be more noticeable. Depending on the type, remove the pitot tube and clean it out. Bugs and other stuff love pitot tubes. MAKE SURE THE PITOT TUBE IS OFF THE AIRPLANE. Static pressure from the back of the panel should be is just fine.
    Russ
    Model IV Speedster
    912UL IVO IFA
    RV6A
    Needle, Ball and Airspeed
    Maybe a little GPS

  3. #3
    Senior Member jiott's Avatar
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    Jun 2010
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    Default Re: Erratic Airspped Indicator

    I think you should be able to get a more stable airspeed reading than what you describe. Mine doesn't vary more than about + or - 2mph during the approach. There are a lot of opinions on this forum as to whether taking static pressure from inside the cab is good or not. Kitfox recommends a static port location low and just forward of the rear stab on the fusalage (on both sides for best accuracy). Forward slips on final can also mess with airspeed indication accuracy. Outside air vents blowing ram air into the cabin and I believe the stab trim slots on the tail area blow ram air into the cabin can increase cabin air pressure compared to outside. I believe every aircraft may react somewhat differently to all these effects.
    Jim Ott
    Portland, OR
    Kitfox SS7 flying
    Rotax 912ULS

  4. #4

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    Oct 2013
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    Petal, Miss
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    Default Re: Erratic Airspped Indicator

    Quote Originally Posted by Russell320 View Post
    Your pitot tube may be partially blocked...
    Thanks a-heap Russ. I guess I should have thought of that, but what tha hey, this 80 year old brain just doesn't tick over as good as it used to.

    Thanks again,
    Chuck Gruby
    Petal, Kit Fox III Flying

  5. #5

    Join Date
    Oct 2013
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    Petal, Miss
    Posts
    187

    Default Re: Erratic Airspped Indicator

    Quote Originally Posted by jiott View Post
    I think you should be able to get a more stable airspeed reading than what you describe...
    Thanks Jim, I've got everything else pretty well done, it's time to clean out the old pitot tube.

    Cheers,
    Chuck Gruby
    Petal, Kit Fox III Flying

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