Results 1 to 6 of 6

Thread: Airspeed indicator

  1. #1

    Join Date
    Sep 2014
    Posts
    12

    Airspeed indicator

    My airspeed indicator is reading 15-20 mph higher than my gps under no wind conditions. How can I determine what is wrong with my airspeed indicator?

  2. #2
    FlyingRon's Avatar
    Join Date
    Aug 2011
    Location
    NC26 (Catawba, NC)
    Posts
    2,627
    I assume you're already taking into account the conversions from indicated to calibrated to true airspeed in your calculation?

    How's your static system? A leak in the static system to the cabin will cause a erroneously high reading.

    A instrument/avionics shop may be able to lend a hand.

  3. #3

    Join Date
    Sep 2014
    Posts
    12
    I will have to check for leaks. My plane is an open cockpit so I left the static port open on the back of the instrument. Do you think that is my problem?

  4. #4
    TedK's Avatar
    Join Date
    May 2013
    Location
    Pax River MD
    Posts
    365
    Quote Originally Posted by LAWise View Post
    I will have to check for leaks. My plane is an open cockpit so I left the static port open on the back of the instrument. Do you think that is my problem?
    Yep. There's your problem.

    Think about out the slipstream line over the cockpit...it curves up at the windscreen then continues over your head until then coming down over the aft fuselage. Looks just like the slipstream line over the top of the wing. What is the result over the wing? Where does that Lift on the top of the wing come from? Low pressure. Same thing in an open cockpit.

    Do you have a VSI? What does it show when you maintain a steady altitude on the Altimeter?

  5. #5

    Join Date
    Aug 2011
    Posts
    282
    The posts above are correct. You should be looking at the static system (pitot pressure cannot read high in an unpressurized airplane). Another check is to check the altimeter, statically on the runway and then compare it to the value when doing a low ("higher speed") pass. This is how tower fly-by airspeed calibrations are done.

  6. #6
    cub builder's Avatar
    Join Date
    Nov 2011
    Location
    North Central AR
    Posts
    456
    Quote Originally Posted by LAWise View Post
    I will have to check for leaks. My plane is an open cockpit so I left the static port open on the back of the instrument. Do you think that is my problem?
    Yes. The windscreen will cause a negative pressure behind it. If your static source is open into the cockpit, the negative pressure will cause your ASI to read high.

    Once upon a time I met a fellow with a little home built biplane that wanted to play tag with me in my Starduster 1. He was quoting performance numbers about the same as my Starduster. First time I got on his tail, I darned near ran over him. He had his static vented into the cockpit, so was drawing a negative pressure there. Made his performance numbers look incredible as compared to his actual performance.

    -Cub Builder
    Last edited by cub builder; 09-30-2014 at 12:02 PM.

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •