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Thread: Determining calibrated airspeed

  1. #1
    Bob Collins's Avatar
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    Determining calibrated airspeed

    In order to determine true airspeed (I don't have one of the fancy EFISes that does this for you), I need to determine the calibrated airspeed. I'm looking for some easy-to-understand (important point for me) process of flight testing that will reveal the difference between IAS and CAS in my system. Any help is, of course, appreciated.

  2. #2

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    I don't know that you need calibrated airspeed to determine true? Isn't calibrated indicated corrected for position error of the pitot? Thus cal is usually very close to indicated, and I don't know how to find it.

    True airspeed can be figured by using indicated airspeed right off the airspeed indicator and correcting for non-standard temp and pressure. Standard pres and temp is 29.92 inches ( 1016 mb or is it 1013? ) and 59 *F at sea level. As you go higher and as you get warmer, then true is higher than indicated. So let's say you are in your P-51 headed to Oshkosh at 15,500 over Las Vegas and indicating 210 knots. Your true airspeed might be 280 knots, ( just a guess, I haven't worked it out).
    I don't have any fancy gizmo, but an E6B does a pretty good job, just as we learned in flight training years ago, and doesn't need batteries.
    Many planes have a ring on the airspeed indicator outside that sort of does the calcualtion for you. Set 29.92 in the altmeter. This gives you density altitude. Set this over the outside air temp, then look at your indicated airspeed and then look just outside of your indicated and there is the figure for true (corrected) airspeed. The E6B makes the numbers more spread out and thus more legible.
    Last edited by Bill Greenwood; 11-15-2012 at 02:44 PM.

  3. #3

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    Funny this topic should appear here when the October issue of the electronic newsletter EAA Experimenter has a comprehensive article on the topic. Ed Kolano probably explains enough to answer your questions.

    Best of luck,

    Wes
    N78PS

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    Bob, I'm guessing you just want to find out how fast your plane is going and are not collecting data for an airspeed calibration chart? If so, you can get a GPS, do some flight testing, collect ground speed data then work backwards to figure TAS.

  5. #5
    EAA Staff / Moderator Hal Bryan's Avatar
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    Quote Originally Posted by WLIU View Post
    Funny this topic should appear here when the October issue of the electronic newsletter EAA Experimenter has a comprehensive article on the topic. Ed Kolano probably explains enough to answer your questions.

    Best of luck,

    Wes
    N78PS
    Here's a direct link to the article Wes is referring to:

    http://experimenter.epubxp.com/i/84816/42

    Hal Bryan
    EAA #638979
    Online Community Manager
    EAA—The Spirit of Aviation

  6. #6
    Bob Collins's Avatar
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    Quote Originally Posted by Hal Bryan View Post
    Here's a direct link to the article Wes is referring to:

    http://experimenter.epubxp.com/i/84816/42
    Fabulous, fellas. Just what I was looking for.

  7. #7

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    Bob: Ed's articles are great! If you need equations let me know, Google them or get a copy of "Aerodynamics for Naval Aviators."

    Down and dirty: Use the whiz wheels

    Better: GPS minus wind (reciprocal headings) is true airspeed.

    Best: Calibrate the pitot static system (mainly the static as pitot is normally okay) and calibrate a temperature probe, too.

    Bonus: A lot of people go from indicated to true, which is not quite correct. Those numbers produce inaccurate numbers ... known in the industry as indicated mach and indicated true airspeed.

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