Paul A. Franz, P.E.
Joined: 02 Dec 2008
Posts: 280
Location: Bellevue WA
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 Posted: Fri Feb 13, 2009 8:34 pm Post subject: Calibrating and instrument |
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On Fri, February 13, 2009 7:34 pm, WurlyBird wrote:
| Quote: |
.... Here is the question, is there a way to calibrate the ASI besides simply
bending the pitot so it is not as directly into the wind? What is the RIGHT way to
do this?
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There is a way to calibrate the airspeed indicator but it doesn't involve bending the
pitot tube.
1) Calibration means correcting the response indication to a known standard.
2) Once the instrument reads within allowable limits of variation to a defined and
known input then you proceed to investigate and correct detection errors.
In the case an airspeed indicator the speed indication is the difference between
static air pressure and velocity pressure. So if it is in error, there are two
detection position anomalies possible. The pressure detection needs to be done out of
the influence of the prop blast and away from flow disturbances that occur when the
wing stalls.
The static pressure is often taken from a line that T's into a static port on each
side of the airplane so that it measures the average static pressure and is less
influenced by slips and skids. Many people have found that a simple static vent to the
airplane interior is fine but when you monkey around with open doors in flight and
various ventilation schemes the interior pressure can change and hence the airspeed
indication.
So, don't go bending your pitot tube thinking it will somehow change your high
indication. If you bend it enough to change the airspeed reading it will be
approaching 90 deg to the relative wind. At the low speeds these aircraft travel,
pitot tube angle has little affect on reading. Calibrate the instrument itself and
then correct pressure sampling problems.
--
Paul A. Franz
Registration/Aircraft - N14UW/Merlin GT
Engine/Prop - Rotax 914/NSI CAP
Bellevue WA
425.241.1618 Cell
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_________________
Paul A. Franz, P.E.
Registration/Aircraft - N14UW/Merlin GT
Engine/Prop - Rotax 914/NSI CAP
Bellevue WA
425.241.1618 Cell
425.440.9505 Office |
|
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