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jcorner(at)shaw.ca
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 Posted: Thu Feb 08, 2007 9:35 am Post subject: ASI's and Altimeter Calibration |
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The current discussion brings up a concern I have never completely
understood regarding calibration of the altimeter and transponder
encoder.
As I understand it the process uses a calibrated vacuum source to
connected to the static line to:
1) ensure that the altimeter reads correctly over its range of operation
2) ensures that the output of the encoder agrees with the altimeter
when corrected to standard atmosphere pressure
My concern is that in actual dynamic operation ( i.e. flying ) the
location of the static source can and does affect ASI and Altimeter
readings.
Is it that the static port location does not effect the altimeter and
encoder readings sufficiently to be of concern even though ASI
readings are affected?
Perhaps the calibration process tests in some way that I am unaware,
the location of the static port?
Am I missing something it the calibration process?
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Float Flyr
Joined: 19 Jul 2006
Posts: 2704
Location: Campbellton, Newfoundland
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| Posted: Fri Feb 09, 2007 8:58 am Post subject: ASI's and Altimeter Calibration |
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Your concern is well founded. No doubt about it a lot of errors can be
caused by having the static ports in the wrong place. In our small planes
the speed isn't critical and few of us get up above 10.000' so some error
can be expected without too much of a problem.
Larger aircraft will have several parts of the fuselage tested for the right
spot to place a static port and some even have the static port built into
the pitot probe. If you have the opportunity to see an Aeronca Champ
sometime have a look at it's static probe. It's right below, or is that
above the pitot tube. The advantage is it's not in the prop wash and also
not affected by slipping.
Noel
[quote] --
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jcorner(at)shaw.ca
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| Posted: Sat Feb 10, 2007 7:37 pm Post subject: ASI's and Altimeter Calibration |
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Further to my previous post I was inspired by Lowel's ASI calibration
spreadsheet to do some testing..
Having ASI and altimeter sitting on my bench waiting for
installation in my new Model 5 I set up a water manometer.
I compared differences in ASI readings for 25, 50, 100, 150 and 200
foot changes in altitude, with the ASI static port connected as they
would be in actual operation.
ASI readings were 30, 40, 55, 60 and 75 mph respectively.
My conclusion is that it would take a very poorly placed Static port
to have much effect on altitude or encoder output. (i.e. a 30 to 40
mph ASI error in order to produce greater than a 50' error in altitude}
Jim
On 8-Feb-07, at 10:32 AM, Jim Corner wrote:
| Quote: |
The current discussion brings up a concern I have never completely
understood regarding calibration of the altimeter and transponder
encoder.
As I understand it the process uses a calibrated vacuum source to
connected to the static line to:
1) ensure that the altimeter reads correctly over its range of
operation
2) ensures that the output of the encoder agrees with the altimeter
when corrected to standard atmosphere pressure
My concern is that in actual dynamic operation ( i.e. flying ) the
location of the static source can and does affect ASI and Altimeter
readings.
Is it that the static port location does not effect the altimeter
and encoder readings sufficiently to be of concern even though
ASI readings are affected?
Perhaps the calibration process tests in some way that I am
unaware, the location of the static port?
Am I missing something it the calibration process?
|
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smokey_bear_40220(at)yaho
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| Posted: Sun Feb 11, 2007 11:33 am Post subject: ASI's and Altimeter Calibration |
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Great job Jim,
This is what the list is all about. "Experimental"
aircraft that we learn from.
I assume you applied a vacuum to the static sides of
the ASI and altimeter together?
That does seem to mean that a very small change at the
static port would change the ASI much more than the
altimeter.
Kurt S.
Do not archive
--- Jim Corner <jcorner(at)shaw.ca> wrote:
| Quote: | Further to my previous post I was inspired by
Lowel's ASI calibration
spreadsheet to do some testing..
Having ASI and altimeter sitting on my bench
waiting for
installation in my new Model 5 I set up a water
manometer.
I compared differences in ASI readings for 25, 50,
100, 150 and 200
foot changes in altitude, with the ASI static port
connected as they
would be in actual operation.
ASI readings were 30, 40, 55, 60 and 75 mph
respectively.
My conclusion is that it would take a very poorly
placed Static port
to have much effect on altitude or encoder output.
(i.e. a 30 to 40
mph ASI error in order to produce greater than a 50'
error in altitude}
Jim
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