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  #1  
Old 09-17-2017, 04:06 PM
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rjcthree rjcthree is offline
 
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Default ASI/TAS calibration - am I done?

I've been chasing a static position error at the start of my phase one that had altitude errors with airspeed, consistent with what many go through. Alt errors and TAS variation vs GPS 3 ways by >100 ft and up to 14kt at highest speed respectavely, reducing with reduced airspeed.

I got the errors down using the Brantel rivet/washer (0.080" proud) to less than 40' and 2kt across the speed range (TAS vs GPS GS using 3 ways). My DA's needed to vary due to WX, but they were within about 500' in all runs (roughly 5500' at 4600' msl).

I think I'm done with this effort. IAS is of course off, but that's just physics.

Does this seem reasonable?
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  #2  
Old 09-17-2017, 04:34 PM
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Yes it's reasonable.

Most of the certified aircraft I've flown have at least that much error in some part of the envelope (look at IAS vs. CAS in the POH)

You could probably tweak it a bit more, especially with a modern EFIS, but is that much absolute accuracy worth the effort? It's an individual preference. Operationally it gets lost in the noise.
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  #3  
Old 09-23-2017, 07:54 PM
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Default Argh!

Ok, so no changes to the airframe, I go out today to lean the engine for the first time. Looking at at data there is a 200' difference between gps alt and alt. GPS > PALT. This is in steady state conditions, 8000' DA. TAS calculated by the Dynon and 3way TAS calls are within 2 kts. How is that possible?

Or are Am I seeing temperature affects of a less than std temp day? I thought those were not impactful on static pressure altitude?

Feeling kinda stupid at the moment. Ant hints are welcome.
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  #4  
Old 09-24-2017, 08:31 AM
Mr Grinch Mr Grinch is offline
 
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GPS altitude is a geometric derived value based on an artificial model of a spherical earth. It isnít relevant to your pressure altitude calibration.

Use a current barometric altimeter setting with the aircraft on a known elevation point, like the touchdown zone height on a runway. Donít use the airport elevation, read the definition and you will understand why.
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  #5  
Old 09-29-2017, 12:58 PM
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Mr Grinch and a whole bunch of data crunching sent be back into the archives and Google. I found this:

http://www.vansairforce.com/communit...ad.php?t=89446

While it wanders a bit, it resolves my concerns and matches my observations. Thanks again all who went there before me, and Mr Grinch for forcing me to reform my hypothosis.
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Dog is my copilot, rest in peace 2015APR28. I miss that dog.

Last edited by rjcthree : 09-29-2017 at 03:37 PM.
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  #6  
Old 09-29-2017, 04:25 PM
BobTurner BobTurner is offline
 
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Quote:
Originally Posted by Mr Grinch View Post
GPS altitude is a geometric derived value based on an artificial model of a spherical earth.
Actually it's a pretty good model of a non-spherical earth....

But to the OP's question. GPS altitude, from a good gps, should be quite close to true altitude - same as you'd get from a vertical tape measure, from sea level to you. Your altimeter should read the same if it's a standard day: 15 C at sea level, dropping 2 C every thousand feet of altitude. Humidity is another variable but that should be pretty well corrected if you have the proper setting in the Kollsman window. If it's not a standard day the altimeter and gps won't agree. Use the altimeter to fly at the correct altitude to help avoid other airplanes; but if you're close to a mountain top make sure your gps altitude is higher than the mountain!
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  #7  
Old 09-29-2017, 04:36 PM
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Default It was a non Standard day!

For the 3-way runs in question, I was seeing 19C at 8100 DA, 6400 on the altimeter. At 792ft MSL field, baro was 30.16, if I remember correctly. At the above alt, I was seeing nearly 200 ft delta.
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