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  #1  
Old 01-19-2020, 05:52 PM
JackZ JackZ is offline
 
Join Date: Nov 2015
Location: neustadt ontario
Posts: 118
Default Speeds, pressures and people smarter than I

Hi
Flying my RV9A and I think I may have some instrument error. I suspect problems with temperature or position error on the static port.

A few observations:

Altitude is about 600 feet higher consistently on my GRT Sports than on my ipad gps derived altitude. (at approx 10000 ft)

The wind when flying into it are always a lot higher than when flying with a tailwind acording to the efis's or is that efi? They are separate ahrs. buit the read the same.

The temperature probe seems to read 4 celcius high.

The pitot static system has recently been tested and adjusted by a certified shop. o leaks and both altimeters and all three air speeds match.

TodayI was indicating 147Kts with 169 ktas and 194kts gs temp -18 c and 10800 ft, the ipad showed 10200 ft

is the problem position errror or temperature error or both?

[i] would appreciate the brighter folks than I to chime in. ( that means just about everybody )

Jack
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  #2  
Old 01-19-2020, 05:58 PM
SPX SPX is offline
 
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With GPS altitude vs indicated altitude, a ~500' split isn't unexpected...
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  #3  
Old 01-19-2020, 06:17 PM
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GalinHdz GalinHdz is offline
 
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This situation is not that uncommon with home built airplanes with even some Certificated airplanes having this issue. There was a LONG write up in the Dynon SkyView Forum that covered this very issue. In order to find the cause, you must do a 3 leg GPS speed run and calculate your actual TAS not the TAS your system is displaying as the first step in the troubleshooting process.

BTW: do not try to use GPS derived altitude to check your system. The altitude displayed on your altimeter is based on barometric pressure while your GPS altitude is not. Apples versus Oranges. Also, no amount of ground checks will uncover the problem if it is a dynamic static source problem. You have to do the GPS test while flying.

I hope this helps get you pointed in the right direction.

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  #4  
Old 01-19-2020, 06:29 PM
Bavafa Bavafa is offline
 
Join Date: Oct 2007
Location: Sacramento, CA
Posts: 3,351
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Quote:
Originally Posted by JackZ View Post
Hi
Flying my RV9A and I think I may have some instrument error. I suspect problems with temperature or position error on the static port.

A few observations:

Altitude is about 600 feet higher consistently on my GRT Sports than on my ipad gps derived altitude. (at approx 10000 ft)

The wind when flying into it are always a lot higher than when flying with a tailwind acording to the efis's or is that efi? They are separate ahrs. buit the read the same.

The temperature probe seems to read 4 celcius high.

The pitot static system has recently been tested and adjusted by a certified shop. o leaks and both altimeters and all three air speeds match.

TodayI was indicating 147Kts with 169 ktas and 194kts gs temp -18 c and 10800 ft, the ipad showed 10200 ft

is the problem position errror or temperature error or both?

[i] would appreciate the brighter folks than I to chime in. ( that means just about everybody )

Jack
A no leak test or any static port test on the ground is not going to show if there is any possible error induced by the static port. A good test would be to use a 3-way or 4-way GPS run and compare the result. Also, it is fairly common that an error induced by the static port location or shape varies at different speed. so, this is where I would start about the accuracy of the TAS and IAS.
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  #5  
Old 01-19-2020, 06:31 PM
ty1295 ty1295 is offline
 
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With the local baro setting correct, does your altimeter read proper field elevation?
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  #6  
Old 01-19-2020, 06:31 PM
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plehrke plehrke is offline
 
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Location: Defiance, MO
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If you are really getting that much error it most likely is not temp related as temperature has a very small impact on TAS Numbers (none on IAS) and on altitude.
I suspect maybe static system error. A lot of RVs don?t follow the recommendation from Van on static port position or shape of port. To check to make sure your static system is really showing correct static check your altitude on the ground then takeoff and circle around and fly down the runway at Anywhere between approach speed and cruise speed about 25 feet above the runway (high speed pass) and check your altitude. It should read less than 100 ft different than your ground altitude was. If more, than your static system is getting in some pitot pressure, altitude will read high and speed will read lower than they should.
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  #7  
Old 01-19-2020, 06:32 PM
BobTurner BobTurner is offline
 
Join Date: Dec 2011
Location: Livermore, CA
Posts: 6,797
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Quote:
Originally Posted by JackZ View Post
A few observations:

Altitude is about 600 feet higher consistently on my GRT Sports than on my ipad gps derived altitude. (at approx 10000 ft)

The wind when flying into it are always a lot higher than when flying with a tailwind acording to the efis's or is that efi? They are separate ahrs. buit the read the same.

The temperature probe seems to read 4 celcius high.

The pitot static system has recently been tested and adjusted by a certified shop. o leaks and both altimeters and all three air speeds match.

TodayI was indicating 147Kts with 169 ktas and 194kts gs temp -18 c and 10800 ft, the ipad showed 10200 ft

is the problem position errror or temperature error or both?

[i] would appreciate the brighter folks than I to chime in. ( that means just about everybody )

Jack
As noted above, gps measures true altitude, altimeter is barometric altitude. They agree only on a ‘standard day’. 600’ difference at 10,000’ seems a bit much, though, unless it was very non-standard.
Does your temperature reading look okay, on the ground, engine off? Is there any way it can see hot exhaust?
The wind calculation depends on the TAS calculation (efis) and the ground speed data (gps). Try this: slowly turn, constant altitude, determine the direction of the wind using gps ground speed (min speed = headwind). Fly into the wind, note ground speed. Turn 180 deg (tail wind), note ground speed. Average the two ground speeds. Result should equal the true airspeed.
Static port check. Set altimeter to field elevation. Do a high speed pass down the runway at a known elevation (maybe the top of the tower as a guide). If the altimeter does not read the tower elevation, there’s a static port error.

Edit. Wow! 6 posts while I was typing (and watching football).

Last edited by BobTurner : 01-19-2020 at 06:36 PM.
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  #8  
Old 01-19-2020, 06:35 PM
JackZ JackZ is offline
 
Join Date: Nov 2015
Location: neustadt ontario
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Ok
I think the GPS speed run in three different directions is the next step. The static port error could affect both indicated speed and altitude, correct?

It will be a couple days maybe to find flyable weather ere maybe.

Jack
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  #9  
Old 01-19-2020, 06:42 PM
BobTurner BobTurner is offline
 
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Location: Livermore, CA
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Quote:
Originally Posted by JackZ View Post
Ok
I think the GPS speed run in three different directions is the next step. The static port error could affect both indicated speed and altitude, correct?

It will be a couple days maybe to find flyable weather ere maybe.

Jack
Correct. Indicated airspeed will be more sensitive to any error.
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  #10  
Old 01-19-2020, 06:42 PM
JackZ JackZ is offline
 
Join Date: Nov 2015
Location: neustadt ontario
Posts: 118
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TY125 Yes

Bob Temp probe is under the win in the inspection plate so no sun or exhaust.

I suspect position error. This could affect alt reporting as well as speed because both depend on accurate static pressure correct?

Jack
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