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Speeds, pressures and people smarter than I

JackZ

Well Known Member
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?

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

Jack
 
With GPS altitude vs indicated altitude, a ~500' split isn't unexpected...
 
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.

:cool:
 
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?

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.
 
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.
 
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?

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).
 
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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
 
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.
 
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
 
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?
Jack
IAS is used for many EFIS calculations so it will throw off other displayed values to include wind direction, wind speed, tail/head wind, cross wind component among others.

Here is a link to an Excel spreadsheet you can use to calculate your actual TAS based on GPS speeds.

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

Jack

An even more likely cause is that the wrong static ports were used.
Are you the original builder?
Do you know what static ports are installed (Van's, aftermarket)?
 
When I say position error I am thinking that the static pressure is slightly different than if the static ports were precisely located as per the plans. They are located precisely in the correct location for a lancair 320. That is where I got my heated pitot tube, and it has the static ports integral to it. The static port is well out on the wing.

I would like to leave it there if possible. Placing a couple layers of tape behind the holes has made the readings a lot closer, but they are not perfect yet. I was mostly concerned with the altitude being correct as this matters to atc.

Jack
 
I would like to leave it there if possible. Placing a couple layers of tape behind the holes has made the readings a lot closer, but they are not perfect yet. I was mostly concerned with the altitude being correct as this matters to atc.

Jack
If a couple of layers of tape behind the hole has made a difference, your static port is in an area of low pressure. That means your IAS, ATC altitude reporting and other EFIS calculated values are wrong. Correct your static source location issue and your other EFIS calculated issues, to include ATC reported altitude, will probably go away.

:cool:
 
Flew today. I added a couple more layers of electrical tape behind the static ports before flight. It?s getting a bit thick and I will replace this with a washer or some epoxy later. Six layers of tape now.

First thing I did was a pass down the runway at 140 Ktias. The altimeter read about 40 feet high. This is a bit of a guess due to the fact that I had to estimate the height above ground.

Next I climbed to 6000 feet and flew directly down wind at 24 square to enter the 200 kit club😎 We had winds in excess of 40 kits up there today.

Back to business I had Otto fly three legs 120 degrees apart. I used the spreadsheet that Galen provided (thanks) and came up with a calculated true airspeed of 156 ktas . I then did the calculations with the spreadsheet to arrive at a calculated ias of 144 Kts . My indicated speed in the flight was 145 on all legs the efis calculated 157ktas.

The difference between the altimeter and the gps derived altitude was also down to about 150 feet, although I did not climb to 10000 today.

I think 1 layer more of tape should be about perfect. It is getting real close and I noted that the wind indication in the Efis is a lot more consistent and believable

Thank you to all for taking the time to assist me with this, I really appreciate it.

Jack
 
If a couple of layers of tape behind the hole has made a difference, your static port is in an area of low pressure. That means your IAS, ATC altitude reporting and other EFIS calculated values are wrong. Correct your static source location issue and your other EFIS calculated issues, to include ATC reported altitude, will probably go away.

:cool:

As mentioned previously, the location of the static port could be fine with the problem simply being that is is the wrong port ( wrong shape).
This is a very common problem which has had a very easy fix by a lot of people. If Jack would supply information of what static ports are installed on his airplane, it may lead to a very simple and durable fix.
 
Scott, the static ports that I am using are in the pitot tube that I acquired in the carcas of a lancair 320. The holes are near the trailing edge of the pitot tube. That is the setup used by Lancair. I thought at the time that it was an elegant alternative to the vans plans. The accuracy is getting very close now.

The system worked very well in the lancair, I spoke with the previous owner of the lancair, and he said it always was fine. It was an ifr setup. I believe that the difference in the shape of our wing causes a little difference that I am nor rectifying. I think I will today measure the thickness of my ?static pressure increasing wall of tape? and replace it with epoxy they is bit thick and by trial I will sand to perfection.

Jack
 
I had Otto fly three legs 120 degrees apart. I used the spreadsheet that Galen provided (thanks) and came up with a calculated true airspeed of 156 ktas . I then did the calculations with the spreadsheet to arrive at a calculated ias of 144 Kts . My indicated speed in the flight was 145 on all legs the efis calculated 157ktas.
Jack
If the TAS displayed by your EFIS and your GPS spreadsheet calculated TAS is within 4kts, your system is fine so having it within 1Kt (156KTAS vs 157KTAS) is excellent. This error will change slightly at different airspeeds so you can wind up chasing your own tail by trying to get it "perfect" at all speeds.

The difference between the altimeter and the gps derived altitude was also down to about 150 feet, although I did not climb to 10000 today.
Don't even try to get your Altimeter and GPS altitudes to be the same. Only under specific atmospheric conditions are they ever the same. Once again, don't wind up chasing your own tail.

Here is how I verified my displayed altitude:
1 - Fly to an airport that has an ILS approach. Make sure your EFIS has the correct altimeter setting.
2 - Fly towards the FAF at normal cruise speed and at the FAF altitude as displayed on your altimeter. Use the autopilot for this. Make sure you are at a stable speed and altitude before arriving at the FAF.
3 - When you are at the FAF according to your GPS, look at the glideslope which should be centered.
4 - If it is not centered (or at least very, very close to being centered) then there is an additional problem unrelated to the static source.

FWIW I used the ILS Rwy31 at KSGJ for my altitude check. I had the autopilot hold 1,600ft (FAF altitude) while tracking the localizer towards the FAF at my normal cruise speed. When my GPS indicated I was at the FAF (HAMGO), the glideslope was centered so I now know my altitude display is correct.

:cool:
 
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Scott, the static ports that I am using are in the pitot tube that I acquired in the carcas of a lancair 320. The holes are near the trailing edge of the pitot tube. That is the setup used by Lancair. I thought at the time that it was an elegant alternative to the vans plans. The accuracy is getting very close now.

The system worked very well in the lancair, I spoke with the previous owner of the lancair, and he said it always was fine. It was an ifr setup. I believe that the difference in the shape of our wing causes a little difference that I am nor rectifying. I think I will today measure the thickness of my ?static pressure increasing wall of tape? and replace it with epoxy they is bit thick and by trial I will sand to perfection.

Jack

Roger that, sorry.
I didn't get that from your other posts when you were saying it was the Lancair standard.

Us caution with the adjusting method you are using. It is very possible to have a non linear indication as a result of you making the high speed end accurate. Doing so may make the low speed end even further off. A very bad thing if it makes the low speed end read high.......
 
Us caution with the adjusting method you are using. It is very possible to have a non linear indication as a result of you making the high speed end accurate. Doing so may make the low speed end even further off. A very bad thing if it makes the low speed end read high.......
If it is a dynamic static source error, which his appears to be, then it is very improbable for that to happen. Yes, these errors are non linear but the error becomes larger as airspeed increases and smaller as airspeed decreases. At low airspeeds it is very possible the airspeed is dead on and it is only affecting the displayed altitude, a real bad situation especially in actual IMC conditions. That is why people wind up thinking they have a fast airplane.

:cool:
 
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If it is a dynamic static source error, which his appears to be, then it is very improbable for that to happen. Yes, these errors are non linear but the error becomes larger as airspeed increases and smaller as airspeed decreases. At low airspeeds it is very possible the airspeed is dead on and it is only affecting the displayed altitude, a real bad situation especially in actual IMC conditions. That is why people wind up thinking they have a fast airplane.

:cool:

From what I have seen in the past, being involved with this issue with lots of different RV's, either one of us could be right.

Only flight testing will prove who is.
 
I had flush static ports on my Rocket. Today I added rivet heads over the ports in a attempt to correct a 10-12 knot airspeed error. Airspeed at stall remained the same. Airspeed at cruise went up about 15 knots. Ground the heads down just a bit and Airspeed is now very close.
G
 
I flew again today after measuring the thickness of my tape stack. It was only a few thousands thinner than a 960 10 washer. I cut a washer in half and glued it on. Speeds are right on, same as yesterday. Today I also flew a 3 way at low speed (65 kias) and it was right on. I went up high and the altimeter and the gps derived altitude were also within a 150 feet or so. On my plane all is now well.

Thanks to all.

Jack

C-FEES RV9A
 
I flew again today after measuring the thickness of my tape stack. It was only a few thousands thinner than a 960 10 washer. I cut a washer in half and glued it on. Speeds are right on, same as yesterday. Today I also flew a 3 way at low speed (65 kias) and it was right on. I went up high and the altimeter and the gps derived altitude were also within a 150 feet or so. On my plane all is now well.

Thanks to all.

Jack

C-FEES RV9A
Glad to hear the good results. FWIW, I would still do the ILS altitude check just to be sure your displayed altitude is accurate and reliable.

:cool:
 
On days when the temp is colder than standard, an altimeter will read ....hummmmm....high I think....so your lower than you think you are. So if it reads 10000 feet you'll run into a 10000' mountain because your really at like 9700'. Crunch.

When you miss the mountain, its usually No big deal. Everyone flying around will be at the same temp and have the same error when they run into each other.....unless they fly at the proper altitudes according to direction flight and they'll miss each other because everyone has the same cold temp error.

The further you are from sea level, the worse the temp induced error gets. Its not uncommon that RNAV RNP approach procedures to have a cold temp limitation. I'm looking at the RNAV RNP Y runway 29 at Newark and its not authorized when its colder than 11F. At LAX the RNAV RNP rwy 24R is not authorized colder than 41F unless a compensated BARO-VNAV system is used. (I don't know what that is.) (I wonder if a cold temp compensated Baro will screw up cruise altitude separation on a really cold day?)

I wrote this about approaches because if your GPS says your at 10,150, and your altimeter says your at 10,000 its more than just inaccurate altimeter settings. It would seem logical to me to ask if the atmosphere is at a standard temperature before declaring things not quite right. You might also ask what the GPS accuracy. The other day I saw 11M. So I think that means its 95% sure the number its gives me is within 11M or what....35 feet.

I really like the idea of flying fast down the runway and just checking that way. I even think shooting the ILS and checking the altitude over the marker to be exposing oneself to temperature related errors.

Don't forget to tap the altimeter too. Knock on it like your checking to see if anyone is in the bathroom. That knock will release some of the resistance of movement of the gears and effect the reading slightly.
 
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