Speeds, pressures and people smarter than I

[GalinHdz]

Originally Posted by JackZ

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.

[plehrke]

Originally Posted by JackZ

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

Jack

What are you referring to as position error?

[rvbuilder2002]

Originally Posted by JackZ

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)?

[JackZ]

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

[GalinHdz]

Originally Posted by JackZ

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.

[JackZ]

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

[rvbuilder2002]

Originally Posted by GalinHdz

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.

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.

[JackZ]

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

[GalinHdz]

Originally Posted by JackZ

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.

Originally Posted by JackZ

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.

[rvbuilder2002]

Originally Posted by JackZ

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.......