IAS too high

[torstenv]

Hi!

I have the impression that my D180 shows an airspeed that's too high.

First I trimmed my plane at 75% Power. IAS was about 145kt, TAS is about 153kt. I oriented the plane to fly into the winds direction (250°, 100% headwind). There's a GPS source connected to the D180. Thus the D180 can calculate the wind direction and speed. In my test that showed me I had a headwind component of 34 kt. Then I turned around to fly the opposite heading (70°). After the turn I should fly with 100% tailwind component. The IAS and TAS indication remain the same (as expected), but the indicated wind component on the D180 now shows the wind to be coming from the left with 20kt. I have reproduced this a few times.

Thinking about the possible sources for this issue I think the calibration of the D180s pitot pressure sensor could be wrong. Or maybe my pitot tube is somewhat wrong ("funnel" formed). However I cannot see that there's anything wrong with my pitot tube.

To verify this I looked at the GPS speed.
Headwind direction GPS ground speed: 121kt
Tailwind direction GPS ground speed: 157kt
Speed difference: 36kt
resulting wind: 250° 18kt

D180 TAS indication: 151kt

Calculation how much the D180 is off:

151kt (D180 TAS indication) - 18kt (headwind component) = 133kt (GS)
133kt (GS) - 121kt (GPS GS) = 12kt

The D180 is showing 12 kt too fast.

The same calculation can be done while flying with tailwind.

151kt + 18kt = 169kt
169kt - 157kt = 12kt

The question is: How can I calibrate the D180 speed indication?
According to the D180 installation manual on page 9-5 I can calibrate the pressure sensors if airspeed is indicated wrong during takeoff speed ("Airspeed is inaccurate by between 1 and 10 knots (or equivalent, in other units) at takeoff speed.") and I did that, but it didn't solve the problem.

Is there anything I can do to fix or calibrate this? Maybe narrowing the opening of the pitot tube?

Thanks in advance!

T.

 

[jakej]

T. This may be helpful from earlier this year, note the last sentence ---

Groundspeed displayed on your GPS or on the Dynon is purely from the GPS. Technically modern receivers calculate this from Doppler shifts, not from two position solutions over X time, but the effect is the same.

The wind arrow and velocity are dependent on:

1) Mag heading
2) OAT
3) Airspeed
4) Altitude
5) GPS track
6) GPS groundspeed

5&6 are basically never wrong assuming you have a GPS lock

2,3,4 all make up TAS, which the original poster said is accurate

Thus, #1 is the issue, and it makes a huge difference. If you do the vector math, just a few degrees error means the winds need to be large. So you need to get your mag as accurate as possible if you want the winds to be useful."

HTH

Jake J

 

[torstenv]

Jake,

thanks for your reply.

If I understand it right, you are quoting a message from another thread. However, in this other message there's the assumption that the airspeed is correct ("which the original poster said is accurate"). This is not what I said. I do believe that the airspeed is not measured correctly. (Though I may not have done the right tests to reliably verify this.)

Is there anything wrong in the method I tried to verify the IAS? I tried to fly the plane in the direction of the wind, compared IAS to GPS speed, turned around 180° and did the same comparison. So one GPS speed is with headwind, one with tailwind.

However, I'm going to ask a certified technician to connect pitotstatic test equipement.

Thanks,
T.

 

[dynonsupport]

The first thing to check is always the static system. You need to check for static errors. In our experience, the vast majority of errors are here. The fun way to do this is to overfly an runway, once fast, once slow, and see if your altitude changes. If it does, static error.

The boring way is to hold a GPS altitude as you change speed and see if your altimeter changes.

Once that's settled, then you can worry about IAS error (before you worry about TAS error). You can't trust the winds on the D180 yet, they are based on accurate TAS! Here's a good overview of how to do the tests, finding the winds manually:

http://members.eaa.org/home/homebuilders/testing/articles/4GPS & Airspeed Calibration.htm

Then worry about TAS, which requires accurate OAT and altitude. TAS is just math off OAT, IAS, and altitude, and has been verified over thousands of flying units. However, junk in, junk out, so if your IAS really is right, then your OAT or altitude is off.

Narrowing the pitot will not do anything besides make the pitot system less reliable.

We have people send in their units now and then, convinced that their airspeed is off, and it's less than 5% of the time that the unit has any error. We fully calibrate the D180 at the factory to be very accurate in airspeed and altitude. It doesn't take much pressure error in your static system to crate a 12 knot error.

 

[GalinHdz]

The first thing to check is always the static system. You need to check for static errors. In our experience, the vast majority of errors are here. The fun way to do this is to overfly an runway, once fast, once slow, and see if your altitude changes. If it does, static error.

The boring way is to hold a GPS altitude as you change speed and see if your altimeter changes.

Once that's settled, then you can worry about IAS error (before you worry about TAS error). You can't trust the winds on the D180 yet, they are based on accurate TAS! Here's a good overview of how to do the tests, finding the winds manually:

http://members.eaa.org/home/homebuilders/testing/articles/4GPS & Airspeed Calibration.htm

Then worry about TAS, which requires accurate OAT and altitude. TAS is just math off OAT, IAS, and altitude, and has been verified over thousands of flying units. However, junk in, junk out, so if your IAS really is right, then your OAT or altitude is off.

Narrowing the pitot will not do anything besides make the pitot system less reliable.

We have people send in their units now and then, convinced that their airspeed is off, and it's less than 5% of the time that the unit has any error. We fully calibrate the D180 at the factory to be very accurate in airspeed and altitude. It doesn't take much pressure error in your static system to crate a 12 knot error.

I could not get the link to work but found this one http://www.challengers101.com/CalASI.html
that worked.