Airspeed calibration

[Littleriverflyer]

I have a d180 and never have been able to get the airspeed and gps speed to match. I have flown many calibration circuits and gps is always 3 to 5 kts slower. Also the wind vector arrow almost always shows more headwind than actual. For example if it is showing 10 kt headwind and I reverse course it does not show a tailwind at all. I have done the zero wind static calibrations several times and experimented with different static port locations and port sizes. Is there a way to calibrate airspeed other than the static zero? Any other ideas?

 

[dynonsupport]

GPS is true airspeed, while the D180 is indicated airspeed on the bar. Unless you're at sea level they shouldn't match.

Are you checking the TAS on the D180 against the GPS speed, and is your OAT accurate? Also, please make sure you are using the same units.

The wind thing may be your compass. We won't know until we get the speeds verified.

There is no user calibration for airspeed except at 0 knots.

 

[Littleriverflyer]

My units are correct, OAT is accurate, compas is pretty good, certainly within 3 or 4 degrees in any heading. The dynon TAS is even higher than the IAS which is higher than the GPS ground speed. I live in Florida and the same relationship holds up even when real close to sea level. I suspect either a static problem or the dynon is off, I am working on building a manometer to test the dynon. If that checks ok then I guess the static must be pulling a little negative pressure at speed.