Correcting Static Port - asking advice

[hevansrv7a]

I may have installed my static ports too far forward (one whole bulkhead's distance). Or perhaps it is the fittings. I used the ones Van's provided in the kit - the ones like big pop rivets.

My IAS is quite accurate at around 90 kts, but it gets progressively higher than CAS as I speed up and is around 4 to 5 kts too high at 8000' and 200 mph.

My TAS is accurate because it is GPS calibrated. However, this could be affecting my reported density altitude a little and I'd rather not have to make all the CAS corrections by hand (The GRT only corrects TAS). My OAT is also accurate, I believe.

The instrument is a GRT Horizon and it is IFR certified. I think there is virtually no chance that the instrument is the problem.

I don't get a warning from the system about the GPS-WAAS altitude being too different from the baro altitude, but that does not rule out an error within its limit setting.

Tape in front? Tape behind? File them down? I know Paul Lipps has suggested corrections for other, similar but not identical problems.

So I am asking Paul and others to suggest how to "fix" this. Thanks.

[dmaib]

Kevin Horton has published some great information on correcting pitot/static errors in Kitplanes magazine. He also has several posts on this forum about this issue.

[elippse]

Use of cellophane tape to build up immediately behind the port will increase pressure at the port and lower the IAS. Conversely, adding tape in front of the port will lower the port pressure and increase IAS. Jim Smith on his RV-6 had to add a washer cut in half surrounding the port on the back side to reduce his 10-12 mph IAS error. I think it was about 1/16" thick.

[Bavafa]

Long answer version
I had an error in my static port which was affecting IAS and altitude. My static port was flush with the fuselage and after much testing, I added a pop rivet style port and brought it as close as I could get it. I was much more concern about altitude (for IFR reason) then IAS. My tests were mostly around altitude by doing a low pass and comparing the indicated altitude with the field altitude. Before the fix, it would show 80 lower then the field altitude, but once fixed the IAS went down to .7 knot at the low speed and 1 knot at high speed, this was confirm with three way GPS test and also GRT TAS calibration.

Short answer,
I suggest by doing a low pass (just above ground affect) and compare your indicated altitude to the field altitude which you have previously set in your altimeter. If this is all good, then it might be some issue with your pitot tube (less likely though).

[hevansrv7a]

Paul gave me exactly what I wanted. Thanks to Paul and to everyone for the quick replies.