I have a 8-10 kt error in my TAS at cruise due to a static position error. The error dissapears at 100 kts KIAS. I verified the error was related to static rather than pitot by using GPS altitude and my altitude hold. The magnitude of the error was verified using GPS ground speed with the widely available formula. I tested as follows:
1. Verified the GPS altitude was accurate while sitting on the ground. It was within 10' of field elevation.
2. Flew to 3000', engage the autopilot and accelerated to cruise.
3. The GPS altitude was indicating around 2870
4. Slowed to 70 KIAS and recorded both indicated altitude and GPS altitude while slowing.
5. They matched at 100 KIAS, after that GPS alt was higher than indicated alt.
6. Repeated the above, but while accelerating from 70 KIAS to cruise, same results.
7. GPS ground speed runs at cruise and 100 KIAS verified that TAS was reading 8-10 kts higher than GPS GS at cruise and GPS GS equaled TAS at 100 KIAS.
I have only identified the problem. I still need to solve it. I have Van's blind rivet port installed exactly where he specified on my RV-6A.