From an RV guy who lurks...

One consistent problem we have had with static ports is in flush mounted  ports. To work correctly the port must sit proud of the surface about 1/16". Look at the static ports STEC supplies with their autopilots.

An easy fix, just glue a small washer over each port and go fly.

To check true airspeed, go to the National Test Pilot School site

http://www.ntps.edu/HTML/Downloads/

and download the GPS PEC excel spreadsheet. Fly a square pattern according to the "Method" document there. When you want to test your speed mods later, fly it again and again and again.

John Huft
Race 18
"Nuisance"