First verify no leaks in the system. I've been told that pitot tube variations only have a tiny effect unless it's WAY off. Then, following up on what RV7Charlie said, I would recommend going to Kevin Horton's site and mine it for flight test procedures. You're describing calibration of your airspeed indication. If you are consistently reading 5 to 10 knots below actual GPS speeds when compensating for wind I think the most likely culprit is the static port. Location is only one suspect. Another is the shape of the port.
Tiny variations in dome height/shape can affect your static related readings. If you're indicating lower than actual the static source is being pressurized. One experiment would be to disconnect the static line and vent it to the cabin. This is an easy experiment if you have an alternate static source
The cabin typically sees slightly negative pressure so there should be a big difference in indicated airspeeds between a slightly pressurized exterior port and a source in the cabin seeing a slight vacuum. There are lots of threads on solutions with pictures of various dams in front, behind and around the port. I was reading too high and wound up filing the round head flatter because mine was getting suction. It was an iterative process...file, fly. File, fly...took about 4 progressions of filing to get it right. You will need to work the other way. Build a dam to pressurize it, fly. Modify, fly until it is right. You're aiming at 0 difference but I wound up settling for a consistent 3 kts higher than actual, from high cruise speeds to about 10 kts above stall. (For me, getting consistent airspeed flight test data gathering was VERY challenging at lower airspeeds
) It would be interesting to see what error people are settling for and whether it's consistent or varies with airpseed.
The fun part is this process puts the "experimental" front and center in Phase 1
Jeremy Constant
