The TAS error is 7-10 KTAS. This has been determined by flying three GPS runs and using the widely available Excel spreadsheet.
I’m sure that my error is due to a static position error. If I engage the altitude hold and vary the IAS from Vs+10 to cruise, the GPS altitude will vary by 110-130’. At cruise, the GPS altitude is lower than indicated altitude, so the altimeter is reading 110-130’ high at cruise. The static system is reading a lower than ambient pressure at these speeds.
You’re correct that a TAS correction would fix the system, and not the problem. I have a stock Van’s static system. Short of randomly drilling holes in the fuselage, I’m unsure how to solve this. The ideal solution would be to increase the pressure at the static port. The documentation that I’ve been able to find talks about determining the error, but not how to correct it.
Have your double checked the location of your static ports against the plans?
Is there anything about your static ports that would cause them to be sticking out further from the skin than on other RVs? Is it possible that Van included the wrong rivets in the static system kit, or that you installed the wrong rivets? Measure the height of your static ports (i.e. the amount they stick out from the skin), and compare it to other RVs.
This may very well be static system position error. An error of 8 kt TAS at 8,000 ft would require an error of about 7 kt CAS. This would imply an error in the static pressure of about 0.11 in HG, which would give about a 130 ft error in the altitude. This is consistent with what you report.
If you want to mess around trying to reduce the error, here are a few things you could try:
1. You could reduce the amount the pop rivets stick out from the skin. Either substitute a flatter rivet, or polish a bit of material off the rivets to make them lower. The magnitude of errors you report is about the same as other RVers have reported with flush static ports (their errors were in the other direction though). Some people fixed their errors by glueing the domes from 3/32 (or 1/8?) rivets on top of the static ports. This suggests that you may eventually need to remove about that much material from your pop rivets, but go at it a bit at a time, with a test flight between each removal of material. I have no idea what this will do to the integrity of the pop rivets. You may want to look for flatter pop rivets to use as a starting point.
2. Alternatively, you could put a small dam just behind the static ports. This would be a small vertical wall just behind the ports that would cause the air to jam up against it, which would cause a small local increase in the static pressure. Easy does it here too, as too big a dam could cause a very large change in airspeed system error. Start small, and work your way towards bigger.
I recommend option 1, as it should be just as effective, and would look a lot better.
If you make either type of change described above, please understand that it could affect the airspeed accuracy at all airspeeds. Thus the IAS at the stall may change, and the optimum IAS for take off and approach may change. This could lead to an accident if you simply use the same IAS you have always used. Be careful. On the first flight with any changes to the airspeed system, it would be prudent to do stalls to check the IAS at the stall, and to do a simulated approach and flare at altitude to confirm adequate handling at your proposed approach IAS.
I am a bit baffled about what is apparently different about your aircraft though. I know that most RV builders don't do extensive static system accuracy testing, so it is possible that many RVers have large errors and don't know it. But, the few that have done testing seem to find relatively small errors, as far as I can tell from the available reports.
Keep us posted. I love a mystery.
, it seems to be a bit of an RV issue with the static port, wouldn't it be better to fix the static port issue instead of correcting the static error on the Dynon? I think to recall that flush static ports did cause that issue on the RV's. Did you check on the vansairforce forum? I just checked