Determining static position error

[sunfish]

I'm finding it difficult to determine if I have a static position error. What is the gold standard test? I tried a shortcut using savvy analysis to plot the difference between pressure height and GPS altitude and that is generally constant within 80 feet no matter what the speed is, specifically, when I slow from 80 knots to 45, there doesnt appear to be any change. Is this good enough? I'm concerned that my speed may be over reading because I cant seem to achieve the low stall speeds specified by the manufacturer.

 

[GalinHdz]

I had this exact issue with my instal back in 2013 and got it corrected. If you go to the maintenance section of my webpage ( www.puertoricoflyer.com ) you can download a .pdf file (SkyView TAS Calibration) of how I figured it out and fixed the issue. It is actually quite simple to do.

BTW completely disregard any GPS altitude numbers while doing this. GPS/Pressure altitude compensation requires data that is significantly more accurate than we typically have available. Remember pressure altitude, not GPS altitude, is what keeps us from colliding with other aircraft while flying.

 

[kurtfly]

Here is an white paper on using GPS to Determine Pitot-Static Errors. It was written by Greg Lewis at the NTPS in Mojave, CA. If you go to their web site they have a free Excel download to do the math. This method has been used and written about many times in this forum and others.

 

[DBRV10]

Fly a GPS box.
You will need to message me your email address so I can send the spreadsheet to you. It will not attach here.

Set a constant speed based on TAS shown on your SV.
Fly 360,90,180,270 and take note of the Ground speed. If there is a +/-1 knot in your TAS just correct the GS accordingly.
Enter the values in the GPS PEC spreadsheet and compare that result with your reference TAS.

If you are within 2 knots....all is good and your static error is insignificant.

 

[-Bob]

This explains how to adjust PITOT errors if, and only if, there is no STATIC error.
The static port position error must be corrected first, before using the GPS speeds to measure the pitot pressure errors.

 

[-Bob]

I'm finding it difficult to determine if I have a static position error. What is the gold standard test? I tried a shortcut using savvy analysis to plot the difference between pressure height and GPS altitude and that is generally constant within 80 feet no matter what the speed is, specifically, when I slow from 80 knots to 45, there doesnt appear to be any change. Is this good enough? I'm concerned that my speed may be over reading because I cant seem to achieve the low stall speeds specified by the manufacturer.

To measure and correct static port position errors, you need to fly past a fixed point at widely varying speeds while reading the altimeter.
I use my airport's tower roofline as a precise visual constant. I can fly past the tower at 70 knots, then 200 knots and note the altitude difference.
If the altitude differs by more than about 20 feet, you have a static error that "should" be corrected. On my Lancair 235, the error was initially about 230 feet. If you know how pressure varies on the surface of your aircraft, you can figure out which direction to move the static port. On the lancair, I moved the port about 2.5" up and forward and then found -40 feet error. The ideal position was about 2.0" up and forward from the initial position.

If you don't want to drill a bunch of holes in your airframe trial-and-error, then consider adding a small dam just upstream or downstream from the static port. Try something, measure the result, then refine your adjustment until the error is tolerable.