Pitot-Static Calibration

Flight testing continues, and I was finally able to sit down and do a data reduction of my pitot-static calibration flights.  I am using my EFIS (a Garmin G3X) to record data for me once per second.  I fly roughly cardinal headings in a box at a target airspeed, and the whole flight was done at 3,500 pressure altitude (density altitude was closer to 4500).  I found I was piss poor at holding an altitude, heading, and airspeed all at once.  Actually, I don't think I managed to hold any of them even one at a time. With the magic of the dense data recording from the EFIS and a spreadsheet, it basically didn't matter, as long as I wasn't really bad at it.

After the flight I was able to sort and eliminate data.  If the 3 data points ahead and behind of a point were not in a heading range of less than 3 degrees, an airspeed range of 3 KTAS, or a vertical speed of less than 100 FPM, the data point was thrown out.  So, each point I was sure that I had 7 seconds of relatively steady data.  Then I sorted the data by indicated airspeed, so similar speeds on different headings could be used to do the calibration.  I used a modified version of this spreadsheet to do all of the trigonometry for me.

I was amazed that taking data points that differed sometimes by seconds and sometimes by nearly the duration of the flight the resultant wind vector was very nearly constant.  The range was 237 to 253 degrees from 12.4 to 16.9 knots.  The reported wind for the day was 280/13G17.  The G3X records magnetic heading without correction for magnetic variation (that is a separate data field) which is -14, which would put the reported wind and the calculated wind very close.  During the flight the G3X calculated a wind vector, which it does by doing vector math with the magnetic heading, the ground track, GPS ground speed, and TAS.  The G3X reported wind was 243 degrees at 16.9 knots.

What I was left with is this:

Basically my pitot-static & OAT system are optimistic.  Starting at 85 KTAS it is indicating 2 knots high (meaning the G3X says 85, but really it is flying at 83), and out at 183 KTAS it is 13 knots high (meaning it says 183, and really it is going 170).

My aim being to minimize the error, I think I have a few things I could try.  First, this could be an error of the OAT probe.  If it was off by a few degrees then the TAS calculation would similarly be off.  I have two OAT probes, and could compare the two, or I could get a third temperature probe of known calibration and try that.  I could also re-do the data analysis, calculating TAS from IAS and OAT, instead of taking the TAS number from the G3X, then fudge the OAT to see how much error would be required to create this.  That is a simple exercise and maybe I'll do that.

But the fact that the error grows over the scale (3% at the low, 7% at the high) indicates to me that it is an error of the static system.  My static ports are not per plans.  I don't seem to have a good picture of them, this is about the best I have.  Basically they are aluminum, roughly 1" round, and create a step that is about 0.080" from the fuselage, with a chamfered edge.  There are two, one on either side of the fuselage, and they are T'ed together.  Because the airspeed indicator is  indicating too high, I tend to believe the static port is measuring a pressure that is too low.  To correct it I am going to try adding layers of tape behind the actual port, in an effort to raise the measured static pressure and reduce the error.  If that were to work I would remove the static probes and machine a similar sized step into them.  

Stay tuned for the next episode of Knuckleheads Pretending to be Flight Test Pilots, where your intrepid pilot/engineer/dufus seeks to determine if flutter testing should be in KIAS or KTAS, right here on the Cozy Forum...