ASI Calibration

I went out and flew some four-sided GPS boxes today to make a calibration table for my airspeed indicator. I am using a Piper "blade-style" heated pitot tube just outboard of the inspection plate on the port wing (about 9 inches outboard of the "factory" pitot position), and fuselage static ports. My GRT EFIS computes TAS in real-time, which is really handy for flying this test!

I was pleasantly surprisedwhen I got home and reduced the data. At speeds from 60 to 180 knots, the biggest error was 1.5 knots - which is essentially as precise as I flew the test anyway. So for all intents and purposes, my ASI is pretty much spot on.

Just curious if everyone gets away this easily, or if I got lucky? Anyone else care to share their results?

Paul

Anyone else care to share their results?

Click to expand...

The first time I did the 4-way GPS run, I was annoyed to find a 3-4% error on the high side, up and down the scale. It explained the higher than Van's stall speeds I was seeing, but it shatttered my illusion that my airplane was somehow way faster than the prototype

Fortunately, before I tweaked the ASI in the EFIS, I made some low passes down a long runway at varying airspeeds, and found significant static error (more than 120' difference in indicated altitude between 100KIAS and 180 KIAS) I replaced the tubing I had originally installed more than 6 years ago with more robust stuff, and now I'm within less than 0.5% on the EFIS, although my backup steam gauge ASI remains a little optimistic.

Another plus is the the EFIS doesn't calculate that I have a headwind everywhere I go anymore.

James Freeman

Van's static ports, or flush?

Ironflight said:

I am using a Piper "blade-style" heated pitot tube just outboard of the inspection plate on the port wing (about 9 inches outboard of the "factory" pitot position), and fuselage static ports.

Click to expand...

Just curious - did you use Van's "pop rivet" static ports, or flush ones? If you used flush ones, who makes them?

Static Ports....

Hi Kevin - I'm just using the pop-rivet things supplied by Van's - nothing fancy at all!

Paul

ASI calibration query

Paul,

I've done a search on calibrating ASI and the picture is still quite fuzzy. Do you know of a good article which explains the process/procedure using a gps for ground speed reference? The topic might have been covered adequately in a previous post, but I have not been able to find it. I'd like to see how accurate my analog ASI and Dynon are- they read within 2-3 mph across the range. Currently doing Phase I, so this is the time to get it done. I have OAT displayed on my EIS.

thanks for the help

Kevin is the expert on this - check his web page for various detailed methods on calibrating your ASI systems:

http://www.kilohotel.com/rv8/phplinks/index.php?&PID=49

Paul

Paul,
I had the same results as you after my calibration runs.
We have 4 RV8's at our field that were finished within 6 months of each other.
All nearly identically equipped. All feature Dynon D180's and Dynon Pitot probes in standard locations. 3 of the 4 have the SafeAir static ports(protruding head style, not the old flush kind). 3 of the 4 have the new "SteinAir" quick couple air line connectors.

When we flew side by side and compared IAS and TAS 3 of the 4 were identical. The oddball had Vans static kit. The oddball also showed to be indicating 10 knots faster.

The oddball assured us that he had checked and double checked his static connections.
The oddball finally pulled up his front floor and checked the only other connection for static and found guess what,..... a connection separated!

Once that was corrected our airspeeds all agree.

The other local RV8 that I built, and flys with us, has Vans static kit and Vans airspeed indicator, when I fly side by side with him, our airspeeds are also identical.

A practical tool

trib said:

Paul,

I've done a search on calibrating ASI and the picture is still quite fuzzy. Do you know of a good article which explains the process/procedure using a gps for ground speed reference? The topic might have been covered adequately in a previous post, but I have not been able to find it. I'd like to see how accurate my analog ASI and Dynon are- they read within 2-3 mph across the range. Currently doing Phase I, so this is the time to get it done. I have OAT displayed on my EIS.

thanks for the help

Click to expand...

Any of the workbooks on my "models" page (3rd link below) contains a page for finding your CAS and correction to ASI at various speeds and has built-in atmosphere corrections to make it easier. In a nutshell, you work from your GPS verified TAS back to the CAS by correcting with pressure altitude and temp. The page does all the work.

I recommend the 3-way NTPS method which Kevin Horton has mentioned before.