Static System Calibration

[turbopilot]

When I get a new airplane I typically look at the systems to see how well they are calibrated. I am working my way through the systems on my new Carbon Cub SS with the G3X panel. So far I have confirmed the compass system is calibrated perfectly. Very little error.

This SS came with the relatively new static ports on the cowl. This was an addition to the SS and I think other models via an SB several years ago. I added the static ports to my 2014 SS and found that they induced a speed related error in that aircraft. Far better than sensing static pressure from the cockpit but not without error. In my 2014 CC SS the error increased linearly with TAS. An error of around 2 mph around 90 mph and up to 5 mph around a TAS of 110. These measurements were made near sea level.

I just finished several runs with the 2020 SS and see about the same airspeed related error. For those who have never calibrated TAS in their airplane it is easy to do with this on line calculator.

I know we are dealing with a cub so why does it matter if the TAS is not accurate? The G3X system is so precise in the calculations it really messes up the winds aloft calculation when the TAS is not accurate. Otherwise no big deal except it shows the airplane flying faster than it really is. The good news is the error is very small near landing speed.

Why is there an error? Cubcrafters uses a raised static port button on each side of the cowl. Most static ports are flush with the skin. Most likely a slight amount of lower pressure is caused by the edge of the static port button as air moves across the cowl. There are tricks to change the shape of the static port to even out these errors.

[Andy]

Why is there an error? Cubcrafters uses a raised static port button on each side of the cowl. Most static ports are flush with the skin. Most likely a slight amount of lower pressure is caused by the edge of the static port button as air moves across the cowl. There are tricks to change the shape of the static port to even out these errors.

Take a look at the airdata parameters when you advance the throttle for takeoff. I think you'll see strong prop wash influence.

[turbopilot]

No need to run a special test or stare at IAS when you should be looking out of the window.

I was really not looking for a ground school lesson. I was curious if anyone else had seen progressive errors in TAS at increasing cruise speeds in Carbon Cubs.

[Andy]

I was curious if anyone else had seen progressive errors in TAS at increasing cruise speeds in Carbon Cubs.

According to the FX-3 AFM IAS reads 20 mph high for CAS 160. If IAS reads higher than calibrated then TAS must be high too. What does your AFM show for IAS calibration?

[turbopilot]

According to the FX-3 AFM IAS reads 20 mph high for CAS 160. If IAS reads higher than calibrated then TAS must be high too. What does your AFM show for IAS calibration?

The SS AFM has not been revised since the static port was moved from the back of the panel in the cockpit to 2 static ports on the side of the aircraft. At 110 mph indicated the CAS is 101 mph in the AFM, so the error was bigger with the cabin static location. Of course static pressure in the cabin changed based on speed and position of the overhead vents.

I fly primarily in the lower and upper deserts of the Southwest with winds constantly changing because of geography. This issue came to my attention when I was flying formation with a G3X equipped KitFox. We were trying to find an area with light winds for photography. He was flying off my wing and we were comparing the calculated winds aloft reported by the G3X in each airplane. The results were very different both in direction and speed.

After we sorted it out we agreed our compasses where accurate but I was seeing an indicated airspeed around 6 mph higher than actual and his Kitfox G3X was showing an indicated airspeed around 5 mph slower than actual.

[Andy]

It's not explicitly stated in the FX-3 AFM what pitot static system was used to derive the IAS/CAS plot. The text does say the static system uses the fuselage ports but the baseline system for the AFM is the jury strut mounted pitot not the Garmin probe.

A couple of points from that plot -

IAS 100 CAS 96
IAS 110 CAS 102

Try fitting these and see if static pressure is better behaved? - https://tinyurl.com/yxubv943

[turbopilot]

It's not explicitly stated in the FX-3 AFM what pitot static system was used to derive the IAS/CAS plot. The text does say the static system uses the fuselage ports but the baseline system for the AFM is the jury strut mounted pitot not the Garmin probe.

A couple of points from that plot -

IAS 100 CAS 96
IAS 110 CAS 102

Try fitting these and see if static pressure is better behaved? - https://tinyurl.com/yxubv943

Those might work. Had a Bonanza with "cheaters" on the static port. Basically a metal lip on the port button just aft of the hole maybe 1/32" think. Just enough to build a little positive pressure over the static port hole.

I tried this on my SS. Got too aggressive and used around a 1/16" lip aft of each static port hole. That dropped at the IAS around 17 mph on a short test flight. So I need to experiment with thinner lips to see if I can eliminate the error by slightly increasing pressure sensed at the port.

[Andy]

If I decided I was not satisfied with my FX-3 static system I think I'd be tempted to mount a Prandtl probe out of the prop wash on the wing.

https://www.unitedsensorcorp.com/pdfs/PSE-CUST.pdf

I don't understand why Garmin only offers the pitot/AOA probe and not a combined pitot/AOA/static probe.

[turbopilot]

I don't understand why Garmin only offers the pitot/AOA probe and not a combined pitot/AOA/static probe.

I think the reason is most airplanes have a static source located in a better position. Combined sources have other problems like icing and redundancy.

For a cub it is not a big deal as long as the speeds are close near stall but the airspeed error gets magnified when you have such precise equipment like the G3X. I have never had a compass system so well calibrated in any airplane as the CC G3X installation. Within 1 degree of actual in all directions.

The current static port position may not the major issue. I suspect the error is induced by not having a flush static port button with a sharp edge in the airstream just in front of the port giving some negative pressure over the sensing hole.



I think there is a way to modify the surface shape of the current static port buttons to eliminate most of the error at cruise speeds. Just takes a lot of trial and error.

[Andy]

I think the reason is most airplanes have a static source located in a better position. Combined sources have other problems like icing and redundancy.

My FX-3 has a heated Garmin AOA/pitot probe and my PA28 has a heated pitot/static probe under the left wing. Piper has been using wing mounted combined pitot/static probes for many years on IFR capable aircraft. Light aircraft fuselage mounted static ports are typically not heated so having a heated static on the wing, out of the prop wash, seems far better to me.

What is the redundancy problem? A single static system has no redundancy whether it is fuselage mounted or combined with a pitot probe.