Vern and Kevin,
We've thought of calibrating TAS, but never IAS. Do you mean an IAS calibration too?
.....
Happy to be convinced otherwise, but for now TAS was what we were thinking about.
To calculate TAS, you need to know the CAS, pressure altitude and OAT. Errors in any of those will lead to errors in the calculated TAS.
CAS - CAS differs from the IAS seen on the ASI (or EFIS) due to instrument error and static system position error. And, if you have an EFIS, maybe there could be errors in the algorithms used to calculate airspeed given static and pitot pressures.
Pressure Altitude - subject to instrument error and static system position error. But, the error is usually less than 100 ft (although I have seen a report of over 150 ft altitude error due to static system position error), and an error of this magnitude has a negligible effect on the TAS. A 100 ft error in the pressure altitude would lead to about a 0.3 kt error in the TAS at typical RV cruise speeds.
Temperature - subject to instrument error, errors in the assumed recovery factor and position error. By position error, I mean that the position chosen may lead to errors in the temperature. For example, many RV builders put the OAT probe in one of the NACA scoops on the forward fuselage. This puts the body of the probe in the cockpit, so it would be heated from the back. I have witnessed one installation where the OAT indication would vary several degrees depending on the setting of the cockpit heat control. And I have read many reports from other builders who found errors in the OAT. At RV cruise speeds, a 5 deg C error in the OAT would lead to about a 1.6 kt error in the calculated TAS.
The OAT indication is affected by ram temperature rise, and the probe's recovery factor should be used to remove this error. Depending on the probe, and how it is mounted, the ram temperature rise could be up to 4 deg C at RV cruise speeds. The amount of ram temperature rise depends on the mach number (or TAS, depending on which formula you want to use). Thus errors in the CAS due to static system position error will lead to error in the OAT, leading to further errors in the calculated TAS. This is a second order effect though, so the additional error is quite small.
Note that all the factors that are needed to calculate TAS are affected to some degree by static system position error. On large aircraft, they use air data computers to remove the effect of static system position error, and this correction is applied to altitude, airspeed, mach, TAS and OAT (admittedly, some installations only apply the static system position correction to the displayed altitude).
It is probably true that most Dynon customers are only worried about the effect of all the above errors on TAS. They aren't asking for corrections to IAS and altitude. But, this is likely because they are not aware of the potential for errors here, nor are they aware of the consequences.
I have communicated with one RV-4 owner who had an aircraft with flush mounted static ports. He set his altimeter to the field elevation one day, took off and then did a high speed pass down the runway. He noted that his altimeter read about 150 ft lower than the field elevation, and was puzzled by this. As an experiment, he glued domed rivet heads with holes in them over his flush static ports. He found that now his altimeter compared well with the field elevation when he flew down the runway, and his cruise IAS had increased about 10 kt. He had been flying for quite some time with this condition, and was not aware that he was probably often flying about 200 ft higher than his assigned altitude, solely due to static system position error. Altitude errors of this magnitude could be significant safety issue if flying IFR (ILS minima are typically 200 ft), or in VFR in airspace with IFR traffic around (the error eats up quite a bit of the assumed 500 ft between IFR and VFR cruising altitudes).
If so, this seems pretty non-standard to me. In an airplane, pitot pressure equals an indicated airspeed. If we change that relationship, the EFIS won't match any other instrument you put in your plane.
Yes, if the aircraft also has conventional round dial ASI and altimeter, having the EFIS show corrected airspeed and altitude would lead to differences between the various displayed data. But, is really a good reason to display erroneous data, just so it agrees with other erroneous data?
It means that if you replace your EFIS you'll need to re-calibrate everything, or if you switch vendors of equipment in the future, nothing will match up. It removes the ability for us to calibrate a system in our shop as we build them.
There should be a way to backup the static system position error corrections to a connected PC, and load them onto a new EFIS if desired. This would be no different than if the corrections were just to TAS.
It removes the ability for us to calibrate a system in our shop as we build them.
The EFIS would leave the factory will corrections set to zero, or disabled via a configuration menu. So the existing calibration routines would still work.
Calibrating static takes it to a whole new level- what's your altitude reference when you're in the air to calibrate against?
There are various ways to do this. For one way, see
http://www.kilohotel.com/rv8/rvlinks/ssec.html
If you are interested in going down the road of correcting for static system position error, I can track down detailed info on how these corrections are normally mechanized in air data computers. I have some contacts at a major manufacturer who should be able to give me more info on the details.
