Kevin I flew it again today and here is the data collected from my Skyview display: I flew the course twice to verify the findings.
Indicated altitude = 5500
Density altitude = 7361
OAT = 70 deg f
RPM = 2450
MP = 22.1
Track 1 = 360
Mag Hdg1 = 004
Ias1 = 152
Tas1 = 169
Gs 1 = 155
Track2 = 270
Mag Hdg2 = 276
Ias2 = 152
Tas2 = 169
Gs 2 = 184
Track3 = 180
Mag Hdg3 = 179
Ias3 = 152
Tas3 = 169
Gs 3 = 190
Track4 = 090
Mag Hdg4 = 086
Ias4 = 152
Tas4 = 169
Gs 4 = 162
It still looks like I have IAS error as the difference from the Skyview TAS of 169 and the spreadsheet 4 way calculation of 173.6 is roughly 5 MPH.
If I remember correctly adding crescents in front of the static port will bring the displayed Tas more into line.
Am I looking at this correct?
Using the NTPS spreadsheet, and converting mph to kt, and deg F to deg C (those are the units used by that spreadsheet), I get an IAS error of about 3.4 kt, or 3.9 mph. That would give a TAS error of about 4.5 mph. These values assume that you were at 5500 ft pressure altitude (i.e. with altimeter setting 29.92). If the 5500 ft was with a different altimeter setting, it will make a small difference. In that case, tell me what the altimeter setting was, and I can correct the altitude.
My calculation assumes a 0.85 recovery factor on the OAT probe - i.e. it recovers about 85% of the ram temperature rise from the TAS. The recovery factor of your probe may be lower than that. My OAT probe has a recovery factor of about 50%, probably because it is mounted in a NACA scoop, and is partially shielded from the ram air. If I assume a recovery factor of 0.5, that lowers the calculated IAS error to about 3.1 kt, or 3.6 mph, or a TAS error of about 4.1 mph.
We can estimate the recovery factor of your OAT probe if you go to some low altitude, and stabilize at max speed until the OAT stabilizes. Record OAT, IAS and pressure altitude. Then slow to the lowest possible speed at the same altitude, and wait for the OAT to stabilize again, and record OAT and IAS. If the OAT reads the same at all speed, the recovery factor would be zero. If the recovery factor is greater than zero, the OAT will read higher at high speed than at low speed. Send me the data, and I can calculate an approximate probe recovery factor.
There are several possible sources of this error:
- Pitot leak, which would lead to IAS reading low,
- OAT error (a small effect, unless the OAT error is huge)
- EFIS ASI instrument error (the instrument error is likely small, unless your EFIS is unserviceable, which is possible). This can be checked using a water manometer.
- Static system position error (the likely cause of most of the error).
Questions:
- Have you done a pitot system leak check? If you do one, keep in mind that many pitot tubes have a small moisture drain, and you must tape over this when doing the leak check.
- Does the indicated OAT match reasoably well with what you would expect?
Assuming that static system position error is the root cause, you need a change in geometry that leads to lower pressure at the static ports. Classical ways to do that are a crescent in front of the ports, or raising the surface of the ports a bit higher above the skin somehow, perhaps by glueing a round disk with a hole in the centre onto the external surface of the protruding part of the port.
If you had an analog ASI, ASI instrument error would be a very possible factor. I've seen ASIs with more than 5 kt instrument error when delivered from the factory.