units of measurement?
 

Kevin, perhaps you should remind folks that many of those formulae work ONLY in Canada, where airspeed and headings are in centipedes per kilojoule!...unless east of the border de la belle province, where all bets are off!:D n'est ce pas?

In principle, assuming you are using an accurate method to calculate TAS from IAS, and Dynon has done their work correctly, the TAS calculated either way should be the same. In any case, you don't need that - the spreadsheet handles that conversion.

You need to record the following data during the box pattern:

OAT
Pressure altitude (should be the same on each leg, plus or minus 100 ft or better)
IAS (should be the same on each leg, as close as humanly possible)
GPS track
GPS ground speed

Static leak check - 45' drop in one minute at 18,000 ft is extremely good. The standard check is done with enough suction to raise the altimeter 1,000 ft above the starting elevation. 18,000 ft would put a lot more pressure differential across the connections, so is an even more severe test.

Note: 18,000 ft of suction on the static system would also cause the ASI to see a delta pressure equivalent to about 520 kt (assuming sea level), unless the suction was also applied to the pitot line. 520 kt worth of delta pressure would not be good for many analog ASIs, but the Dynon should handle it OK.

Flew again today
 

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?

That data looks better. I calculate 174.4 TAS with a much smaller std deviation of 1.3. Still looks like your indicating about 5 low.

This is using the NTPS spreadsheet.

TJ

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.

I also calculate a std deviation of 1.3, but only if I put his heading values in the track column.

If I put track values in the track column (as you should), I calculate a std deviation of 0.6, which is a reasonable quality test point. Ideally, you'd prefer std deviation less than that, but it is a matter of luck to do much better. It is easy to do worse too.