D10A TAS reads high

[Carl_Froehlich]

I repeated a four leg GPS run in my RV-8A today. The error I previously noted in the Dynon D-10A TAS indication has remained constant. The D10A TAS reading was 9 knots higher than actual at 4500 feet (194 versus 185) and 6 knots higher than actual at 7500 feet (191 versus 185).

I checked the D10A outside air temperature reading with a second instrument. The readings were within one degree of each other (33 degrees F). The D10A altimeter reading was the same as a second instrument, and compared reasonably well with GPS altitude. The D10A indicated airspeed reading was the same as a second instrument.

As you would expect, the D10A wind speed and direction indications carried over the TAS error. As I really like having wind speed and direction feature on the D10A, this error is the most annoying.

Any suggestions on fixing this? Am I the only one with this problem?

 

[dynonsupport]

Carl,

The TAS math in the EFIS isn't wrong. We have thousands of units flying, and this isn't a problem with them.

As you know, only three things factor into TAS: altitude, airspeed, and OAT.

So one or more of them have to be wrong.

It sounds like OAT is ok since a second instrument agrees, and it has a totally independent sensor.

With IAS and static, agreement between instruments just means each instrument is reading pressure correctly, since they are both sharing "sensors" (the pitot and static)

It's hard to have a pitot fail in a way that it reads too high. A leak or position error will usually cause low readings. Unless you've placed your pitot behind your propeller

So first thing to check is static error. There's an easy way to do this: Go find a runway that you can overfly with a low pass. Do the pass at 90 and then 150+ IAS. Check your altitude at both speeds. There should be no variation.

I'm guessing you do have some static position error, and this is reinforced by your difference in error at altitude. Static error is affected by IAS, not TAS. So when you did your 185 knot TAS runs, your IAS was lower at the higher altitude, and so any static problem would also be less.

As an FYI, static errors are common on RV's where the stock Van's static ports are not used. Building a small dam or lip behind/in front of the port is often needed to get accurate readings.

One final thing you can do is write down the actual IAS / OAT / ALT that is giving you a TAS and do the calculation by hand to show that it is correct based on the given inputs.

Do you have an EMS networked to the EFIS? There are ways to get the EMS to lie to the EFIS about OAT.

 

[Carl_Froehlich]

Thanks for the note.

I agree on the RV static error issue. I installed a static fix on my plane last year as I had a 300' altitude error. The altitude error went away but it looks like I still have a small static error as IAS apprears to be high.

I just did the math using IAS, altitude and OAT to calculate TAS. I get a TAS of 190 knots using data from either the 4500 or 7500 foot runs (actual TAS from the GPS runs was 185 knots). This translates to a 3 knot indicated airspeed error (indicated reading high) at both 4500 and 7500 feet.

This explains some of the actual versus D10A TAS difference. Correcting for this IAS error, I calculate the D10A TAS indication is 5 knots high at 4500 feet (not 9 knots as in my first note) and 2 knots high at 7500 feet (not 6 knots as in my first note).

So the D10A TAS is closer to truth than I orginally thought, but still indicates high. Other thoughts?

Carl

 

[khorton]

Carl - Can I check your math?  Give me the data for one or more cases.  IAS on the EFIS, altitude on the EFIS, altimeter setting, OAT and the TAS indicated on the EFIS.  

I've worked as an engineering test pilot for 20 years, so I know how to calculate TAS.

 

[Carl_Froehlich]

Kevin,

Here are the average values.

At 4500'
IAS = 183 knots
OAT = 33F
Altimeter = 31.2"
From this data I compute TAS as 190 knots. Actual TAS from the 4 leg GPS run was 185 knots. From this I calculate a 4 knot IAS error (IAS reading higher than actual). TAS indicated on the D10A was 195 knots.

At 7500'
IAS = 172 knots
OAT = 30F
Altimeter = 31.2"
Using this data I compute TAS as 188 knots. Actual TAS from the 4 leg GPS run was 185 knots. From this I calculate a 3 knot IAS error (IAS reading higher than actual). TAS indicated on the D10A was 190 knots.

Thanks,
Carl

 

[khorton]

Kevin,

Here are the average values.

At 4500'
IAS = 183 knots
OAT = 33F
Altimeter = 31.2"
From this data I compute TAS as 190 knots.  Actual TAS from the 4 leg GPS run was 185 knots.    From this I calculate a 4 knot IAS error (IAS reading higher than actual).  TAS indicated on the D10A was 195 knots.

At 7500'
IAS = 172 knots
OAT = 30F
Altimeter = 31.2"
Using this data I compute TAS as 188 knots.  Actual TAS from the 4 leg GPS run was 185 knots.  From this I calculate a 3 knot IAS error (IAS reading higher than actual).  TAS indicated on the D10A was 190 knots.

Thanks,
Carl

Carl - for the first case, I calculate a pressure altitude of 3337 ft, and a TAS of 189.3 kt.  Close enough to your calculation.  I wonder if Dynon's TAS calculation is taking altimeter setting into account.  183 kt CAS at 4500 ft pressure altitude gives a TAS of 193.4 kt.

For the second case, I calculate a pressure altitude of 6337 ft, and a TAS of 187.5 kt - your 188 kt calculation is right on the money.

Note: There is a good chance that your indicated OAT is warmer than the actual OAT, due to ram temperature rise on the OAT probe.  Depending on the details of your OAT probe installation, it may have a recovery factor as high as 0.95, which means that the indicated OAT could be as much as 8 deg warmer than the actual OAT.  If this is in fact the case, your IAS error may not be as much as you think.  You can attempt to get a rough idea of the recovery factor of your OAT probe by flying at several different airspeeds at the exact same altitude.  You want the widest TAS range possible, from just above the stall, to maximum speed in level flight.  Fly at each speed long enough to be sure that the indicated OAT has stabilized.  Note the indicated OAT, the IAS, altitude, and altimeter setting.  If your OAT probe installation is not affected by ram temperature rise, you will have the same indicated OAT at all speeds.  If there is ram temperature rise the indicated OAT will get higher as the speed increases.  Give me the data and I'll give you a rough idea on the recovery factor of your OAT probe.

Also, some OAT probes are mounted in unfortunate locations that allow cockpit heat to warm the back side of the probe.

For the four leg GPS run - please confirm that you are using a known test technique, such as the "NTPS spreadsheet", and not simply averaging four groundspeeds.  Some people do not realize that the average of ground speeds on four legs of a box will only equal the TAS if the wind is zero.

 

[Carl_Froehlich]

Kevin,

Thanks for the reply. Yes, I am using a spreadsheet to crank the GPS data. I also logged the D10A wind speed and direction for each leg. This shows a 10 knot or so error (D10A showing wind speed higher than actual). The GPS calculated winds aloft also compared favorably with the local forcast.

I assumed the wind speed and direction indication problem was fully contribitable to the TAS indication error. I now see some pitot/static problem as presented by the 3 knot or so high IAS, and perhaps another 2-4 knots somewhere in the D10A TAS process. I don't think these two combined however cover the D10A wind speed indication error.

I also considered an OAT probe reading higher than actual. In this case however, to get 185 knots TAS actual OAT would have to be 12F at 4500' (instead of my indicated 33F) and 15F at 7500 feet (instead of my indicated 30F). It just was not that cold out there today.

Carl