TAS calibration

[crowder]

After completing multiple GPS patterns, I have found that my displayed TAS is consistently 5 Knots high at cruise speed.  I know that a couple of years ago this was discused and Dynon said there was merit to adding a TAS calibration to the EFIS.  Since TAS is used in the calculation of wind data, this would make that data more accurate/useful as well.  I was wondering if this is still being looked at, or if not, I would love to see it added to a future release.  

Mark

 

[dynonsupport]

It gets talked about often, but never makes the cut for the next software release. So maybe someday...

Of course, if you are always 5 knots high, it's a 90% chance that your static is wrong and you have both and alt and airspeed error. We generally recommend you focus on the root cause of the problem.

 

[crowder]

You may be right about the assumption of a static system error, but that can be a hard nut to crack.  The aircraft is an RV8, and the static system was built according to plans with the parts supplied by Van's aircraft.  A 5 knot error at high speed cruise seems to be worst case from the testing I've done so far.  Digging out my old aeronautical engineering texts, if we assume that the error is 100% due to a low static pressure value, the altitude error would be 87 feet.  (8k 156 KIAS)  In a VFR airplane at altitude and cruise speed, I can live with that.  The truth is the only reason I'm even aware of the error is because of the advent of GPS and other relatively new technology that gives that degree of accuracy.  When I started flying jets 20+ years ago, we used to calculate GS with a stop watch by watching the DME count down.   After 36 seconds, we would take the DME change and multiply by 10.  If the result was accurate to within 10-15 knots, we were doing pretty good.  The only reason I brought the topic up, is that it seems that it wouldn't be too difficult to add some code to the EFIS for a calibration factor, and then those who were interested could really get some precise readings.

Mark

 

[DBRV10]

Problem I see with CAL functions that are not really calibrating, they are FUDGE FACTORing, is that some people will end up tangled in knots (pardon the pun) by fudging things that they think look right, rather than fix the problem. And in doing so only end up with several functions not accurate.

Do you not mean the delta DME x 100........otherwise you will be going really slow!!!

DB

 

[crowder]

Yeah DB, I meant to multiply by 100, shows you were reading the post!    I understand what you're saying about the fudge factor, but let me add another data point.  I have two airspeed indicators.  One is the Dynon EFIS, the other is an analog round dial.  Both indicators agree within a knot at pattern speed, at cruise speed the Dynon indicates about 5 knots higher.  This is not a big deal, but if the TAS were calculated from the round ASI, it would be spot on.  This is exactly where a calibration function makes sense.  To simply not include it because someone might use it incorrectly, rubs me wrong.  If someone can be trusted to build and fly their own airplane, we probably should give them the credit to calibrate a TAS indicator.  I'm not sugesting monkeying around with the IAS, only calibrating the TAS to match the wind adjusted ground speed.

Mark

 

[DBRV10]

Gooday Mark,

Yes I did read it..... but I had seen another old timer.... I mean very experienced pilot make that same x10 comment on another aviation forum and hence my eyes were alerted!  

So yes I do agree with you, and myself being in the industry I am in, I would be happy to have access to all parameters in the Dynon gear via a laptop............ but they do have a market which I dare say with all due respect is not made up entirely of folks you can unleash that kind of access too.

My comment based on my old plane (new RV10 is coming) is that the D10 (old D10 model) is dead accurate accross the range and the old steam driven ASI is a few knots out. Maybe thats the problem for you also.

Another RV10 down here just had its avionics checked as required and the techo claimed the D100 and ASI both over read exactly the same, yet no static leaks etc..................makes you wonder hey!  : I think the tech's equipment is wrong!!!!! 2 x devices agree across the range one a D100 and a steam device and he says they are both wrong yet in agreement!  :-?

If in doubt take it to Dynon and have it checked, but my experience says they will most likely be 100% on the money. Sure they may have the odd one..... and they admit it can happen, but they fix it!

Cheers from Downunder!

DB

PS Static vents are a funny issue too............. a mystery of sorts of which I am not an expert!

 

[Mel]

Both indicators agree within a knot at pattern speed, at cruise speed the Dynon indicates about 5 knots higher.  
Mark

I had the exact same situation. I tried another analog ASI and guess what; The new one matches the Dynon perfectly.

 

[Etienne]

It's not uncommon to have a page in the POH that gives the conversion between IAS and CAS. Since CAS is used to calculate TAS, and the D100 does this internally with no IAS to CAS lookup table, I'd expect that even in a certified aircraft, that for some reason were to have a D100 in it, there would be some form of static-based error (and these aircraft are the benchmark!).

A simple page, even only available on a laptop, where a user could enter in the same information as is in a POH i.e. a table with IAS and CAS value pairs, would go a long way to giving an accurate TAS and therefore wind indication.

It's not always possible to fix a static port to be perfect, but in software, it is possible to use constant correction values for any given airspeed.

By all means, limit the correction value by +-5 kts - it seems that this is about the average error being experienced by most people. This can fix the wind indication by up to 10kts, which is quite a significant amount!