AA Butterfly versus CNv LCD wind calculation

On Tuesday, September 20, 2016 at 5:52:15 AM UTC-4, Per Carlin wrote:
This error in reading the IAS in the glide computer is not calibrated...
Plenty of instruments provide calibration (ie, SN10).
So, CAS is available.

therefore can it not use the difference TAS / GS to calculate the wind.

Just plain Wrong. In addition to input IAS-CAS calibration,
wind algorithm can (should be) be designed to self-calibrate and
be relatively intolerant of airspeed errors.

We did all this 2 decades ago, and now you're saying it is not possible???

Really now.

On Tuesday, 20 September 2016 12:52:15 UTC+3, Per Carlin wrote:
A thought regarding usage of IAS, TAS and GS for wind calculations: I have been a long term abuser of this, when flying with the Zander GP940 who only calculate the wind during circling. The wind component calculation is of great information on final glide to detect any changes of wind when getting closer to the ground. But I just got a second thought about this.

When reading this tread and putting 1+1 together do I realize that using the difference between TAS and GS is not an useful approach to calculate the wind. Read any of the Johnsson reports and figure out why he is so keen on calibrating the IAS readings!
It is not uncommon that the IAS differs from TAS(at ground level) with more than 5%, it is individual both between types of gliders and most likely also within gliders of the same type depending on which static probe you are using.
This error in reading the IAS in the glide computer is not calibrated, therefore can it not use the difference TAS / GS to calculate the wind. The error would be of dignity 5-15 km/h.

You are correct but all of this can be taken into consideration. TAS can be calculated with the help of pressure and temperature, computers (Zander incl.) can be calibrated for pitot-static errors. At least GlideS does this, I would think that it is same with other systems.

You are correct but all of this can be taken into consideration. TAS can be calculated with the help of pressure and temperature, computers (Zander incl.) can be calibrated for pitot-static errors. At least GlideS does this, I would think that it is same with other systems.

Yes I did the Zander TAS calibration, but how good was it? The IAS has the same error in reading as it uses the same static port.

To clarify/confirm a few things about CNv...

CNv has hardware on board for inertial sensing and magnetic compass. These are not used at the present time. The goal was to build the best possible pneumatic + GPS vario first.

Wind is determined by an algorithm that compares computed TAS to GPS ground track many times per second. So a determined effort to fly a constant ground track can inhibit wind updates. In real life, on all but the straightest ridges, it updates automatically, also shows the time elapsed since last update. As well, one can force the wind to reset at any time.

If you don't get vector wind on tow with CNv, then you need to check your installation.

Some features, including the thermal assistant and quick wind are available only with the XC license. The thermal assistant graphics just don't fit on the LCD included with the mechanical pointer display, which is why that feature is NAV display only.

Best,
Evan Ludeman for CNi

tiistai 20. syyskuuta 2016 15.40.52 UTC+3 Per Carlin kirjoitti:

You are correct but all of this can be taken into consideration. TAS can be calculated with the help of pressure and temperature, computers (Zander incl.) can be calibrated for pitot-static errors. At least GlideS does this, I would think that it is same with other systems.

Yes I did the Zander TAS calibration, but how good was it? The IAS has the same error in reading as it uses the same static port.

I guess IAS-CAS difference is pretty linear in most glider pitot-static systems, at least with normal flying speeds. I cannot remember Zander calibration procedure anymore, but Air Glide S let you correct IAS for several speeds and probably does linear interpolation between. Maybe some computers let you do only one point of correction? TAS is result of CAS + temp + pressure, that cannot be calibrated.

On Monday, September 19, 2016 at 11:15:18 AM UTC-6, Dave Nadler wrote:

That is incorrect.
Multiple observations on different headings allow wind calculation.

Dave,
Can you explain a little more how this works?

On Wednesday, 21 September 2016 19:02:11 UTC+3, K m wrote:
On Monday, September 19, 2016 at 11:15:18 AM UTC-6, Dave Nadler wrote:

That is incorrect.
Multiple observations on different headings allow wind calculation.

Dave,
Can you explain a little more how this works?

You get windspeed component in flight track direction comparing TAS and GS. Doing this to different bearings gives you several wind component vectors, after that it is basic vector math. Optimal vectors would be to widely different bearings, and flying only straight line gives you nothing.

On Wednesday, September 21, 2016 at 10:24:25 AM UTC-6,

You get windspeed component in flight track direction comparing TAS and GS. Doing this to different bearings gives you several wind component vectors, after that it is basic vector math. Optimal vectors would be to widely different bearings, and flying only straight line gives you nothing.

Thanks, This is as I understand. I was confused when Dave posted that you can not use track and heading. Seems that these (Along with TAS and GS) would make instantaneous cross wind calculations possible.

keskiviikko 21. syyskuuta 2016 19.48.27 UTC+3 K m kirjoitti:
On Wednesday, September 21, 2016 at 10:24:25 AM UTC-6,

You get windspeed component in flight track direction comparing TAS and GS. Doing this to different bearings gives you several wind component vectors, after that it is basic vector math. Optimal vectors would be to widely different bearings, and flying only straight line gives you nothing.

Thanks, This is as I understand. I was confused when Dave posted that you can not use track and heading. Seems that these (Along with TAS and GS) would make instantaneous cross wind calculations possible.

Yes, with compass you get the bearing that is not available from any other source. Using bearing, track vector, TAS & GS gives you instant wind vector in straight flight.

I think you mean Heading- a compass shows your heading.
On Wednesday, September 21, 2016 at 2:09:50 PM UTC-4, krasw wrote:
keskiviikko 21. syyskuuta 2016 19.48.27 UTC+3 K m kirjoitti:
On Wednesday, September 21, 2016 at 10:24:25 AM UTC-6,

You get windspeed component in flight track direction comparing TAS and GS. Doing this to different bearings gives you several wind component vectors, after that it is basic vector math. Optimal vectors would be to widely different bearings, and flying only straight line gives you nothing.

Thanks, This is as I understand. I was confused when Dave posted that you can not use track and heading. Seems that these (Along with TAS and GS) would make instantaneous cross wind calculations possible.

Yes, with compass you get the bearing that is not available from any other source. Using bearing, track vector, TAS & GS gives you instant wind vector in straight flight.