AA Butterfly versus CNv LCD wind calculation

Interesting, would you mind sending me more detailed information? In the
mean time, I'll be sure my CNv is electronically compensated and see how
that works.

On 9/27/2016 2:13 AM, bumper wrote:
It's not undisturbed air. On my ex Stemmes , serial 11-18, the nose TERRY was severely under compensatedevelopment due to the bow wave. I increased the TE "washer" size to 3/4" IIRC and that helped a lot. Other Stemmed owners followed suit and agreed. Stemmes did not come stock with fin mounted probes then, some owners got them when the issue became known.

--
Dan, 5J

On Tuesday, September 27, 2016 at 6:52:44 AM UTC-7, wrote:
In the past I flew an old Stemme S10V for many hours. Loved it, but vario was very undercompensated.
Would love you to expand on the "washer" mod. I just can't figure what you mean.

thanks!

Aldo Cernezzi


Also (and Dan),

As mentioned, the Stemme TE probe is adversely affected by the nose bow wave. You can see this with the classic stick pull push as the vario needle follows showing lift or sink - "stick thermal".

The TE probe is the "top hat" style with the brim of the hat creating a low pressure area directly behind the probe - the negative pressure is reduced by the bow wave. Increasing the top hat diameter compensates (mostly) for the bow wave and dramatically reduces stick thermal indication.

To increase diameter, I used a 3/4" OD nylon washer with an ID to closely match the horizontal barrel of probe. The washer is cut from OD to ID at an angle to allow the washer to be "threaded" onto the probe just ahead of he top hat flange - thus effectively increasing it's diameter and further reducing the pressure behind the probe in flight.

I did a bunch of ground testing too, using a leaf blower and various means to smooth the airflow to the probe while using a manometer to monitor pressure in an attempt to increase compensation further still. Larger washer did not work. Going to a cone shape instead of a washer to further smooth flow also made no improvement. Best I could come up with was that washer . . . and it works!

IIRC, I found the washers at West Marine years ago, I'm sure they can be found on-line.

bumper

Den tirsdag den 27. september 2016 kl. 15.19.03 UTC+2 skrev krasw:
On Tuesday, 27 September 2016 11:15:02 UTC+3, Tango Whisky wrote:
Why would a mechanical vario give you more information on airmass?

That is interesting question. If it would give identical information, the varios should read exactly the same. They obviously are not, so they are either measuring TE pressure differently, or one of them is measuring same thing but poorly (which I don't agree with, in some gusting thermals my Bohli reacts more realistically than electric vario). I feel getting two "opinions" from same thing is advantage (three actually if you count inertial variometer). Heck, I would gladly install fourth variometer if that tells me something new, instead of duplicating something I already have.

I think the mechanical vario and the electrical transducer based vario should have different responses simply due to physics.

Consider a sudden small pressure drop in the TE line (like hitting very sudden lift). The mechanical vario measures the flow rate from the flask to the TE line. This flow is proportional to the pressure difference between the flask and the TE pressure. The pressure difference, and hence the vario reading, is largest right after the pressure drop and then decays exponentially back to zero. That explanation is consistent with a simple experiment done a few years ago with a 57 mm Winter vario, a syringe and a video camera. The vario reading peaked after just 0.3 s. The subsequent decay fitted perfectly with an exponential decay with a time constant of 3 s.

The electrical vario indicates the time derivative of the measured TE pressure after some low pass filtering. The response depends on the actual filtering, but the response to a sudden small pressure drop will peak after some time given by the filter type and time constant. The lab experiment with a syringe and a video camera showed that a late generation LX5000 on minimum time constant (0.5 s) would peak after 2.5 s. On the other hand, the following decay to zero was much faster than for the Winter vario.

The tests were repeated with both the Winter and the LX5000 on the same TE line in order to evaluate the influence of the mechanical vario and flask on the LX5000 response. Surprisingly, the LX5000 response to a sudden small pressure drop was essentially unchanged, and if anything it measured a bit faster.

Another test with an LX7007 showed similar results, except now the Winter on the same TE line did seem to slow the LX7007 a bit when operated on the shortest possible time constant of 0.2 s (peak after 2.0 s without Winther, peak after 2.5 s with Winther on TE line).

I prefer the electrical vario, but I know one regular WGC pilot who keeps a mechanical vario claiming that it has a faster response to sudden changes in the air mass. He doesn’t know why – and I don’t know if the considerations above are relevant at all in the real world…

-J

That is why, if it is in the budget, buy the latest generation instruments. I have compass module on LX 90XX, compass on Butterfly, and Bolhi compass, They all work within two degrees of each other. The LX90XX and butterfly always agree.

On Tuesday, September 27, 2016 at 8:21:44 AM UTC-7, krasw wrote:

Even Zander compass was very sensitive to installation. I installed one Zander system to a LS8 for WGC years ago and calibrated it with factory calibration compass. It worked fantastically giving very accurate readings. After that my club bought similar system to D2 and it didn't work at all, even after many calibration attempts. I have flown third glider with similar setup, and that also did not work. We never understood why. With current knowledge of compass sensor installation problems, there might have been speaker or something too close to compass.

Capito!
thanks!
aldo

The TE probe is the "top hat" style with the brim of the hat creating a low pressure area directly behind the probe - the negative pressure is reduced by the bow wave. Increasing the top hat diameter compensates (mostly) for the bow wave and dramatically reduces stick thermal indication.

We tried for wave today and all we got was high on the engine. Since I
was in smooth air, I tried pushing over and pulling up and found the
Winter mechanical vario in my Stemme to be very well compensated.
Likewise the CNv was well compensated. Checking its setup, I found that
it is set for electronic compensation, I did not look at the percentage.

On 9/26/2016 4:43 PM, Dan Marotta wrote:
Thanks Evan, I was gonna try that. I already have the TCs set at 0.5
sec. I'll report back if I see any difference.

Dan

On 9/26/2016 12:14 PM, Tango Eight wrote:
On Monday, September 26, 2016 at 1:19:46 PM UTC-4, Dan Marotta wrote:
On 9/26/2016 10:15 AM, Tango Eight wrote:
snip
Mechanical varios don't play well with modern pressure transducer
type varios and should not share TE sources with them.

Evan Ludeman / T8
I have a 57 mm Winter mechanical and a CNvXC connected to the triple
probe in my Stemme. The needles on each track perfectly with each
other.
Which illustrates what goes wrong :-).

The effect of the mechanical vario is to slow the whole system down
to its own speed.

If you are using the triple probe pitot and static for CNv's pitot
and static inputs, and the static source is not shared with any
mechanical instrument (pitot source may be shared with an ASI since
the pitot aneroid volume is small), try setting CNv to "no probe" and
see if you don't like the response of the instrument better. I
suggest setting the audio & pointer time constants to 0.5 sec.

best,
Evan for CNi




--
Dan, 5J

tiistai 27. syyskuuta 2016 22.23.28 UTC+3 kirjoitti:
Den tirsdag den 27. september 2016 kl. 15.19.03 UTC+2 skrev krasw:
On Tuesday, 27 September 2016 11:15:02 UTC+3, Tango Whisky wrote:
Why would a mechanical vario give you more information on airmass?

That is interesting question. If it would give identical information, the varios should read exactly the same. They obviously are not, so they are either measuring TE pressure differently, or one of them is measuring same thing but poorly (which I don't agree with, in some gusting thermals my Bohli reacts more realistically than electric vario). I feel getting two "opinions" from same thing is advantage (three actually if you count inertial variometer). Heck, I would gladly install fourth variometer if that tells me something new, instead of duplicating something I already have.

I think the mechanical vario and the electrical transducer based vario should have different responses simply due to physics.

Consider a sudden small pressure drop in the TE line (like hitting very sudden lift). The mechanical vario measures the flow rate from the flask to the TE line. This flow is proportional to the pressure difference between the flask and the TE pressure. The pressure difference, and hence the vario reading, is largest right after the pressure drop and then decays exponentially back to zero. That explanation is consistent with a simple experiment done a few years ago with a 57 mm Winter vario, a syringe and a video camera. The vario reading peaked after just 0.3 s. The subsequent decay fitted perfectly with an exponential decay with a time constant of 3 s.

The electrical vario indicates the time derivative of the measured TE pressure after some low pass filtering. The response depends on the actual filtering, but the response to a sudden small pressure drop will peak after some time given by the filter type and time constant. The lab experiment with a syringe and a video camera showed that a late generation LX5000 on minimum time constant (0.5 s) would peak after 2.5 s. On the other hand, the following decay to zero was much faster than for the Winter vario.

The tests were repeated with both the Winter and the LX5000 on the same TE line in order to evaluate the influence of the mechanical vario and flask on the LX5000 response. Surprisingly, the LX5000 response to a sudden small pressure drop was essentially unchanged, and if anything it measured a bit faster.

Another test with an LX7007 showed similar results, except now the Winter on the same TE line did seem to slow the LX7007 a bit when operated on the shortest possible time constant of 0.2 s (peak after 2.0 s without Winther, peak after 2.5 s with Winther on TE line).

I prefer the electrical vario, but I know one regular WGC pilot who keeps a mechanical vario claiming that it has a faster response to sudden changes in the air mass. He doesn’t know why – and I don’t know if the considerations above are relevant at all in the real world…

-J

That is very interesting, thanks. I think many pilots recognize good mechanical variometers fast peaking / slow decay behaviour vs. electricals somewhat opposite response. I've often considered would it be possible to build a vario that has adjustable, separate time constants/filters for peak and decay. Cannot think any reason why not.

Sorry, but comparing the two varios mentioned in the original post:

When flying through turbulence that nets neither up nor down and not worth circling in, ever wonder what it is? A mechanical or old school vario won't soon display anything useful about the change in airmass horizontal vector..
Correction, a mechanical vario will never tell you about it.

Today's flight was in wind from SE then NW then SW then SE then NW then SW then S then SW then SE then SW then NW. When approaching convergence lines the wind change was indicated on the Air-Glide S before the airmasses converged, by showing velocity reduction then change of direction then - on both mentioned varios - up indication.
Jim

On Thursday, September 29, 2016 at 9:29:18 PM UTC-4, JS wrote:
Sorry, but comparing the two varios mentioned in the original post:

When flying through turbulence that nets neither up nor down and not worth circling in, ever wonder what it is? A mechanical or old school vario won't soon display anything useful about the change in airmass horizontal vector.
Correction, a mechanical vario will never tell you about it.

Today's flight was in wind from SE then NW then SW then SE then NW then SW then S then SW then SE then SW then NW. When approaching convergence lines the wind change was indicated on the Air-Glide S before the airmasses converged, by showing velocity reduction then change of direction then - on both mentioned varios - up indication.
Jim

JS...do you use electronic compensation on both varios?

On Thursday, September 29, 2016 at 7:35:50 PM UTC-7, wrote:
On Thursday, September 29, 2016 at 9:29:18 PM UTC-4, JS wrote:
Sorry, but comparing the two varios mentioned in the original post:

When flying through turbulence that nets neither up nor down and not worth circling in, ever wonder what it is? A mechanical or old school vario won't soon display anything useful about the change in airmass horizontal vector.
Correction, a mechanical vario will never tell you about it.

Today's flight was in wind from SE then NW then SW then SE then NW then SW then S then SW then SE then SW then NW. When approaching convergence lines the wind change was indicated on the Air-Glide S before the airmasses converged, by showing velocity reduction then change of direction then - on both mentioned varios - up indication.
Jim

JS...do you use electronic compensation on both varios?

Using electronic on the CNv, a (50/50?) mix in the Air-Glide.
Jim