vario behavior mystery

[Moshe Braner]

So we have this plain mechanical vario in a glider, connected to a TE
probe and a flask. The TE probe is also connected to an electronic
vario with a pressure sensor, which is not ideal for the electronic
vario, but shouldn't affect the mechanical one much. While thermaling,
the mechanical vario shows lift, varying similarly to what the
electronic one shows. But while cruising, the mechanical vario is
continuously PEGGED DOWN. Well sometimes for a little while it is not
quite pegged, but still way down, around -8 knots. At the same time the
electronic one (if the Speed Control function is turned off) is showing
reasonable and varying sink rates. What could cause that behavior?

[Martin Gregorie]

Where is the T split in the line from the TE probe.

In http://www.borgeltinstruments.com/?page_id=248 there's a note that
says:

"Note that other instruments with flasks can affect the response of the
BORGELT variometers which can be avoided by splitting the TE line well aft
from the instrument panel. Under the rear of the seat pan will work well.
This does not apply for instruments without flasks like the B21/B40/B50/
B100/B400/B500/B600/B700/B800/B900 and other variometers which work on the
silicon pressure sensor principle."

I know that this is the opposite problem to what you're seeing but it may
still be relevant. Could there be a leak on the flask side of the
mechanical vario? (unsupported guess by me).


--

Martin | martin at
Gregorie | gregorie dot org

[Moshe Braner]

I forget where the T is, but it's reasonable aft. Anyway, it's not the
pressure-sensor e-vario that has a problem. An air leak is of course
possible, but where would a leak cause the mechanical vario to show an
endless indication of strong sink while cruising? The vario shows sink
when air is moving from the TE probe into the flask. The TE probe
pressure is LOWER than the static, thus a disconnected or leaky flask
should indicate lift not sink. A leak on the TE probe line should
affect the e-vario too, and would make the TE compensation partial, but
would not result in endless "sink".

[Hank Nixon]

Don't outthink this one.
1)Never connect varios in series.
2) If using a common TE source connect as far away from variometers as possible.
3) Leak check circuits independently.
4) Verify correct connections.
UH

[john firth]

Check for vario case leaks; glass seal.
JMF

[Jay Campbell]

IMHO "checking for leaks" without the right equipment and knowing exactly where leaks can occur is an exercise in futility. At a minimum, you need a combo pressure-vacuum gauge, designed plugs, syringe, valve, and several hours of watching time to detect tiny leaks. (And don't forget to cut off both ends of the suspect tubing where they stretch over their fittings.) Any over or under pressure of a device can ruin it, so know what you are doing. Easier and cheaper: Replace the suspect lines. Last comment: assure that the length of the tubing after the split to the mechanical and pressure varios are the same and consider lengthening them both by adding a couple of loops to the tubing to diminish the effect of the different flow rates between the two types of devices. Good luck!

[krasw]

Sounds like you have problem with mechanical vario. Hard to imagine scenario where static leak would not be visible on both instruments.

[Moshe Braner]

On 8/17/2022 10:28 AM, krasw wrote:
> Sounds like you have problem with mechanical vario. Hard to imagine scenario where static leak would not be visible on both instruments.
>

Yup. I can't think of any point in the system where just an air leak
would explain the behavior we're seeing. Unless the e-vario pumps air
out of its TE port the whole time you're cruising... Would be nice to
have an hypothesis on what COULD be the problem. Otherwise it's
shooting in the dark. Perhaps we should start by inserting a different
mechanical vario into the same air lines and compare the behavior?

[John Galloway]

I don't use all that equipment to check for leaks big enough to affect function. Observing the caveats about isolating the relevant tube from other instruments etc. just block off the source end (e.g. TE probe, static, pitot etc) then suck on the other end and plug it with your tongue. If your tongue stays stuck for 20 seconds then it is OK. As recommended by ESA Systems probe manufacturers for checking TE systems for leaks.

[Eric Greenwell]

Does TE seem to work? Can you duplicate the problem on the ground?

My guess is the mechanical vario has a leak. Substitute another mechanical vario, and test
again.

You might be able to test the vario without flying, by blocking the port to the flask with
short, plugged piece of tubing, and gently blowing on a tube connected to the TE/Static
port. The needle on a leaky vario will show sink; the needle on a good vario should remain
still.

--
Eric Greenwell - USA
- "A Guide to Self-launching Sailplane Operation"
https://sites.google.com/site/motorgliders/publications

[kinsell]

I'd replumb it to eliminate the electronic vario temporarily. It's most
likely a problem inside the mechanical vario or flask, and this would
help narrow it down.

[Hank Nixon]

Every glider comes with a pressure gage that will work for trouble shooting. It is known as an ASI. With some tubing and a couple of tees the whole system can be leak checked. Care must be taken not to over pressure. Alternatively a piece of tubing and a stick to hold it vertical makes a useful manometer.
Be careful applying pressure to mechanical varios.
I have found the connection fittings on the back of Winter varios loose on more than one occasion.
UH

[Moshe Braner]

On 8/17/2022 12:23 PM, Eric Greenwell wrote:
> On 8/17/2022 7:41 AM, Moshe Braner wrote:
>> On 8/17/2022 10:28 AM, krasw wrote:
>>> Sounds like you have problem with mechanical vario. Hard to imagine
>>> scenario where static leak would not be visible on both instruments.
>>>
>>
>> Yup.  I can't think of any point in the system where just an air leak
>> would explain the behavior we're seeing.  Unless the e-vario pumps air
>> out of its TE port the whole time you're cruising...  Would be nice to
>> have an hypothesis on what COULD be the problem. Otherwise it's
>> shooting in the dark.  Perhaps we should start by inserting a
>> different mechanical vario into the same air lines and compare the
>> behavior?
>>
> Does TE seem to work? Can you duplicate the problem on the ground?
>
> My guess is the mechanical vario has a leak. Substitute another
> mechanical vario, and test again.
>
> You might be able to test the vario without flying, by blocking the port
> to the flask with short, plugged piece of tubing, and gently blowing on
> a tube connected to the TE/Static port. The needle on a leaky vario will
> show sink; the needle on a good vario should remain still.
>

Right. But if instead you could SUCK on the TE/Static port, the leaky
vario would show LIFT. In flight, the pressure from the TE probe is
LOWER than the static, or cabin pressure. I.e., a leaky vario, or
flask, or connection to flask, should cause the instrument to show LIFT.
Why is the vario in question showing massive SINK (pegged down)? And
why only in cruise? It is showing reasonable lift while circling in a
thermal.

I suppose we should carefully try a slow cruise, at speeds used in
thermaling, and see if what determines the vario's "cruise" behavior is
the air speed. But so far we haven't noticed a relationship to
airspeed. So perhaps what causes the instrument to peg down "in cruise"
is the fact that it is in sink at all? Could the instrument be
defective in a way that causes the needle deflection to be much too
large in sink, but normal (or not very large) in lift?

To be specific, it is a standard Winter brand 80mm mechanical vario, and
it is probably some 40 years old.

[Eric Greenwell]

The difference in behavior between thermalling and cruise might be due to cockpit
pressure, which might affect a leak.

[Mark Mocho]

Mike Borgelt of Borgelt Instruments has a good overview about pneumatic leaks in glider instruments. It covers detection, diagnosis and repair of leaks. The article can be found here: http://www.borgeltinstruments.com/?page_id=252. Additionally, Lee Kuhlke had an article in Soaring magazine from around 2005 that covered the same topic. I have referred to both when chasing down problems in several gliders.

[Rakel]

On Wednesday, August 17, 2022 at 2:09:43 PM UTC-4, Mark Mocho wrote:
> Mike Borgelt of Borgelt Instruments has a good overview about pneumatic leaks in glider instruments. It covers detection, diagnosis and repair of leaks. The article can be found here: http://www.borgeltinstruments.com/?page_id=252. Additionally, Lee Kuhlke had an article in Soaring magazine from around 2005 that covered the same topic. I have referred to both when chasing down problems in several gliders.

Caution with this link. I got a security alert.

[Steve Leonard]

Rakel, you may want to try a second time. Using Chrome, and a Windows 10 machine, the first time I selected the link Mark provided, I was taken to some other site asking me to "allow" it to "show notifications". I blocked, closed, and selected the link a second time, and it went to the Borgelt page. Computers are weird! Especially if running Chrome on Windows 10!

Steve Leonard

[Darren Braun]

mmm. It's actually that the webserver at www.borgeltinstruments.com does not support encryption so technically packets transmitted and received are exposed and not private. I would not send any personal information etc, ..certainly not buy anything from the site directly. Just be aware if you choose to visit the page.
Darren

[Moshe Braner]

Followup: We replaced the mechanical vario with another mechanical
vario we had on hand, using the existing TE probe, flask, and plumbing.
In flight it correctly showed lift and sink, the same as the
electronic vario. The TE compensation seems good.

So presumably the old vario itself is faulty. I wonder how it's built
inside, and what internal malfunction would make it work normally for
lift but greatly over-react to sink.

[Mark Mocho]

> Followup: We replaced the mechanical vario with another mechanical
> vario we had on hand, using the existing TE probe, flask, and plumbing.
> In flight it correctly showed lift and sink, the same as the
> electronic vario. The TE compensation seems good.
>
> So presumably the old vario itself is faulty. I wonder how it's built
> inside, and what internal malfunction would make it work normally for
> lift but greatly over-react to sink.

I would inspect the gasket sealing the glass to the bezel. Variometers are dependent on the case being sealed from the outside air pressure. Assuming the fittings are sound and the case is not cracked, the glass to bezel seal is suspect.

[Moshe Braner]

Why would that affect the reading asymmetrically, with the ups showing
normally and the downs greatly amplified?

[Mark Mocho]

> Why would that affect the reading asymmetrically, with the ups showing
> normally and the downs greatly amplified?

It is possible that a broken gasket could "flap" in one direction on inflow to the case and seal when the airflow direction is reversed. At some point, there comes a time that old instruments just need to be retired. Nothing lasts forever, with the possible exceptions of Model A Fords, DC-3s. 1-26s and Volkswagens. But they take a lot of dedicated maintenance from some certifiably insane people.

[Hank Nixon]

One possibility is that it was worked on by someone. Mechanical variometers, such as Winter varios, have a light spring that centers them that is connected to the needle assembly and an adjustment disc. It is used to center the instrument at assembly.
Over time the needle can become lighter, due to drying out of the paint, and even flaking off. Rotating the adjustment disc can re center the instrument but it unbalances the response. Holding the instrument face up will indicate if the adjustment is off. if it changes when going to face vertical it is an indication that the needle weight is not balanced. Correction is to center with the face up and then add a small amount of weight to the needle. I use a tiny drop of epoxy. Iyt is delicate work , but can be done. The other choice is use as wall decoration.
Also while doing all of this is make sure there are no case leaks.
UH

[Moshe Braner]

Thank you UH. Indeed, now that I have the instrument in hand out of the
glider, the needle is very close to zero when the face is vertical, but
holding it face-up makes the needle show -3.5 knots. I suppose there's
no harm in trying to fix it rather than retiring it. Would nail polish
work instead of epoxy?

I also tried this: attached a short hose to one port, blow on it gently,
the needle moves. Using one port it moves up some. Using the other
port it moves down a lot more for the same strength of blowing (not
precise, but convincing). As for case leaks, blowing on the hose while
plugging the other port, the needle does not move at all. In both
directions.

[kevin anderson]

I have a friend that has sent several back to Winter to be rebuilt, but I do not know the cost of the work.

Kevin
92

[Tony]

I had a 57mm overhauled & new case 2 years ago. Can't find the exact amount but it was cheaper than buying a replacement. I subsequently found someone selling a used one for about the same price but was happy to get mine back with new papers🤷‍♂️

[Moshe Braner]

On 9/18/2022 9:42 PM, Tony wrote:
> I had a 57mm overhauled & new case 2 years ago. Can't find the exact amount but it was cheaper than buying a replacement. I subsequently found someone selling a used one for about the same price but was happy to get mine back with new papers🤷‍♂️
>

And, as they say, we've had this same ax in our family for 5
generations. We've replaced the handle twice, and the head once, but
it's still the same ax. :-)

Meanwhile, we've apparently fixed the vario using the method UH
mentioned. Will report after testing it in flight.

Also, while the face glass was off the instrument, found that blowing
air (gently!) in the "up" direction (into the flask port) showed an
indication, but it did not respond to blowing in the "down" direction
(into the TE probe port). I interpret that to mean that the main air
space inside the instrument is on the TE probe side of the air flow
path. Thus, a leaky case would degrade the TE compensation, by mixing
it somewhat with "cabin static", but would not affect the operation
otherwise. It should still indicate almost-correct vertical speed
(whether ascending and descending) as long as the horizontal speed is
constant.

[Moshe Braner]

On 9/19/2022 9:13 AM, Moshe Braner wrote:
>
> Meanwhile, we've apparently fixed the vario using the method UH
> mentioned.  Will report after testing it in flight.

Worked fine in an actual flight. Yay!

[John DeRosa OHM Ω http://aviation.derosaweb.net]

I'm late in this conversation. Glad that this problem was fixed.

I put together some accumulated knowledge in my
pneumatic air line presentation you can find at
Http://aviation.derosaweb.net/presentations/#airlines

Let me know of any comments, additions, or dunder-headed corrections.

- John (OHM)