Speed of a vario

There seems to be a misconception about time constant of variometer. Many (all?) think that 2 sec time constant means that it takes 2 seconds after hitting thermal for variometer to show any lift. This is not true. Time constant means that within that time variometer needle reaches 100% of the pressure change signal. That does not mean that variometer needle does not start to show at least something (needle starts to rise) before that 2 secs.

On Tuesday, August 11, 2020 at 12:01:37 AM UTC-4, krasw wrote:
There seems to be a misconception about time constant of variometer. Many (all?) think that 2 sec time constant means that it takes 2 seconds after hitting thermal for variometer to show any lift. This is not true. Time constant means that within that time variometer needle reaches 100% of the pressure change signal. That does not mean that variometer needle does not start to show at least something (needle starts to rise) before that 2 secs.

What is the advantage of that 2 sec time constant, i.e. taking 2 seconds to reach 100% of the pressure change? Is the advantage that setting the time constant to a lower value (say, 1 sec or a half sec) means that the vario NEVER reaches 100% of the pressure change? i.e. you never get to know how good the thermal really is?

Ben Ethridge

On Tuesday, 11 August 2020 at 13:43:12 UTC+3, wrote:
On Tuesday, August 11, 2020 at 12:01:37 AM UTC-4, krasw wrote:
There seems to be a misconception about time constant of variometer. Many (all?) think that 2 sec time constant means that it takes 2 seconds after hitting thermal for variometer to show any lift. This is not true. Time constant means that within that time variometer needle reaches 100% of the pressure change signal. That does not mean that variometer needle does not start to show at least something (needle starts to rise) before that 2 secs.
What is the advantage of that 2 sec time constant, i.e. taking 2 seconds to reach 100% of the pressure change? Is the advantage that setting the time constant to a lower value (say, 1 sec or a half sec) means that the vario NEVER reaches 100% of the pressure change? i.e. you never get to know how good the thermal really is?

Ben Ethridge

If lower time constant would be better, then everyone would set electrical vario time constant to zero.

Adjusting time constant is just a tool to filter out gusts from TE signal. Amount of disturbances would depend on your glider pitot static system, TE-probe, plumbing, variometers connected and most importantly, your personal preference.

There is no right or wrong here. I have my inertial variometer tc is set to 1 sec, electrical to 5 sec, and mechanical somewhere between. They have all different settings, because way of measuring signal is profoundly different between instruments.

On Tuesday, August 11, 2020 at 11:45:19 AM UTC-4, krasw wrote:
There is no right or wrong here. I have my inertial variometer tc is set to 1 sec, electrical to 5 sec, and mechanical somewhere between. They have all different settings, because way of measuring signal is profoundly different between instruments.

Interesting. There may be no right or wrong, but what's the best way to use the instruments when you set the three of them differently this way, in your opinion? i.e. how do you get good useful info out of them, so that they don't just add instrument confusion?

On Wednesday, 12 August 2020 at 01:53:16 UTC+3, wrote:
On Tuesday, August 11, 2020 at 11:45:19 AM UTC-4, krasw wrote:
There is no right or wrong here. I have my inertial variometer tc is set to 1 sec, electrical to 5 sec, and mechanical somewhere between. They have all different settings, because way of measuring signal is profoundly different between instruments.
Interesting. There may be no right or wrong, but what's the best way to use the instruments when you set the three of them differently this way, in your opinion? i.e. how do you get good useful info out of them, so that they don't just add instrument confusion?

TC between instruments is set according to their capability to measure reliable variometer data. Inertial variometer does not see the pitot-static errors, thus very fast tc. A good mechanical variometer is almost always best traditional way of showing TE signal, but is affected by gusts, thus couple of sec tc. Electrical variometer can be at best as good as (good) mechanical, but usually not. Mine is set up to show kind of "fast average" of thermal, thus 5 sec tc. I wan't to setup the instruments to complement their strength and hide the weaknesses. No reason to tune them to show identical signal (which is impossible with three different ways of doing things).

Fast vario example: https://youtu.be/XAjwVPRTxQE?t=120

CNv set to "0.5s" for both pointer and audio. Panel is badly underexposed, but you can see the orange needle of the old CNv display on left side. I have the vario set to suppress sink tones (I don't need to be scolded when I'm not climbing).

What the TC refers to in this case is the low pass filter on the back end of the data stream. 0.5s effectively turns the low pass "off". There is plenty of digital filtering in front of this, and imo, it works quite well. I think it is about as fast as is useful for sailplanes. There's a reliable, repeatable sequence of boot in the bum followed a couple tenths of a second later by the audio for real thermals. If the boot is missing, that's a gust. It's very easy to train yourself to detect this difference (a good vario helps).

Not yet released CNv color display at upper right (it's quite visible in real life), should be out soon.

OT: At 2:25 you get a nice look at the Appalachian Trail as it ascends Old Speck Mountain just before plunging into Grafton Notch. We fly this route fairly often... but normally a bit higher :-).

T8

On Monday, August 10, 2020 at 9:01:37 PM UTC-7, krasw wrote:
There seems to be a misconception about time constant of variometer. Many (all?) think that 2 sec time constant means that it takes 2 seconds after hitting thermal for variometer to show any lift. This is not true. Time constant means that within that time variometer needle reaches 100% of the pressure change signal. That does not mean that variometer needle does not start to show at least something (needle starts to rise) before that 2 secs.

The traditional meaning of "time constant" is exponentially based: after one time constant you reach 63% of its final value, after two time constants this is up to 86%, after three it is 95%. You never reach the final value of 100%.