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

Well, I'm 72 so I guess that's to be expected...

On 8/10/2020 7:57 PM, SoaringXCellence wrote:
Dan,

It means you're very slow, only faster than 16% of the test subjects. I got an average of 210, that's the 76 percentile.

MB

--
Dan, 5J

Le mardi 11 août 2020 Ã* 06 h 47 min 51 s UTC-4, Dan Marotta a écritÂ*:
Well, I'm 72 so I guess that's to be expected...
On 8/10/2020 7:57 PM, SoaringXCellence wrote:
Dan,

It means you're very slow, only faster than 16% of the test subjects. I got an average of 210, that's the 76 percentile.

MB
--
Dan, 5J

If I remember ok it is the time it takes to reach 80% of the thermal strenght.
S6

At 11:23 11 August 2020, s6 wrote:
If I remember ok it is the time it takes to reach 80% of the
thermal strength.

Actually (1− 1/e) (approximately 63 percent)
J.

On Tuesday, August 11, 2020 at 6:45:05 AM UTC-7, wrote:
At 11:23 11 August 2020, s6 wrote:
If I remember ok it is the time it takes to reach 80% of the
thermal strength.

Actually (1− 1/e) (approximately 63 percent)
J.
Also, the time constant of the electronics is after mechanical lags, so the delay may be somewhat longer.

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?

jfitch wrote on 8/11/2020 8:22 AM:
On Tuesday, August 11, 2020 at 6:45:05 AM UTC-7, wrote:
At 11:23 11 August 2020, s6 wrote:
If I remember ok it is the time it takes to reach 80% of the
thermal strength.

Actually (1− 1/e) (approximately 63 percent)
J.
Also, the time constant of the electronics is after mechanical lags, so the delay may be somewhat longer.

And then there is the time constant of the glider, which does not reach a steady
upward speed immediately; ie, the glider is a mechanical filter for lift with, I'm
guessing, a time constant between 1 second (low wing loading with stiff wings) and
3 seconds (high wing loading with bendy wings). Jon may have included the glider
lag in his "mechanical lags".
.
--
Eric Greenwell - Washington State, USA (change ".netto" to ".us" to email me)
- "A Guide to Self-Launching Sailplane Operation"
https://sites.google.com/site/motorg...ad-the-guide-1

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).