TE probe installation

On Saturday, December 6, 2014 12:27:42 AM UTC-5, krasw wrote:
... variometer does not care about the absolute pressure.

Really? What happens at 10,000 feet vs. sea level?
Discuss amongst yourselves...

On Saturday, December 6, 2014 2:59:38 PM UTC-8, Dave Nadler wrote:
On Saturday, December 6, 2014 12:27:42 AM UTC-5, krasw wrote:
... variometer does not care about the absolute pressure.

Really? What happens at 10,000 feet vs. sea level?
Discuss amongst yourselves...

Well, let's see. The air is less dense at 10,000 MSL than it is at sea level. One formula for force is F = ma. This suggests the air flow through the variometer would produce less force at 10,000MSL than
it would at sea level. If the variometer is a vane-type variometer perhaps the variometer vane would not be moved as much at 10,000 MSL than it would be at sea level. So... the variometer would read less lift/drag at 10,000 MSL than it would at sea level? Just a guess. I really have no idea.

At 22:59 06 December 2014, Dave Nadler wrote:
On Saturday, December 6, 2014 12:27:42 AM UTC-5, krasw wrote:
... variometer does not care about the absolute pressure.

Really? What happens at 10,000 feet vs. sea level?
Discuss amongst yourselves...

In it's most simplistic for a vario measures differences in pressure over
time. Again in the most simplistic form the instrument measures the rate of
flow of air from a sealed flask trying to equalise the pressure within it
to the outside air (static). It converts the rate of flow to give an
indication of the rate of climb/descent. In that respect it does not care
about the absolute pressure. Whether the reduced air density in the flow
will cause any detectable change in the indication is debatable, I can't
see why it should.

Modern electronic instruments calculate a rate by comparing minute
differences in pressure directly. If the instrument is measuring changes in
pressure then it relies on the absolute pressure value entirely. In this
type of instrument the altitude will make no difference.

On Saturday, December 6, 2014 12:55:05 PM UTC-5, Jim Lewis wrote:
I understand now that the TE probe (and the pitot probe,
for that matter) cannot help but sense static pressure -
in addition to dynamic pressure.

Right! This is easy to understand by considering the pressure
sensed by TE or pitot probes with glider on the ground.

Seems to me that if the vario measures a volume rate of flow, then at
higher altitudes, for a given change in altitude, the change of pressure
would be less (recall that pressure change is non linear with altitude),
hence a lower volume flow, hence a lower rate of change reading (lift or
sink).

So, if I'm climbing at 10 kts at sea level and my vario reads 10 due to
some finite number of moles or molecules of gas exiting the flask, past
a sensor, to the atmosphere, I'd think that, at 18,000' MSL where the
static pressure is roughly half what it is at sea level, then the volume
of air leaving the flask and passing the sensor would be roughly half of
what it was at sea level, then an absolute climb rate of 10 kts would be
displayed at something more like 5 kts. It probably doesn't make a damn
bit of difference anyway.

Flash update! I passed this by my wife (who's a current student glider
pilot) and she opened the topic of indicated vs. true speed. I think
that's the answer. We'll be reading an indicated 10kts regardless of
altitude but our true rate of climb will depend upon altitude. I think
that supports what I said above but I'd still like a mathematical
explanation.

Would somebody with an actual education in the subject please explain it
for the rest of us?

On 12/7/2014 7:39 AM, Dave Nadler wrote:
On Saturday, December 6, 2014 12:55:05 PM UTC-5, Jim Lewis wrote:
I understand now that the TE probe (and the pitot probe,
for that matter) cannot help but sense static pressure -
in addition to dynamic pressure.

Right! This is easy to understand by considering the pressure
sensed by TE or pitot probes with glider on the ground.


--
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Dan Marotta

On Sunday, December 7, 2014 11:46:38 AM UTC-5, Dan Marotta wrote:
Seems to me that if the vario measures a volume rate of flow, then at
higher altitudes, for a given change in altitude, the change of pressure
would be less (recall that pressure change is non linear with altitude),
hence a lower volume flow, hence a lower rate of change reading (lift or
sink).

So, if I'm climbing at 10 kts at sea level and my vario reads 10 due to
some finite number of moles or molecules of gas exiting the flask, past
a sensor, to the atmosphere, I'd think that, at 18,000' MSL where the
static pressure is roughly half what it is at sea level, then the volume
of air leaving the flask and passing the sensor would be roughly half of
what it was at sea level, then an absolute climb rate of 10 kts would be
displayed at something more like 5 kts. It probably doesn't make a damn
bit of difference anyway.

Flash update! I passed this by my wife (who's a current student glider
pilot) and she opened the topic of indicated vs. true speed. I think
that's the answer. We'll be reading an indicated 10kts regardless of
altitude but our true rate of climb will depend upon altitude. I think
that supports what I said above but I'd still like a mathematical
explanation.

Would somebody with an actual education in the subject please explain it
for the rest of us?

On 12/7/2014 7:39 AM, Dave Nadler wrote:
On Saturday, December 6, 2014 12:55:05 PM UTC-5, Jim Lewis wrote:
I understand now that the TE probe (and the pitot probe,
for that matter) cannot help but sense static pressure -
in addition to dynamic pressure.

Right! This is easy to understand by considering the pressure
sensed by TE or pitot probes with glider on the ground.


--
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Dan Marotta

What you have described is generally correct for FLOW type variometers such as mechanical instruments and early electric instruments. Almost all modern electric varios use pressure transducers and do not have the change in flow volume to consider.
UH

On Sunday, December 7, 2014 12:55:57 PM UTC-5, wrote:
What you have described is generally correct for FLOW
type variometers such as mechanical instruments and early
electric instruments.

How you sense the pressure is unimportant.

The pressure change between 0 and 1 feet MSL is much larger
than the pressure difference between 10,000 and 10,001 feet MSL.

Discuss amongst yourselves...

On Sunday, December 7, 2014 10:16:45 AM UTC-8, Dave Nadler wrote:
How you sense the pressure is unimportant.

The pressure change between 0 and 1 feet MSL is much larger
than the pressure difference between 10,000 and 10,001 feet MSL.

Discuss amongst yourselves...

Looking at my decade old code for a transducer based variometer, I see that I went to a bit of trouble to convert pressure to ISA altitude before differentiating to get rate of climb. For even more amusement, consider the relationships between "altitude corrected" rate of climb, IAS, TAS, calculated total energy compensation, and speed to fly...

Marc

On Sunday, December 7, 2014 1:56:25 PM UTC-5, wrote:
Looking at my decade old code for a transducer based variometer,
I see that I went to a bit of trouble to convert pressure to ISA
altitude before differentiating to get rate of climb.

Good, but its much easier just to correct by one_over_sigma ;-)

For even more amusement, consider the relationships between
"altitude corrected" rate of climb, IAS, TAS, calculated total
energy compensation, and speed to fly...

Right, proper instruments consider MC as a 'true' input as
well as correcting vario, IAS-TAS, and of course polar.

Discuss amongst yourselves ;-)

....And it's not even officially winter yet...

OK, I understand about correcting the transducer based altitude,
corrected for ISA, so I guess you're using your static and OAT inputs to
calculate altitude before calculating and displaying the rate of change.

So am I to understand, then, that my mechanical vario is displaying
*indicated* rate of lift/sink while my electronic vario is displaying
*true* rate of lift/sink? I guess I'll take a close look on my next
wave flight.

On 12/7/2014 12:03 PM, Dave Nadler wrote:
On Sunday, December 7, 2014 1:56:25 PM UTC-5, wrote:
Looking at my decade old code for a transducer based variometer,
I see that I went to a bit of trouble to convert pressure to ISA
altitude before differentiating to get rate of climb.

Good, but its much easier just to correct by one_over_sigma ;-)

For even more amusement, consider the relationships between
"altitude corrected" rate of climb, IAS, TAS, calculated total
energy compensation, and speed to fly...

Right, proper instruments consider MC as a 'true' input as
well as correcting vario, IAS-TAS, and of course polar.

Discuss amongst yourselves ;-)


--
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Dan Marotta