TE probe installation

Good point! I'm sad to say that it has taken me several days to remember that the variometer, unlike the altimeter, is not concerned with absolute pressure, only with the relative pressure on both sides of the variometer.

Thank you to all who have helped me sort out my understandings of how TE probes work.

My understanding has been blocked by my confusion over how a TE probe senses environmental static pressure. I know, dumb. This confusion led me down a dead end path of wondering just what pressure (total, dynamic, static) a TE probe presents to the static side of a variometer.

I read several souces on the subject and was further stumped by repeated statements that a TE probe must produce a pressure equal in magnitude but opposide in sign to the dynamic pressure sensed by the pitot probe. This just fed my confusion.

The veil has been lifted from my eyes - I hope. 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. Static pressure, in a sense, is everywhere. It is, in a sense, the background pressure. The TE probe does sense static pressure; it cannot help but do so. But it also senses dynamic pressure (the inverse of it, actually). The TE probe 'adds' the inverse of dynamic pressure to the static pressure to moderate the static pressure changes that may accompany dynamic pressure changes. If the dynamic pressure is not changing the TE probe presents to the variometer the pressure it does sense - the unmodified static pressure, and so functions as a simple static probe. Again, in a sense, the TE probe, in the absence of dynamic pressure changes, 'adds' zero to the static pressure.

I hope I finally have this sorted out. If not, just shoot me.

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.


--
---
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.


--
---
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