Why are TE probes so long?

Robert Ehrlich wrote:
...
P - S is total - static = (static + dynamic) - dynamic) = dynamic).
...

correcting myself, should read:

.... P - S is total - static = (static + dynamic) - static = dynamic.

Hank Nixon wrote:

Easier to learn not to bend probe than try to make TE system work with
stubby probe.

Last year, our Astir lost its TE probe due to a rough landing. We did not
find it and invented a replace from an old glider that was out of service.
We had to bend it to make it fit to the Astir. After this had worked fine
for several weeks, we found the original probe laying in the grass just
beside another landed glider, in perfect shape. So we could replace it
again! But the bended probe had done a good job.
Eggert

On 5 May 2004 16:53:06 GMT, Derrick Steed
wrote:

I have driven many miles with an automobile test rig consisting of a
pitot, a static and a TE probe connected to a pair of side by side
U-tube water manometers. The manometers share the same static and one
uses the pitot signal while the other uses the TE signal. A properly
calibrated TE probe will cause exactly the same pressure difference as
the pitot, just with the opposite sign.

I understand we need the TE probe to generate the difference
static-dynamic because classical instruments are pure mechanical and
need the TE information to compensate the vario. But given electronic
devices/computers, do we really need the TE probe at all? The
information is given, when static and dynamic pressure are known. The
rest can be calculated. Wrong?
Eggert

Eggert, right. You are correct and some electronic instruments do just that
- they take the pitot and the static (instead of static and TE) and subtract
them electronically, and it's not just mechanical instruments which use TE,
the B50 has three inputs: pitot, TE, and static.

A TE probe isn't the only way of compensating for airspeed changes, ancient
glider pilots used diaphrams to achieve the same thing.

I sent some notes to Eckhard yesterday relating to how you might try and
achieve the calculation in Cumulus.

Rgds,

Derrick.


The B50 does not do electronic TE from the pitot/static. The TE probe
provides TE for the vario and the pitot/static measures airspeed for
the TAS/speed to fly/relative netto computation.

The reason for this is that is is *much* easier to get satisfactory TE
this way for the users.

The diaphragm compensator was one of the giant setbacks to the cause
of good glider instruments. Read any of the stuff by Moffat et al from
the 1960's and you will be treated to many stories of trying to get
good TE(and mostly failing). They only work properly at one altitude
too.

Althaus's revival of the venturi type probe in about 1969 was a great
improvement and they provide correct compenation at all altitudes.

It seems to me most TE probes are somewhat shorter than this thread!

:-)

Mike Borgelt
Borgelt Instruments

d b wrote:

Perhaps it isn't obvious. The airspeed indicator is a DELTA pressure guage.
It is not an absolute pressure guage. If the indicator reads the same delta
pressure, the two devices have to be producing the same delta pressure.
One sucking, the other blowing.

Or, if you have a good static system, one that neither blows nor sucks,
but just sits there!
--
Change "netto" to "net" to email me directly

Eric Greenwell
Washington State
USA

Todd Pattist wrote:

Eggert Ehmke wrote:


I understand we need the TE probe to generate the difference
static-dynamic because classical instruments are pure mechanical and
need the TE information to compensate the vario. But given electronic
devices/computers, do we really need the TE probe at all? The
information is given, when static and dynamic pressure are known. The
rest can be calculated. Wrong?


Not "wrong" but incomplete. The static pressure (from the
static sensor) and the total pressure (static plus dynamic
from the pitot tube) are large numbers. You would need to
calculate the dynamic pressure which is a small number by
subtracting the two large numbers to be able to calculate
the TE pressure that you want. Since the large numbers
come from two different sensors (static and pitot) they have
different errors, and the difference shows those errors. It
is simpler/cheaper and you get better data by using a single
TE sensor at a single point where the errors tend to cancel
out.

Or you could use the semiconductor equivalent of the airspeed gauge
(delta pressure sensor), connect it to the pitot and static, and measure
the dynamic pressure directly. This avoids the need to subtract two
large numbers. Even if the pressure is measured accurately, the quality
of the reading depends on the quality of the pitot/static system. This
quality is generally easier to ensure on a TE probe than a sailplane.

Nonetheless, using a Cambridge 302 on my ASH 26 E with a probe and with
electronic TE, I couldn't tell the difference. I stayed with the
electronic TE because it is much less affected while the engine is
running, since the pitot and static are not in the prop wash.

--
Change "netto" to "net" to email me directly

Eric Greenwell
Washington State
USA

No. The ASI measures the dynamic pressure as the difference between the
total pressure and the static pressure. The pitot measures total pressure.

Thanks, I mispoke. It's what I was thinking as I wrote, but not what I wrote.

Mike Borgelt wrote:
On 5 May 2004 16:53:06 GMT, Derrick Steed
wrote:

I have driven many miles with an automobile test rig consisting of a
pitot, a static and a TE probe connected to a pair of side by side
U-tube water manometers. The manometers share the same static and one
uses the pitot signal while the other uses the TE signal. A properly
calibrated TE probe will cause exactly the same pressure difference as
the pitot, just with the opposite sign.

I understand we need the TE probe to generate the difference
static-dynamic because classical instruments are pure mechanical and
need the TE information to compensate the vario. But given electronic
devices/computers, do we really need the TE probe at all? The
information is given, when static and dynamic pressure are known. The
rest can be calculated. Wrong?
Eggert

Eggert, right. You are correct and some electronic instruments do just
that
- they take the pitot and the static (instead of static and TE) and
subtract
them electronically, and it's not just mechanical instruments which use
TE,
the B50 has three inputs: pitot, TE, and static.

A TE probe isn't the only way of compensating for airspeed changes,
ancient
glider pilots used diaphrams to achieve the same thing.

I sent some notes to Eckhard yesterday relating to how you might try and
achieve the calculation in Cumulus.

Rgds,

Derrick.


The B50 does not do electronic TE from the pitot/static. The TE probe
provides TE for the vario and the pitot/static measures airspeed for
the TAS/speed to fly/relative netto computation.

The reason for this is that is is *much* easier to get satisfactory TE
this way for the users.

The diaphragm compensator was one of the giant setbacks to the cause
of good glider instruments. Read any of the stuff by Moffat et al from
the 1960's and you will be treated to many stories of trying to get
good TE(and mostly failing). They only work properly at one altitude
too.

Althaus's revival of the venturi type probe in about 1969 was a great
improvement and they provide correct compenation at all altitudes.

It seems to me most TE probes are somewhat shorter than this thread!

:-)

Mike Borgelt
Borgelt Instruments

I should correct the impression given above - I wasn't trying to imply the
B50 did it electronically, but I did, although it seemed obvious to me when
I wrote my bit that if it took the TE input then it used it as just that in
the conventional way.

I cherish memories of a british army nationals pilot that I crewed for back
in '69 and '70 trying to repair his diaphram compensator with a sheet of
latex material cut from a male contractive - I think he might have got a lot
more pleasure from using it in the conventional manner.

I think the thread on high energy pull ups with or without water was longer
- there was just as misconception, bogus physics, and appalling arithmetic
there too!

Still, it's a good laugh innit?

Rgds,

Derrick.

Todd Pattist schrieb:

Not "wrong" but incomplete. The static pressure (from the
static sensor) and the total pressure (static plus dynamic
from the pitot tube) are large numbers. You would need to
calculate the dynamic pressure which is a small number by
subtracting the two large numbers to be able to calculate
the TE pressure that you want. Since the large numbers
come from two different sensors (static and pitot) they have
different errors, and the difference shows those errors. It
is simpler/cheaper and you get better data by using a single
TE sensor at a single point where the errors tend to cancel
out.

But the TE probe also gives a large value (static-dynamic) as the
pitot gives (static+dynamic). So we have a third probe that has it's
own error. It may still be the case that the TE probe gives the better
result because if is in clean air, what may not be the case for the
pitot.
Eggert

On Thu, 06 May 2004 15:21:05 +0200, Asbjorn Hojmark
wrote:


Previously, I had an Avionic GS500 (designed by Stig Øye, who I'm
sure you know), and it's by far the best variometer I've ever
flown with. (It's not in production any more). In particular, the
fact that on glides, it told you exactly what you were going to
get if you started turning, worked extremely well. The GS500 was
purely electronically compensated.


That feature is called relative netto . The B50 does that. You do not
need to have electronic TE to do this but you do need an airspeed
sensor.

I might add that I fly almost entirely in flat old Denmark, so
the variations with altitude has never hit me as a problem.

Altitude variations probably aren't a problem in Denmark. In
Australia, NZ and the western US they certainly are particularly when
the cruising speeds are higher and greater demands are placed on the
TE system.

Oh, I have a B40, which also works fine, but it's of cause not as
advanced as the GS500 was, and it uses a TE probe.

As designed. The B50 has the advanced features.

Mike Borgelt

On 6 May 2004 06:35:06 GMT, Derrick Steed
wrote:


I should correct the impression given above - I wasn't trying to imply the
B50 did it electronically, but I did, although it seemed obvious to me when
I wrote my bit that if it took the TE input then it used it as just that in
the conventional way.

I cherish memories of a british army nationals pilot that I crewed for back
in '69 and '70 trying to repair his diaphram compensator with a sheet of
latex material cut from a male contractive - I think he might have got a lot
more pleasure from using it in the conventional manner.

I think the thread on high energy pull ups with or without water was longer
- there was just as misconception, bogus physics, and appalling arithmetic
there too!

Still, it's a good laugh innit?

Rgds,

Derrick.



Sure is. Are you frightened too by the fact that we have a high tech
civilization that might as well be running on magic as far as most
people are concerned? I do know the quote about "any sufficiently
advanced technology is indistinguishable from magic". Arthur C. Clarke
wasn't it?

Mike