Sharing static and pitot line

Roger Druce wrote:
While on the matter of line sharing, the following is relevant to sharing
the Total Energy line between a number of flow variometers, mechanical
(Winter, PZL, etc.) and / or electrical (Cambridge CAV II etc.)

Each of the variometers fed by the one TE source have flow through the
instrument to their own flask.

All flasks running off one TE source must be the same physically and
thermodynamically. If you mix flasks with different characteristics then
you will get cross flow between the instruments. You can try this test on
the bench provided you do it carefully. Get two identical pneumatic varios
(with hopefully little inherent instrument error) and two diferent flasks,
say one vacuum flask with heat sink material inserted and the other a same
type vacuum flask without heat sink material inside. Apply a signal via a
Tee junction to the two variometers. Note the variometer readings of the
two variometers at different flow rates, ie calibrate one vario relative to
the other. They won't read the same! Then swap the flasks between the
variometers and repeat the calibration. The relative calibration will swap
over between the varios showing that the flasks are influencing things
strongly due to their different characteristics. Install identical flasks
and the varios will resume responding together.

So use identical flasks to avoid cross flow when using a common TE source.

If you have to worry about the flasks cross-flowing, you have another
problem: your TE source isn't good enough to supply the flow the two
flasks need, and both your varios will operate more slowly than they
would alone. The TE system must be able to supply the TE pressure to the
vario tee, regardless of the flow the varios require. If it does this,
then the flasks can't affect the pressure at the tee, and it won't
matter what size each is. This might require a probe with a larger
hole(s) in it, shorter or larger diameter tubing from the TE probe to
the tee for the varios, or varios that use smaller flasks (i.e., require
less flow).

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Eric Greenwell
Washington State
USA

On Thu, 10 Mar 2005 19:33:57 -0800, Eric Greenwell wrote:

Roger Druce wrote:
While on the matter of line sharing, the following is relevant to sharing
the Total Energy line between a number of flow variometers, mechanical
(Winter, PZL, etc.) and / or electrical (Cambridge CAV II etc.)

Each of the variometers fed by the one TE source have flow through the
instrument to their own flask.

All flasks running off one TE source must be the same physically and
thermodynamically. If you mix flasks with different characteristics then
you will get cross flow between the instruments. You can try this test on
the bench provided you do it carefully. Get two identical pneumatic varios
(with hopefully little inherent instrument error) and two diferent flasks,
say one vacuum flask with heat sink material inserted and the other a same
type vacuum flask without heat sink material inside. Apply a signal via a
Tee junction to the two variometers. Note the variometer readings of the
two variometers at different flow rates, ie calibrate one vario relative to
the other. They won't read the same! Then swap the flasks between the
variometers and repeat the calibration. The relative calibration will swap
over between the varios showing that the flasks are influencing things
strongly due to their different characteristics. Install identical flasks
and the varios will resume responding together.

So use identical flasks to avoid cross flow when using a common TE source.

If you have to worry about the flasks cross-flowing, you have another
problem: your TE source isn't good enough to supply the flow the two
flasks need, and both your varios will operate more slowly than they
would alone. The TE system must be able to supply the TE pressure to the
vario tee, regardless of the flow the varios require. If it does this,
then the flasks can't affect the pressure at the tee, and it won't
matter what size each is. This might require a probe with a larger
hole(s) in it, shorter or larger diameter tubing from the TE probe to
the tee for the varios, or varios that use smaller flasks (i.e., require
less flow).

If the varios all require the same sized flask, why not connect the
varios in series to a single flask ? The small additional volume seen
by the early instruments in the chain, due to the volume of the later
varios will be a source of error, but shouldn't be a large error
unless a small flask is used.
Cheers, John G.

There are two problems with electronic pitot/static TE. The magnitude of
the compensation signal and its phase.

The first is commonly adjustable on the instrument, the second isn't.

Your pressure port and the combined flow resistance and capacity of
the tubing form a low pass filter. If the pitot and static system
filter time constants are greatly different you can get large transient
indications on the vario during pull ups or push overs.

So if you have a system that works adding another instrument and
connecting it to the static system will usually cause a problem.

Leaks will also cause problems, as would partially blocking the static
port with wax.(flow resistance increases.)

For optimum performance of electronic TE the pitot and static should be
on the same side of the TE(nose or tail) and the pitot should be twice
as far from the TE as the static is. (Former World champion Stig Oye
told me of this and a couple of minutes thought convinced me)

A combined pitot/static probe on the fin does not fulfill this
condition and still results in "g" change effects the same as a TE
probe but is more sensitive to pitch and sideslip than the common two
hole Irving pattern probe.

If you aren't having apparent problems with electronic TE you are lucky
or you've got your vario on a very slow response speed where the
unwanted transients are heavily damped.

If you are having problems you want to get a good handle on the physics
of what is going on

in order to fix the problem.

Mike Borgelt


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mborgelt
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mborgelt wrote:

For optimum performance of electronic TE the pitot and static should be
on the same side of the TE(nose or tail) and the pitot should be twice
as far from the TE as the static is. (Former World champion Stig Oye
told me of this and a couple of minutes thought convinced me)

Mike, could you elaborate on this? It the vario is using electronic TE,
it's not using a TE probe, so I'm confused about the "pitot and static
should be on the same side of the TE(nose or tail)". What is TE
referring to in your statement?

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Eric Greenwell
Washington State
USA