Sharing static and pitot line

Dear All,
I have seen several posts about how to share the TE line between
e-vario and mechanical one. Maybe I am wrong but I can not see any
post about sharing the pitot pressure (dynamic) between ASI, e-vario
(computer) and and the only one static port between e-vario, altimeter
and mechanical vario.
Any good advice or this is not a problem at all ?

On 7 Mar 2005 00:08:47 -0800, Istvan Csonka wrote:

Dear All,
I have seen several posts about how to share the TE line between
e-vario and mechanical one. Maybe I am wrong but I can not see any
post about sharing the pitot pressure (dynamic) between ASI, e-vario
(computer) and and the only one static port between e-vario, altimeter
and mechanical vario.
Any good advice or this is not a problem at all ?

The altimeter doesn't need to be connected to a static port unless you
plan on pressurising your glider ;-)
The most important instrument on the static port is the ASI, as this
measures very small changes in pressure, and the fluctuations in
cockpit pressure will make the ASI reading almost useless.
A vario on the static port will give you a completely uncompensated
vario. This is sometimes done for motorgliders to provide a VSI for
climbing under power, but would be unusual these days in a pure
glider. A vario using a capacity on the static line could give
transient errors on the ASI due to resistance in the tubing. If you
really want to do this, separate the tubes for ASI and vario as close
to the static port as possible to reduce common impedance.
Sharing the pitot line seems to be less of a problem, particularly if
the "e-vario" is a pressure one rather than a capacity type. However
if the e-vario is a flow-meter type with a typical 1 pint capacity, it
would be a good idea to separate the tubes as close to the pitot tube
as possible, for the same reason as separating the tubes to different
instruments from the static port or TE probe.
Cheers, John G.

I have run in to problems sharing a pitot between ASI and Cambridge
302, resulting in several very frustrating days at Hobbs one year where
it seemed as if I had forgotten to thermal. Separate pitots for the two
instruments solved the problem. Luckily, I have a tail pitot and a nose
pitot so each can have its own. The 302 seems particularly fussy about
having its own pitot and static sources however.

John Cochrane

BB wrote:
I have run in to problems sharing a pitot between ASI and Cambridge
302, resulting in several very frustrating days at Hobbs one year
where
it seemed as if I had forgotten to thermal. Separate pitots for the
two
instruments solved the problem. Luckily, I have a tail pitot and a
nose
pitot so each can have its own. The 302 seems particularly fussy
about
having its own pitot and static sources however.

John Cochrane

John,
I'm flying a 302 in my LS8 and cannot recall any problems in having it
attached to the same pitot that supplies pressure to the ASI. How
would you describe the behavior of the 302, just weird
deflections/audio signals?

Herb, J7

I went through that learning curve with the 302 in Hobbs last summer.
The working combination was to have the 302's static and TE/static
ports connected to the same source, and (very important!) changing the
302's internal setup to use electronic TE. Once I did that, the 302's
vario and my B-40 danced to the same tune.

2NO

BB wrote:
I have run in to problems sharing a pitot between ASI and Cambridge
302, resulting in several very frustrating days at Hobbs one year where
it seemed as if I had forgotten to thermal. Separate pitots for the two
instruments solved the problem. Luckily, I have a tail pitot and a nose
pitot so each can have its own. The 302 seems particularly fussy about
having its own pitot and static sources however.

Based on my experience with my 302, I don't think it was the 302. On my
ASH 26 E, I went from a tail mounted TE feed to the 302 to using the ASI
pitot/static (electronic TE) with no detectable change in ASI or 302
operation. Just guessing: perhaps your connection involved a leak, or
maybe the 302 was not happy using a nose pitot and a TE probe and static
at the rear of the glider.


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

Eric Greenwell
Washington State
USA

Hello,
Maybe there was a misunderstanding.
I am asking advices for the following:
On a PIK20D, there are 4(four) tubes comming up from the cockpit at the panel:
1. Pitot from the nose
2. TE from the fin
3. Left rear fuselage static
4. Right rear fuselage static
What would be the best (optimal) tubing for the following instruments ?
1. Simple ASI
2. Simple Altimeter
3. Simple PZL mech vario
4. Peschges VP9

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.

Roger Druce

"Istvan Csonka" wrote in message
om...
Dear All,
I have seen several posts about how to share the TE line between
e-vario and mechanical one. Maybe I am wrong but I can not see any
post about sharing the pitot pressure (dynamic) between ASI, e-vario
(computer) and and the only one static port between e-vario, altimeter
and mechanical vario.
Any good advice or this is not a problem at all ?

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).

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

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.