Electronic versus Pneumatic compensation (follow-up)

[[email protected]]

For you instrument installation experts out there, I'd like to pick
your brain if you have a circular tuit.

My new glider (a Ventus 2C) came with a Sage (model CV) vario, Winter
ASI and altimeter, and nothing else that connects to pressure sources.
Last week a Cambridge 302 and a new LX7000 were installed.

When the new hardware was installed, all the tubing was replaced except
for the one from the capacity bottle (behind the pilot seat) to the
Sage, which is also connected to static pressure. No restrictor was
installed in the latter. So the Sage setup wasn't modified in any way.

The C-302 and LX7000 were both installed on the TE source and
configured internally for TE (not electronic) compensation, so at the
time of my first flight Saturday, those were the only two instruments
connected to the TE probe.

The Sage was virtually useless during my two flights on Saturday (my
first in the glider). It swung wildly and rapidly in all directions.
The 302 and the LX seemed to work fine.

A fellow pilot suggested that I add a restrictor in the form of a
cigarette filter to the Sage's pressure source. The result yesterday
was a very, very calm needle, maybe too calm (some of the filter will
be removed). But I still found its reading to be quite different from
the 302's. Some to be expected, of course, since the Sage was on
static source while the 302 was on the TE. On light pull-ups, the stick
thermals were 10 to 15 knots!

Anyway, all that's the background. I want to switch the Sage from
static to TE pressure source so it acts more like the B-40 of my
previous glider (which was on the TE). But I have been warned not to
put more than one or two instruments on the TE pressure source, because
they have a certain amount of built-in "leak" or "loss" as part
of the PFM they do.

This is what I would like feedback on - if it's really inadvisable
to put 3 or more instruments on the TE. If it's a Bad Thing, my plan
will be to move the C-302 from TE to electronic compensation, and leave
the LX7000 with the Sage on the TE. Any thoughts on this too? ... or
should I put the LX7000 on electronic compensation too and leave the
Sage alone on TE?

Advance thanks

-ted/2NO

[Paul Remde]

Hi Ted,

The strange behavior of the Sage vario suggests that you may not be using
the correct size capacity flask. There are several size capacity flasks
available. Please check the documentation for the vario. I give dimensions
for a few Winter capacity flasks on my Winter page here:
http://www.soarmn.com/cumulus/winter.htm#W-9014

I think the idea of using a filter in line with the vario is a bad one.

It may work OK to connect all 3 to the TE source as long as the connection
to the Sage is a long way (3m? - behind your seat) from the connection to
the LX7000 and 302. But I think that if you want the Sage to be a TE vario
you may want to use an independent TE source for it.

I have never tried to connect 3 TE sources to a single TE so I can't say for
certain whether that will work or not.

Good Soaring,

Paul Remde
Cumulus Soaring, Inc.
http://www.cumulus-soaring.com

> wrote in message
ups.com...
> For you instrument installation experts out there, I'd like to pick
> your brain if you have a circular tuit.
>
> My new glider (a Ventus 2C) came with a Sage (model CV) vario, Winter
> ASI and altimeter, and nothing else that connects to pressure sources.
> Last week a Cambridge 302 and a new LX7000 were installed.
>
> When the new hardware was installed, all the tubing was replaced except
> for the one from the capacity bottle (behind the pilot seat) to the
> Sage, which is also connected to static pressure. No restrictor was
> installed in the latter. So the Sage setup wasn't modified in any way.
>
> The C-302 and LX7000 were both installed on the TE source and
> configured internally for TE (not electronic) compensation, so at the
> time of my first flight Saturday, those were the only two instruments
> connected to the TE probe.
>
> The Sage was virtually useless during my two flights on Saturday (my
> first in the glider). It swung wildly and rapidly in all directions.
> The 302 and the LX seemed to work fine.
>
> A fellow pilot suggested that I add a restrictor in the form of a
> cigarette filter to the Sage's pressure source. The result yesterday
> was a very, very calm needle, maybe too calm (some of the filter will
> be removed). But I still found its reading to be quite different from
> the 302's. Some to be expected, of course, since the Sage was on
> static source while the 302 was on the TE. On light pull-ups, the stick
> thermals were 10 to 15 knots!
>
> Anyway, all that's the background. I want to switch the Sage from
> static to TE pressure source so it acts more like the B-40 of my
> previous glider (which was on the TE). But I have been warned not to
> put more than one or two instruments on the TE pressure source, because
> they have a certain amount of built-in "leak" or "loss" as part
> of the PFM they do.
>
> This is what I would like feedback on - if it's really inadvisable
> to put 3 or more instruments on the TE. If it's a Bad Thing, my plan
> will be to move the C-302 from TE to electronic compensation, and leave
> the LX7000 with the Sage on the TE. Any thoughts on this too? ... or
> should I put the LX7000 on electronic compensation too and leave the
> Sage alone on TE?
>
> Advance thanks
>
> -ted/2NO
>

[John Galloway]

I would expect a non-TE-compensated sensitive vario
like a Sage to be useless for soaring even with the
right flask and no leaks.

There is no theoretical reason not to attach 3 varios
to the one TE line with the exception of the consideration
of not wanting to mix a flow sensor vario (the Sage)
with pressure sensor varios (the other two) unless
the TE line is divided as far aft as is humanly possible
- as Paul says. In practice under the rear of the
seat pan is about as far aft as it is easy to reach
in a Schempp.

A big mistake to avoid is to have a restrictor in the
TE line aft of the Y division in the line. In that
case airflows flow from the Sage will be even more
encouraged to go back and forth between that and the
other 2 varios because the flow resistance is less
than the resistance back down the rear fuselage tube
to the TE probe. If you decide to use any sort of
flow restrictor then make sure it is forward of the
Y junction - but better still forget it. There is
no flow from the two other varios so the branch of
the TE line to them can be divided close to the instruments.

A good TE probe is a very clever device that produces
the inverse of pitot minus static pressure and not
the inverse of total pitot pressure (as is commonly
mistakenly believed) and it also has good performance
with yaw and pitch changes. It is amazing to me that
instrument manufacturers and pilots opt to use 2 pressure
sensors one of which (the static source) is highly
yaw sensitive and then electronically combine these
poorer sources to get electronic TE compensation when
a good quality correctly sited TE probe provides the
correct pressure correction at source. So it is better
and faster - always - to use pneumatic TE unless there
is something wrong with the probe or the installation
or the pneumatic tubing.

So - check for leaks, right connections and correct
flask, do without nasty restrictors, make sure you
have a good quality TE probe, keep the varios on pneumatic
TE and divide the TE line between the Sage and the
other 2 varios as far aft as possible - IMHO.

John Galloway

At 17:00 19 September 2005, Paul Remde wrote:
>Hi Ted,
>
>The strange behavior of the Sage vario suggests that
>you may not be using
>the correct size capacity flask. There are several
>size capacity flasks
>available. Please check the documentation for the
>vario. I give dimensions
>for a few Winter capacity flasks on my Winter page
>here:
>http://www.soarmn.com/cumulus/winter.htm#W-9014
>
>I think the idea of using a filter in line with the
>vario is a bad one.
>
>It may work OK to connect all 3 to the TE source as
>long as the connection
>to the Sage is a long way (3m? - behind your seat)
>from the connection to
>the LX7000 and 302. But I think that if you want the
>Sage to be a TE vario
>you may want to use an independent TE source for it.
>
>I have never tried to connect 3 TE sources to a single
>TE so I can't say for
>certain whether that will work or not.
>
>Good Soaring,
>
>Paul Remde
>Cumulus Soaring, Inc.
>http://www.cumulus-soaring.com
>
> wrote in message
ups.com...
>> For you instrument installation experts out there,
>>I'd like to pick
>> your brain if you have a circular tuit.
>>
>> My new glider (a Ventus 2C) came with a Sage (model
>>CV) vario, Winter
>> ASI and altimeter, and nothing else that connects
>>to pressure sources.
>> Last week a Cambridge 302 and a new LX7000 were installed.
>>
>> When the new hardware was installed, all the tubing
>>was replaced except
>> for the one from the capacity bottle (behind the pilot
>>seat) to the
>> Sage, which is also connected to static pressure.
>>No restrictor was
>> installed in the latter. So the Sage setup wasn't
>>modified in any way.
>>
>> The C-302 and LX7000 were both installed on the TE
>>source and
>> configured internally for TE (not electronic) compensation,
>>so at the
>> time of my first flight Saturday, those were the only
>>two instruments
>> connected to the TE probe.
>>
>> The Sage was virtually useless during my two flights
>>on Saturday (my
>> first in the glider). It swung wildly and rapidly
>>in all directions.
>> The 302 and the LX seemed to work fine.
>>
>> A fellow pilot suggested that I add a restrictor in
>>the form of a
>> cigarette filter to the Sage's pressure source. The
>>result yesterday
>> was a very, very calm needle, maybe too calm (some
>>of the filter will
>> be removed). But I still found its reading to be quite
>>different from
>> the 302's. Some to be expected, of course, since the
>>Sage was on
>> static source while the 302 was on the TE. On light
>>pull-ups, the stick
>> thermals were 10 to 15 knots!
>>
>> Anyway, all that's the background. I want to switch
>>the Sage from
>> static to TE pressure source so it acts more like
>>the B-40 of my
>> previous glider (which was on the TE). But I have
>>been warned not to
>> put more than one or two instruments on the TE pressure
>>source, because
>> they have a certain amount of built-in 'leak' or 'loss'
>>as part
>> of the PFM they do.
>>
>> This is what I would like feedback on - if it's really
>>inadvisable
>> to put 3 or more instruments on the TE. If it's a
>>Bad Thing, my plan
>> will be to move the C-302 from TE to electronic compensation,
>>and leave
>> the LX7000 with the Sage on the TE. Any thoughts on
>>this too? ... or
>> should I put the LX7000 on electronic compensation
>>too and leave the
>> Sage alone on TE?
>>
>> Advance thanks
>>
>> -ted/2NO
>>
>
>
>

[Eric Greenwell]

John Galloway wrote:

> It is amazing to me that
> instrument manufacturers and pilots opt to use 2 pressure
> sensors one of which (the static source) is highly
> yaw sensitive

I thought static sources were carefully chosen to be yaw insensitive.
The one on my ASH 26 E seems to be that way - I have to yaw a large
amount to affect the ASI, for example.

> and then electronically combine these
> poorer sources

These days, the sensors are very good.

> to get electronic TE compensation when
> a good quality correctly sited TE probe provides the
> correct pressure correction at source. So it is better
> and faster - always - to use pneumatic TE unless there
> is something wrong with the probe or the installation
> or the pneumatic tubing.

This was true years ago, but I don't think it is universally true
anymore. I know several pilots with Cambridge 302s, including myself,
that find the electronic TE works just as well as the TE probe.

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

Eric Greenwell
Washington State
USA

[[email protected]]

Hi
I have electronic TE on my LX 5000. I do not notice any yaw
sensitivity, maybe it is because front and rear static source are used
togetter on my LS6 with this instrument. It is a very good system. This
installation was on the aircraft when I got it last year.Not at all
different from the Lnav I had for 10 years on my previous glider, which
was very good. The damping was set at 7 in the instument by the
previous owner.
I did not touch anything it is perfect as is. I do not have any other
vario. I have been flying like this for 16
years now. No problem.
Gilles