Pneumatic switching?

[bumper]

On a self-launch with pylon mounted prop, the prop wash can interfere with a
probe mounted on the vertical fin. One way to get around this is to use a
pneumatic switch to change from tail probe to nose pitot/fuselage static
when running the engine. Question is, what plumbing is best to switch,
pitot, static, TE or combination of these?

Glider in question is equipped with a 302 using pneumatic compensation (not
electronic) and Tasman electronic back-up vario (no flask). Currently tail
TE is plumbed to both varios direct (no switching). Two ganged pneumatic
switches are used for the 302 pitot and static inputs to switch these from
fuselage when motoring, to tail probe when soaring. ASI is connected to nose
pitot/fuselage static (no switching). This configuration seems to work okay,
but is this really the optimal configuration? For more sensitive detection
of lift during self-launching, would it be better to switch the vario/s TE
input from tail TE to cockpit or fuselage static, and forgo switching
either pitot or static to the 302?

Thanks for any input on this.

all the best,

bumper

[Marc Ramsey]

bumper wrote:
> This configuration seems to work okay,
> but is this really the optimal configuration? For more sensitive detection
> of lift during self-launching, would it be better to switch the vario/s TE
> input from tail TE to cockpit or fuselage static, and forgo switching
> either pitot or static to the 302?

If it were my glider, I'd always run the 302 off the nose pitot and
fuselage static, using the 302s (excellent) electronic total energy
compensation. The Tasman would always be connected to the TE probe, and
just ignored when the engine is running. There is no need to add the
complication of pneumatic switches...

Marc

[5Z]

I have my varios plumbed as Marc suggests, but in my case the other
vario is a Winter. It bounces all over the place while under power,
while the 302 provides an excellent aid in finding and centering a
thermal ASAP after launch.

-Tom

[soarski]

That's how my TwinIII SL was working. With a Peschges VP 6, that did
it's own thing with Pitot and static for TE compensation.
The ship did have a Pneumatic switch, and simply turned the Probe on
and off in the back. I think I was able to also feed the VP 6 with the
probe, but never did, and compared

Dieter

[Marc Ramsey]

T o d d P a t t i s t wrote:
> Marc Ramsey > wrote:
>>If it were my glider, I'd always run the 302 off the nose pitot and
>>fuselage static, using the 302s (excellent) electronic total energy
>>compensation.
>
>
> As MIke Borgelt recently pointed out, this produces two in
> phase error signals. Running with the pitot and static both
> on the nose or both on the tail, preferably with the pitot
> twice as far from your CG, will produce canceling errors for
> electronic TE.

Yes, I'm sure it makes sense in theory. In practice, I'm flying a
glider with an electronic TE vario (C302) on nose pitot and tail boom
static (which is all that is available), and I've never seen it shown
any "errors" that weren't also present on the B40 running on the TE
probe by itself. But, YMMV...

I've had problems in the past, in several different gliders, with the
combined pitot/static/TE tail probes (I've got 3 different types of
multi-probes gathering dust in the closet, right now) measuring
airspeeds that vary by several knots from the nose pitot, fuselage
static setup. It's, of course, hard tell which was more accurate
(unless you're Dick Johnson), but given that the POHs inevitably require
use of latter setup for the ASI and altimeter, I'd rather have
everything working off the same source...

Marc

[bumper]

"T o d d P a t t i s t" > wrote in message
...
> Marc Ramsey > wrote:
>
>>>>If it were my glider, I'd always run the 302 off the nose pitot and
>>>>fuselage static, using the 302s (excellent) electronic total energy
>>>>compensation.
>>I'm flying a
>>glider with an electronic TE vario (C302) on nose pitot and tail boom
>>static (which is all that is available), and I've never seen it shown
>>any "errors"
>
> I have very limited personal experience with electronic TE.
> I'll take your word for it that it works for you on the nose
> pitot and tail static. That said, however, I have found
> Mike Borgelt's comments to be right on target every time, so
> even if it works for you, others may see problems.


When I ordered my 302, I talked with Dave Ellis about using the tail probe
inputs vs. using the 302's electronic compensation. This was several years
ago, so I may be leaving something out. The gist of it was that he felt the
302's electronic compensation was very good, but not quite as good as a well
installed fin mounted TE probe with good plumbing.

I also discussed pneumatic switching for a self-launch with Dave . . . and
he agreed this to be an acceptable approach. That's were the old brain
starts to fade a bit, as I can't remember if we discussed exactly *what* to
switch (g) . . . and that's why I posted my original question.

I am aware that I can simply use the 302's very good electronic TE. However,
since I went to the effort to design and install a remote mount for two
Clippard pneumatic switches near the forward end of the glareshield (where
they take no panel space), I'll continue to use them *if* that turns out to
be the best solution. The switching works fine BTW, no leaks and so far
reliable. Absent a better suggestion, I'll leave it the way it is.

all the best,

bumper