airspeed indicator problem after TT2 transponder install [Archive]

Frank Paynter[_2_]

March 3rd 11, 03:26 PM

I installed a TT21 transponder over the winter, and as per the
installation instructions, tapped into my ship's static line for
altitude reporting. Yesterday I checked the transponder with Orlando
ATC and they reported a good signal and proper altitude reporting. So
far, so good.

Then I noticed my airspeed indicator seemed to be reading
significantly higher than normal, and I confirmed this with a stall
series. My Ventus 2bx, which normally stalls in the 40-45kt range
depending on bank angle, now is apparently stalling in the 50-55kt
range. The ship actually feels pretty much the same as before,
although I haven't flown for 3 months so that's a bit hard to tell for
sure.

My dry flying weight is probably about 5-10lbs higher this year than
last, as I added the TT21 itself, plus a 5-lb battery to support it.
However, I'm pretty sure that by itself doesn't explain the
significantly higher apparent stall speed.

As a troubleshooting step, I removed and plugged the static line from
the airspeed indicator, so now the ASI takes it's static input from
cabin air, and everything else should be unchanged. I haven't flown
it yet to try it out, but I will today. I also have a spare airspeed
indicator that I can swap in if necessary.

Anything else I should be considering?

TIA,

Frank

Tony[_5_]

March 3rd 11, 03:36 PM

On Mar 3, 9:26*am, Frank Paynter > wrote:
> I installed a TT21 transponder over the winter, and as per the
> installation instructions, tapped into my ship's static line for
> altitude reporting. *Yesterday I checked the transponder with Orlando
> ATC and they reported a good signal and proper altitude reporting. *So
> far, so good.
>
> Then I noticed my airspeed indicator seemed to be reading
> significantly higher than normal, and I confirmed this with a stall
> series. *My Ventus 2bx, which normally stalls in the 40-45kt range
> depending on bank angle, now is apparently stalling in the 50-55kt
> range. *The ship actually feels pretty much the same as before,
> although I haven't flown for 3 months so that's a bit hard to tell for
> sure.
>
> My dry flying weight is probably about 5-10lbs higher this year than
> last, as I added the TT21 itself, plus a 5-lb battery to support it.
> However, I'm pretty sure that by itself doesn't explain the
> significantly higher apparent stall speed.
>
> As a troubleshooting step, I removed and plugged the static line from
> the airspeed indicator, so now the ASI takes it's static input from
> cabin air, and everything else should be unchanged. *I haven't flown
> it yet to try it out, but I will today. *I also have a spare airspeed
> indicator that I can swap in if necessary.
>
> Anything else I should be considering?
>
> TIA,
>
> Frank

Frank,

My guess is the new static line that you plumbed in for the
transponder introduced a leak into the static system. It should've
been leak tested when the transponder check was completed. Do you know
if that was done?

Andy[_10_]

March 3rd 11, 04:06 PM

On Mar 3, 7:36*am, Tony > wrote:
> On Mar 3, 9:26*am, Frank Paynter > wrote:
>
>
>
> > I installed a TT21 transponder over the winter, and as per the
> > installation instructions, tapped into my ship's static line for
> > altitude reporting. *Yesterday I checked the transponder with Orlando
> > ATC and they reported a good signal and proper altitude reporting. *So
> > far, so good.
>
> > Then I noticed my airspeed indicator seemed to be reading
> > significantly higher than normal, and I confirmed this with a stall
> > series. *My Ventus 2bx, which normally stalls in the 40-45kt range
> > depending on bank angle, now is apparently stalling in the 50-55kt
> > range. *The ship actually feels pretty much the same as before,
> > although I haven't flown for 3 months so that's a bit hard to tell for
> > sure.
>
> > My dry flying weight is probably about 5-10lbs higher this year than
> > last, as I added the TT21 itself, plus a 5-lb battery to support it.
> > However, I'm pretty sure that by itself doesn't explain the
> > significantly higher apparent stall speed.
>
> > As a troubleshooting step, I removed and plugged the static line from
> > the airspeed indicator, so now the ASI takes it's static input from
> > cabin air, and everything else should be unchanged. *I haven't flown
> > it yet to try it out, but I will today. *I also have a spare airspeed
> > indicator that I can swap in if necessary.
>
> > Anything else I should be considering?
>
> > TIA,
>
> > Frank

You need to install a Dittel FSG-2T radio with the new capacitor
arrangement. Apparently that fixes all types of seemingly unrelated
glider gremlins.

;-)

Your experiment sounds like the right first step. A second step would
be to put the static back into the original configuration and then to
try a new ASI if the above doesn't work (which would be odd). You
also could check and see if you can try different static sources -
consistent with what the certification allows.

9B

Gary Boggs

March 3rd 11, 04:07 PM

But wouldn't a leak in the system cause the airspeed to read lower?

> My guess is the new static line that you plumbed in for the
> transponder introduced a leak into the static system. *It should've
> been leak tested when the transponder check was completed. Do you know
> if that was done?

Grider Pirate

March 3rd 11, 05:27 PM

On Mar 3, 8:07*am, GARY BOGGS > wrote:
> But wouldn't a leak in the system cause the airspeed to read lower?
>
>
>
> > My guess is the new static line that you plumbed in for the
> > transponder introduced a leak into the static system. *It should've
> > been leak tested when the transponder check was completed. Do you know
> > if that was done?- Hide quoted text -
>
> - Show quoted text -

That depends on whether his cabin pressure is higher or lower than
static. Easy to check with a manometer connected to static on one side
and nothing (cabin) on the other. My fancy manometer is some acrylic
tube in a 'U' shape, and some water.

jsbrake[_2_]

March 3rd 11, 05:31 PM

Perhaps the curse I put on CP for abandoning his Canadian club (not
the whiskey!) for warmer climes (and climbs!) has had a spill-over
effect to Frank's ASI? I wouldn't have thought the sacrifice of a
PW-5 could be so powerful.

Hagbard Celine

March 3rd 11, 07:25 PM

It also depends upon whether the ships static system shows true
static or not. If you read through Richard Johnson's tests you'll see
that many gliders have static systems with significant errors built in
to them. If there is more than one static source the manual will
usually specify which one to connect to as that is the one which was
used for certification testing and the placard speeds will take any
error into account.

I would reconnect everything the way it's supposed to be and then get
the pitot-static system checked out with a test set.
>
> > But wouldn't a leak in the system cause the airspeed to read lower?

>
> That depends on whether his cabin pressure is higher or lower than
> static.

Tim Taylor

March 3rd 11, 08:42 PM

Mike the Strike

March 3rd 11, 09:36 PM

On Mar 3, 1:42*pm, Tim Taylor > wrote:
> On Mar 3, 10:26*am, Frank Paynter > wrote:
>
>
>
> > I installed a TT21 transponder over the winter, and as per the
> > installation instructions, tapped into my ship's static line for
> > altitude reporting. *Yesterday I checked the transponder with Orlando
> > ATC and they reported a good signal and proper altitude reporting. *So
> > far, so good.
>
> > Then I noticed my airspeed indicator seemed to be reading
> > significantly higher than normal, and I confirmed this with a stall
> > series. *My Ventus 2bx, which normally stalls in the 40-45kt range
> > depending on bank angle, now is apparently stalling in the 50-55kt
> > range. *The ship actually feels pretty much the same as before,
> > although I haven't flown for 3 months so that's a bit hard to tell for
> > sure.
>
> > My dry flying weight is probably about 5-10lbs higher this year than
> > last, as I added the TT21 itself, plus a 5-lb battery to support it.
> > However, I'm pretty sure that by itself doesn't explain the
> > significantly higher apparent stall speed.
>
> > As a troubleshooting step, I removed and plugged the static line from
> > the airspeed indicator, so now the ASI takes it's static input from
> > cabin air, and everything else should be unchanged. *I haven't flown
> > it yet to try it out, but I will today. *I also have a spare airspeed
> > indicator that I can swap in if necessary.
>
> > Anything else I should be considering?
>
> > TIA,
>
> > Frank
>
> As with all design work:
>
> 1. *Only make one change at a time.
> 2. If there is a problem, undo the change.
> 3. If the system returns to normal start testing other things.
> If you start chasing options without checking if the original system
> is still the same you may spend lots of time chasing the incorrect
> problem.
>
> I would put the system back to the pre trig state and leave the trig
> in cabin static for the first test.

Are you sure you hooked it up to Static and not the TE probe?

Mike

Frank Paynter[_2_]

March 4th 11, 12:21 AM

On Mar 3, 10:26*am, Frank Paynter > wrote:
> I installed a TT21 transponder over the winter, and as per the
> installation instructions, tapped into my ship's static line for
> altitude reporting. *Yesterday I checked the transponder with Orlando
> ATC and they reported a good signal and proper altitude reporting. *So
> far, so good.
>
> Then I noticed my airspeed indicator seemed to be reading
> significantly higher than normal, and I confirmed this with a stall
> series. *My Ventus 2bx, which normally stalls in the 40-45kt range
> depending on bank angle, now is apparently stalling in the 50-55kt
> range. *The ship actually feels pretty much the same as before,
> although I haven't flown for 3 months so that's a bit hard to tell for
> sure.
>
> My dry flying weight is probably about 5-10lbs higher this year than
> last, as I added the TT21 itself, plus a 5-lb battery to support it.
> However, I'm pretty sure that by itself doesn't explain the
> significantly higher apparent stall speed.
>
> As a troubleshooting step, I removed and plugged the static line from
> the airspeed indicator, so now the ASI takes it's static input from
> cabin air, and everything else should be unchanged. *I haven't flown
> it yet to try it out, but I will today. *I also have a spare airspeed
> indicator that I can swap in if necessary.
>
> Anything else I should be considering?
>
> TIA,
>
> Frank

Results of todays experiment:
With the ASI static port open to the cabin, the problem remained
(reads too high by about 10-15kt). I believe this eliminates the
plumbing changes associated with the TT21 install.

I happened to remember that my CAI-302 has an airspeed readout as
part of it's calibration/debug screens, and I was able to get it to
that display while in flight today. The CAI-302 (static also open to
cabin pressure) display was much more believable than the ASI, reading
about 38kt at stall in thermalling configuration, and consistently
about 10-13kt lower than the ASI at all airspeeds. I believe this
eliminates cabin pressure differential as a possible cause, as both
the CAI-302 and ASI were using the same static reference, and the same
pitot line.

My conclusion (to be tested tomorrow) is that the ASI has decided to
depart for an extended lunch. I happen to have a spare (actually the
original ASI for this ship) that was overhauled about a year ago. My
prediction is that the replacement ASI and the CAI-302 will be very
close - any bets?

Regards,

Frank

PS: Two nice streeting days here, but very windy (20+kt out of
east). Got 3hrs in yesterday, and another 3 in today, all street
running.

JJ Sinclair[_2_]

March 4th 11, 03:09 PM

On Mar 3, 4:21*pm, Frank Paynter > wrote:
> On Mar 3, 10:26*am, Frank Paynter > wrote:
>
>
>
>
>
> > I installed a TT21 transponder over the winter, and as per the
> > installation instructions, tapped into my ship's static line for
> > altitude reporting. *Yesterday I checked the transponder with Orlando
> > ATC and they reported a good signal and proper altitude reporting. *So
> > far, so good.
>
> > Then I noticed my airspeed indicator seemed to be reading
> > significantly higher than normal, and I confirmed this with a stall
> > series. *My Ventus 2bx, which normally stalls in the 40-45kt range
> > depending on bank angle, now is apparently stalling in the 50-55kt
> > range. *The ship actually feels pretty much the same as before,
> > although I haven't flown for 3 months so that's a bit hard to tell for
> > sure.
>
> > My dry flying weight is probably about 5-10lbs higher this year than
> > last, as I added the TT21 itself, plus a 5-lb battery to support it.
> > However, I'm pretty sure that by itself doesn't explain the
> > significantly higher apparent stall speed.
>
> > As a troubleshooting step, I removed and plugged the static line from
> > the airspeed indicator, so now the ASI takes it's static input from
> > cabin air, and everything else should be unchanged. *I haven't flown
> > it yet to try it out, but I will today. *I also have a spare airspeed
> > indicator that I can swap in if necessary.
>
> > Anything else I should be considering?
>
> > TIA,
>
> > Frank
>
> Results of todays experiment:
> * * * With the ASI static port open to the cabin, the problem remained
> (reads too high by about 10-15kt). *I believe this eliminates the
> plumbing changes associated with the TT21 install.
>
> * * I happened to remember that my CAI-302 has an airspeed readout as
> part of it's calibration/debug screens, and *I was able to get it to
> that display while in flight today. *The CAI-302 (static also open to
> cabin pressure) display was much more believable than the ASI, reading
> about 38kt at stall in thermalling configuration, and consistently
> about 10-13kt lower than the ASI at all airspeeds. *I believe this
> eliminates cabin pressure differential as a possible cause, as both
> the CAI-302 and ASI were using the same static reference, and the same
> pitot line.
>
> My conclusion (to be tested tomorrow) is that the ASI has decided to
> depart for an extended lunch. *I happen to have a spare (actually the
> original ASI for this ship) that was overhauled about a year ago. *My
> prediction is that the replacement ASI and the CAI-302 will be very
> close - any bets?
>
> Regards,
>
> Frank
>
> PS: *Two nice streeting days here, but very windy (20+kt out of
> east). *Got 3hrs in yesterday, and another 3 in today, all street
> running.- Hide quoted text -
>
> - Show quoted text -

Check the pitot line for a kink. if you have a kink it may be trapping
higher pitot pressure between the kink and the ASI and the kink wont
allow it to dissipate as you slow down.
JJ

Tim Mara

March 4th 11, 04:51 PM

Is this in a motor glider?
If it is and the motor was run with the Tail Pitot in use rather than the
alternate Pitot (some gliders will have front and rear Pitot and static
inputs if they come with motors today) you may have damaged the ASI from the
high pressure .......just a thought
tim
Please visit the Wings & Wheels website at www.wingsandwheels.comtim

"Frank Paynter" > wrote in message
...
On Mar 3, 10:26 am, Frank Paynter > wrote:
> I installed a TT21 transponder over the winter, and as per the
> installation instructions, tapped into my ship's static line for
> altitude reporting. Yesterday I checked the transponder with Orlando
> ATC and they reported a good signal and proper altitude reporting. So
> far, so good.
>
> Then I noticed my airspeed indicator seemed to be reading
> significantly higher than normal, and I confirmed this with a stall
> series. My Ventus 2bx, which normally stalls in the 40-45kt range
> depending on bank angle, now is apparently stalling in the 50-55kt
> range. The ship actually feels pretty much the same as before,
> although I haven't flown for 3 months so that's a bit hard to tell for
> sure.
>
> My dry flying weight is probably about 5-10lbs higher this year than
> last, as I added the TT21 itself, plus a 5-lb battery to support it.
> However, I'm pretty sure that by itself doesn't explain the
> significantly higher apparent stall speed.
>
> As a troubleshooting step, I removed and plugged the static line from
> the airspeed indicator, so now the ASI takes it's static input from
> cabin air, and everything else should be unchanged. I haven't flown
> it yet to try it out, but I will today. I also have a spare airspeed
> indicator that I can swap in if necessary.
>
> Anything else I should be considering?
>
> TIA,
>
> Frank

Results of todays experiment:
With the ASI static port open to the cabin, the problem remained
(reads too high by about 10-15kt). I believe this eliminates the
plumbing changes associated with the TT21 install.

I happened to remember that my CAI-302 has an airspeed readout as
part of it's calibration/debug screens, and I was able to get it to
that display while in flight today. The CAI-302 (static also open to
cabin pressure) display was much more believable than the ASI, reading
about 38kt at stall in thermalling configuration, and consistently
about 10-13kt lower than the ASI at all airspeeds. I believe this
eliminates cabin pressure differential as a possible cause, as both
the CAI-302 and ASI were using the same static reference, and the same
pitot line.

My conclusion (to be tested tomorrow) is that the ASI has decided to
depart for an extended lunch. I happen to have a spare (actually the
original ASI for this ship) that was overhauled about a year ago. My
prediction is that the replacement ASI and the CAI-302 will be very
close - any bets?

Regards,

Frank

PS: Two nice streeting days here, but very windy (20+kt out of
east). Got 3hrs in yesterday, and another 3 in today, all street
running.

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Darryl Ramm

March 4th 11, 07:22 PM

On Mar 4, 8:51*am, "Tim Mara" > wrote:
> Is this in a motor glider?
> If it is and the motor was run with the Tail Pitot in use rather than the
> alternate Pitot (some gliders will have front and rear Pitot and static
> inputs if they come with motors today) you may have damaged the ASI from the
> high pressure .......just a *thought
> tim
> Please visit the Wings & Wheels website atwww.wingsandwheels.comtim
[snip]

I don't see how the pressure at a tail mounted pitot in a motorglider
will ever be enough to damage an ASI. Not compared the pressure from
airspeed they'll experience from normal flight at different speeds.
And the main ASI would normally be permanently plumbed to the nose
pitot and the ships main static (not a TE or triple probe).

Darryl

Andy[_1_]

March 4th 11, 09:09 PM

On Mar 4, 12:22*pm, Darryl Ramm > wrote:
>I don't see how the pressure at a tail mounted pitot in a motorglider
>will ever be enough to damage an ASI.

Agree - A pitot pressure high enough to damage the ASI would result in
an airspeed reading that would cause a change, perhaps several
changes, of underwear. No such event was reported ;)

Andy

Tim Mara

March 4th 11, 09:44 PM

it's not just a simple pressure if it's in the tail and there's a fan
blowing on it, you don't find aircraft with the Pitot mounted behind an
engine either.there is a reason they put alternative Pitot and statics on
motor gliders today...most newer gliders do have the multi probes
(pitot-static-TE) in the fin.they all work typically "best" there but they
intend these to be switched to alternative sources when the motor is being
used.....
next time you ask someone to test your pitot are you going to ask them to
blow really hard ?? or just lightly?.and the reason for this would be????
tim

"Darryl Ramm" > wrote in message
...
On Mar 4, 8:51 am, "Tim Mara" > wrote:
> Is this in a motor glider?
> If it is and the motor was run with the Tail Pitot in use rather than the
> alternate Pitot (some gliders will have front and rear Pitot and static
> inputs if they come with motors today) you may have damaged the ASI from
> the
> high pressure .......just a thought
> tim
> Please visit the Wings & Wheels website atwww.wingsandwheels.comtim
[snip]

I don't see how the pressure at a tail mounted pitot in a motorglider
will ever be enough to damage an ASI. Not compared the pressure from
airspeed they'll experience from normal flight at different speeds.
And the main ASI would normally be permanently plumbed to the nose
pitot and the ships main static (not a TE or triple probe).

Darryl

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database 5926 (20110304) __________

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Don Johnstone[_4_]

March 4th 11, 10:47 PM

At 21:44 04 March 2011, Tim Mara wrote:
>it's not just a simple pressure if it's in the tail and there's a fan

>blowing on it, you don't find aircraft with the Pitot mounted behind an

>engine either.there is a reason they put alternative Pitot and statics on

>motor gliders today...most newer gliders do have the multi probes
>(pitot-static-TE) in the fin.they all work typically "best" there but
they
>
>intend these to be switched to alternative sources when the motor is
being
>
>used.....
>next time you ask someone to test your pitot are you going to ask them to

>blow really hard ?? or just lightly?.and the reason for this would
be????
>tim
>
Man who teach me to fly he say, "Never ever blow in pitot, altimeters are
not designed to deal with being blown"
If you must test it seal the tube with your finger and rub the tube. The
heat produced will warm the air enough to increase the pressure and cause
a small indication or better yet put a manometer on it, that will tell you
that the instrument is work AND if you have a leak. Of course if you have a
pot pitot gnore what I have written.

Frank Paynter[_2_]

March 4th 11, 10:53 PM

On Mar 3, 10:26*am, Frank Paynter > wrote:
> I installed a TT21 transponder over the winter, and as per the
> installation instructions, tapped into my ship's static line for
> altitude reporting. *Yesterday I checked the transponder with Orlando
> ATC and they reported a good signal and proper altitude reporting. *So
> far, so good.
>
> Then I noticed my airspeed indicator seemed to be reading
> significantly higher than normal, and I confirmed this with a stall
> series. *My Ventus 2bx, which normally stalls in the 40-45kt range
> depending on bank angle, now is apparently stalling in the 50-55kt
> range. *The ship actually feels pretty much the same as before,
> although I haven't flown for 3 months so that's a bit hard to tell for
> sure.
>
> My dry flying weight is probably about 5-10lbs higher this year than
> last, as I added the TT21 itself, plus a 5-lb battery to support it.
> However, I'm pretty sure that by itself doesn't explain the
> significantly higher apparent stall speed.
>
> As a troubleshooting step, I removed and plugged the static line from
> the airspeed indicator, so now the ASI takes it's static input from
> cabin air, and everything else should be unchanged. *I haven't flown
> it yet to try it out, but I will today. *I also have a spare airspeed
> indicator that I can swap in if necessary.
>
> Anything else I should be considering?
>
> TIA,
>
> Frank

Too bad I didn't get any takers on yesterday's prediction, because
sometimes the cause of a faulty airspeed reading is just the airspeed
indicator. Replaced the indicator for today's flight, and lo and
behold, the airspeed dropped back to what it should be, matching the
CAI-302 indication almost exactly.

Interestingly enough, this little exercise taught me to trust what the
glider was telling me rather than what the ASI was saying. Before I
figured out the ASI problem, I was having trouble getting the glider
slowed down enough at -1 and -2 flaps - it seemed to want to run hands-
off at the fully ballasted speeds even though I was flying dry. Of
course today with the faulty ASI changed out, the glider wants to fly
hands-off pretty much in the correct (dry) speed range for all cruise
flap settings (-1 through S1)

So now all I have to do is figure out what to do with the 'extra'
posts associated with this topic. Do I still need to replace the
Dittel capacitor? Can I recover from the effects of the PW-5
sacrifice? Will the feds swoop down on me for replacing the ASI in my
experimental glider? Stay tuned for next week's episode ;-).

Regards,

Frank

Darryl Ramm

March 4th 11, 11:38 PM

On Mar 4, 1:44*pm, "Tim Mara" > wrote:
> it's not just a simple pressure if it's in the tail and there's a fan
> blowing on it, you don't find aircraft with the Pitot mounted behind an
> engine either.there is a reason they put alternative Pitot and statics on
> motor gliders today...most newer gliders do have the multi probes
> (pitot-static-TE) in the fin.they all work typically "best" there but they
> intend these to be switched to alternative sources when the motor is being
> used.....
> next time you ask someone to test your pitot are you going to ask them to
> blow really hard ?? or just lightly?.and the reason for this would be????
> tim

Which manufacturer intends the ships's ASI to ever be connected to (or
switchable) to a tail mounted pitot and/or static? The ASI should only
be connected to the nose pitot and fuselage main static. A vario/
flight computer (like my C302) might be switchable between both (as it
is in my motorglider).

Even if it was connected that way, which still seems awfully wrong for
an ASI, I do not expect the "fan" wind speed plus actual airspeed
induced pressures to damage an ASI.

Darryl

Tim Mara

March 5th 11, 12:26 AM

several new gliders now have auxiliary nose and tail inputs

"Darryl Ramm" > wrote in message
...
On Mar 4, 1:44 pm, "Tim Mara" > wrote:
> it's not just a simple pressure if it's in the tail and there's a fan
> blowing on it, you don't find aircraft with the Pitot mounted behind an
> engine either.there is a reason they put alternative Pitot and statics on
> motor gliders today...most newer gliders do have the multi probes
> (pitot-static-TE) in the fin.they all work typically "best" there but they
> intend these to be switched to alternative sources when the motor is being
> used.....
> next time you ask someone to test your pitot are you going to ask them to
> blow really hard ?? or just lightly?.and the reason for this would be????
> tim

Which manufacturer intends the ships's ASI to ever be connected to (or
switchable) to a tail mounted pitot and/or static? The ASI should only
be connected to the nose pitot and fuselage main static. A vario/
flight computer (like my C302) might be switchable between both (as it
is in my motorglider).

Even if it was connected that way, which still seems awfully wrong for
an ASI, I do not expect the "fan" wind speed plus actual airspeed
induced pressures to damage an ASI.

Darryl

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BruceGreeff

March 5th 11, 11:01 AM

AHEM - my T59D has a pitot on the tail fin - sharing space with the TE
probe...

That was way back in 1971 and is actually not that uncommon an arrangement.
Apparently they thought that it would spoil the laminar flow having a
pot pitot in the nose.

Bruce

On 2011/03/05 1:38 AM, Darryl Ramm wrote:
> On Mar 4, 1:44 pm, "Tim > wrote:
>> it's not just a simple pressure if it's in the tail and there's a fan
>> blowing on it, you don't find aircraft with the Pitot mounted behind an
>> engine either.there is a reason they put alternative Pitot and statics on
>> motor gliders today...most newer gliders do have the multi probes
>> (pitot-static-TE) in the fin.they all work typically "best" there but they
>> intend these to be switched to alternative sources when the motor is being
>> used.....
>> next time you ask someone to test your pitot are you going to ask them to
>> blow really hard ?? or just lightly?.and the reason for this would be????
>> tim
>
>
> Which manufacturer intends the ships's ASI to ever be connected to (or
> switchable) to a tail mounted pitot and/or static? The ASI should only
> be connected to the nose pitot and fuselage main static. A vario/
> flight computer (like my C302) might be switchable between both (as it
> is in my motorglider).
>
> Even if it was connected that way, which still seems awfully wrong for
> an ASI, I do not expect the "fan" wind speed plus actual airspeed
> induced pressures to damage an ASI.
>
>
> Darryl

--
Bruce Greeff
T59D #1771 & Std Cirrus #57

Kevin Neave[_2_]

March 5th 11, 12:33 PM

At 23:38 04 March 2011, Darryl Ramm wrote:
>Which manufacturer intends the ships's ASI to ever be connected to (or
>switchable) to a tail mounted pitot and/or static? The ASI should only
>be connected to the nose pitot and fuselage main static. A vario/
>flight computer (like my C302) might be switchable between both (as it
>is in my motorglider).
>
>Darryl
>

Schempp Hirth.
My Discus only has fin pitot.

Presumably this is so that the ASI still works when the nose hook is used
for aerotow.
(Not sure why knowing the speed on tow is important?)

Kevin

Darryl Ramm

March 6th 11, 03:22 AM

On Mar 5, 3:01*am, BruceGreeff > wrote:
> AHEM - my T59D has a pitot on the tail fin - sharing space with the TE
> probe...
>
> That was way back in 1971 and is actually not that uncommon an arrangement.

Darryl Ramm

March 6th 11, 03:42 AM

On Mar 5, 4:33*am, Kevin Neave > wrote:
> At 23:38 04 March 2011, Darryl Ramm wrote:
>
> >Which manufacturer intends the ships's ASI to ever be connected to (or
> >switchable) to a tail mounted pitot and/or static? The ASI should only
> >be connected to the nose pitot and fuselage main static. A vario/
> >flight computer (like my C302) might be switchable between both (as it
> >is in my motorglider).
>
> >Darryl
>
> Schempp Hirth.
> My Discus only has fin pitot.
>
> Presumably this is so that the ASI still works when the nose hook is used
> for aerotow.
> (Not sure why knowing the speed on tow is important?)
>
> Kevin

I was talking about motorgliders. AFAIK Schempp-Hirth motorgliders
have nose mounted pitots for the ASI and do not switch the ASI pitot
source to a tail pitot.

I've seen the ASI on some gliders like my old club's DG-1000S bouncing
around a fair bit as we aerotowed at times with a wiffle ball on the
tow rope (to protect the rope from abrasion damage). That tow operator
liked to use a round wiffle ball followed by a half cut wiffle ball
that could get a pretty good cup over the nose of the glider and the
actual impact on the ASI would depend on how exactly that half cut
ball sat on the nose. But since you are following a towplane this is
less of a potential problem than would be using an ASI accidentally
switched to a tail mounted pitot when self launching a motorglider
(not that motorglider manufactures actally would intend anybody to do
that, sigh.).

Darryl

Andy[_1_]

March 6th 11, 03:45 PM

On Mar 4, 3:47*pm, Don Johnstone > wrote:

>Man who teach me to fly he say, "Never ever blow in pitot, altimeters are
>not designed to deal with being blown"

Are you sure that's what he said? How would blowing into a pitot have
any impact on the altimeter?

Applying pressure to pitot tubes is a standard method of testing
airspeed systems. You can buy quite expensive test equipment to do
just that. The same test equipment will also vary the static port
pressures although that's typically a suction.

For comparison testing of airspeed indicators, such as a mechanical
ASI and the Cambridge 302, one can either gently blow on the pitot and
gently suck on the static while observing the instrument readings. If
the mechanical ASI does not go off scale no harm can be done.

Andy