airspeed indicator problem after TT2 transponder install

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

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?

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

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?

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.

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.

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.

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.

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

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.