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.