ASW-20C Fuselage mounted TE probe question

I am having trouble with static/TE input from my fuselage mounted TE probe. I think the tubing is original issue (1983 ASW-20C, experimental) and not sure if the probe is getting a good tight seal when installed. Instrument indications are minimal on mechanical and electronic varios compared to info from my seat and altimeter winding up/down. (I have gotten pretty good estimating thermals based on rate of altimeter winding up-bonus?) Instruments have been tested and are in good shape.

My first idea would be to replace all tubing to the probe. It is a long way back from any access to get hands on the inside of the fuselage. How would one approach getting new tubing pressed onto the barb on the inside of the fuselage?

My second idea would be to replace the pitot probe on the vertical stab with a modern triple sense probe this winter. It is relatively easy to string three new tubes through the fuselage using the existing tubes to pull through. It would require some composite work to core out existing pitot tube receiver and replace with new triple sensor.

Has anybody tackled this problem? If so, can you share your insights. Reply on forum or at generein located at good old AOL(e mail account from the last century, actually compuserve . . .)

Thanks
GR

I'd check the system first. Put tape on the hole and draw some vacuum where the tubing enters the instrument area. An ASI hooked in line is a good pressure gauage. Pull about 40 knots and see if it holds or bleeds off.

Agreed. Replacing the fin probe with a triple probe is probably the last resort, and replacing tubing not far behind it.

Does the probe itself work? The small holes can clog. Some models have joints; is there a leak there?

Is the probe or socket clogged? Pretty common with spiders, water, dirt, etc. I might go here first. No or minimal instrument indications would suggest a closed system. Having the TE line open to static pressure from a big leak near the probe would make the varios behave like they're connected to static and uncompensated, I think.

Then go looking for the leak as Karl describes. If not in one of the instruments, start closing or clamping off sections of tubing and then retesting.

You can convert the tail pitot to a TE socket, tape it each time you insert the probe, and move the pitot to the nose. What source does your handbook spec for the ASI? Tail pitot was added originally, in part, to reduce errors from the signals from different sources arriving at sensitive instruments at different times (e.g., nose pitot, aft statics) but if that's a problem, you can adjust the timing of the pitot signal with restrictors. I know the instrument experts will cringe when they see this! Moving the TE probe to the fin is desirable; the fuselage-mounted ones are said to not work as well and be more prone to errors because they're not in relatively undisturbed flow.

You mentioned static/TE input. Is this a model with two signals: static and TE? If so, there's likely an o-ring in there that can go bad. I've never replaced one but some guys have done so on the fin mounts: frustrating but possible without cutting anything.

If there truly is a leak in the aft tubing, make sure it's not somewhere within reach. If it is, just replace the offending section. If there's a leak where the tubing fits over the barbed end of the socket (process of elimination), you might be able to get to that to slip on a few wraps of safety wire or cable tie or whatever to tighten it up.

This was just stream of consciousness stuff and some of it may not be on the mark. My point is that there are lots of things to try and test before cutting holes...unless you just want to install a triple probe. They're standard equipment now but that hasn't always been the case and you can do just fine without one.

Chip Bearden
ASW "JB"

On Friday, August 19, 2016 at 2:08:36 PM UTC-4, wrote:
Agreed. Replacing the fin probe with a triple probe is probably the last resort, and replacing tubing not far behind it.

Does the probe itself work? The small holes can clog. Some models have joints; is there a leak there?

Is the probe or socket clogged? Pretty common with spiders, water, dirt, etc. I might go here first. No or minimal instrument indications would suggest a closed system. Having the TE line open to static pressure from a big leak near the probe would make the varios behave like they're connected to static and uncompensated, I think.

Then go looking for the leak as Karl describes. If not in one of the instruments, start closing or clamping off sections of tubing and then retesting.

You can convert the tail pitot to a TE socket, tape it each time you insert the probe, and move the pitot to the nose. What source does your handbook spec for the ASI? Tail pitot was added originally, in part, to reduce errors from the signals from different sources arriving at sensitive instruments at different times (e.g., nose pitot, aft statics) but if that's a problem, you can adjust the timing of the pitot signal with restrictors. I know the instrument experts will cringe when they see this! Moving the TE probe to the fin is desirable; the fuselage-mounted ones are said to not work as well and be more prone to errors because they're not in relatively undisturbed flow.

You mentioned static/TE input. Is this a model with two signals: static and TE? If so, there's likely an o-ring in there that can go bad. I've never replaced one but some guys have done so on the fin mounts: frustrating but possible without cutting anything.

If there truly is a leak in the aft tubing, make sure it's not somewhere within reach. If it is, just replace the offending section. If there's a leak where the tubing fits over the barbed end of the socket (process of elimination), you might be able to get to that to slip on a few wraps of safety wire or cable tie or whatever to tighten it up.

This was just stream of consciousness stuff and some of it may not be on the mark. My point is that there are lots of things to try and test before cutting holes...unless you just want to install a triple probe. They're standard equipment now but that hasn't always been the case and you can do just fine without one.

Chip Bearden
ASW "JB"

The tube in the vertical stabilizer has o rings that harden or get dirty. I sprayed my probe with ACF50 (WILL NOT DAMAGE THE O RINGS) before inserting several times. Problem solved. Vaseline may help however it could block the small holes inside

J Smiley
AH

Thanks guys. Gives me a lot to work with. I'll post what I find out.
GR

perjantai 19. elokuuta 2016 2.43.08 UTC+3 GeneReinecke kirjoitti:
I am having trouble with static/TE input from my fuselage mounted TE probe. I think the tubing is original issue (1983 ASW-20C, experimental) and not sure if the probe is getting a good tight seal when installed. Instrument indications are minimal on mechanical and electronic varios compared to info from my seat and altimeter winding up/down. (I have gotten pretty good estimating thermals based on rate of altimeter winding up-bonus?) Instruments have been tested and are in good shape.

My first idea would be to replace all tubing to the probe. It is a long way back from any access to get hands on the inside of the fuselage. How would one approach getting new tubing pressed onto the barb on the inside of the fuselage?

My second idea would be to replace the pitot probe on the vertical stab with a modern triple sense probe this winter. It is relatively easy to string three new tubes through the fuselage using the existing tubes to pull through. It would require some composite work to core out existing pitot tube receiver and replace with new triple sensor.

Has anybody tackled this problem? If so, can you share your insights. Reply on forum or at generein located at good old AOL(e mail account from the last century, actually compuserve . . .)

Thanks
GR

Fuselage location is far from ideal, so even new fuselage probe with airtight sealing will not get you that far.

I changed my 20's vertical stabilizer adapter to modern ESA ST-type (2hr job) and put modern TE-probe in it (type DN/ST). Made some test flights comparing new Schleicher Prandtl fuselage probe and stab-mounted DN/ST. DN/ST probe is much better in every respect.

You can take static and dynamic pressure from normal fuselage locations, there is no need to install triple probe IMO. It would be major pain in the ass to route three pneumatic tubes to rear fuselage *and secure everything properly*.

there is no need to install triple probe IMO.
It would be major pain in the ass to route
three pneumatic tubes to rear fuselage
*and secure everything
properly*.

Just remember that the elevator pushrods
and bellcranks run through the fin and
tailcone.