Pitot tube issues

[bogmonster]

On my 1994 built KF4 Speedster, my airspeed has been slowly been getting slower and slower. I took a small syringe and applied a little bit of pressure and low and behold, it didn't hold. I disconnected the pitot line from the airspeed indicator, blocked off the pitot tube and applied a couple lbs of pressure and there is a noticeable leak right where the pitot connects to the tubing. Now the question. The builder didn't put any access panels in near the pitot tube. BTW: I did test the other direction and the airspeed indicator doesn't have any leaks.

1) add an access panel (or two) to fix the leak. I'll probably never be able to match the automotive paint the builder used.
2) move the pitot further out on the wing that I can reach with the wing tip removed.
3) just live with it for now since it works good enough through most of the speed range (up until about 70 mph)

I'm tempted to do 2 just so I don't have to mess with fixing/painting the fabric, though I'm debating 3 and just punt until winter and maybe replacing the 94 vintage gas tank in the right wing and adding a wing tank to the left tank anyways.

Thoughts, comments?

Mike

[jiott]

Personally, I would do #1. Its quite easy to add a round access panel. Trace around the plastic ring and roughen up the paint (or remove it with MEK if its Polytone), glue the ring on with PolyTak, cover it with a light fabric doily extending out about 1". Do the usual coatings with PolyBrush and PolySpray. Match the finish paint as best you can; the paint stores can do an amazingly accurate job if you bring them a small sample to match. Since it will be on the underside of the wing and a fairly small round area, a slight mismatch will not be noticeable. Then cut out the fabric layers from the inside of the plastic ring and install the metal cover plate.

I have never seen a pitot tube located out near the wingtip, so I would be leery about doing that, possible turbulence from tip vortices, etc.

[109JB]

I agree #1 is what I would do. If it is a standard auto color (try to ask the builder) then you can order 2K urethane in spray cans nowadays.

[avidflyer]

Here is another idea. Take a plastic inspection cover ring for a pattern, and make a couple of pieces of aluminum the same size as that plastic ring. Drill about 8 holes though the two alum. rings so you will be able to rivet them together. Put a slice in one of the rings so you can get it through a smaller hole and work it though as small a hole as you can where you want the inspection cover to be. Rivet the two rings together with the fabric sandwiched in-between them. You could then cut the fabric the rest of the way up to the rings about every 3/8". Fold that fabric up and over the top ring, and glue it to that top ring. Do that with all the bits of fabric. You could even glue a doily on the top, and cut it and bring it around and glue it to the bottom ring. If you're careful, the only painting you will have to do is the inspection cover. I actually did this about a month ago on the belly of a plane. I was able to get at the top though, and that helped a lot. But if I was trying to do what you are doing, I would give it a try. YMMV JImChuk

[bogmonster]

good idea, i might try that. Problem with be that I'm not sure I can do what I need to do one handed and i would hate to have to make two inspection holes.

Mike

[t j]

How did you determine that " there is a noticeable leak right where the pitot connects to the tubing." if there is no inspection hole there?

My kit was delivered in 1994. The tubing supplied was 1/4 inch ID yellow Tygon. After a few years it turned to sticky goo I replaced it with blue Bing fuel line. Also, my airspeed indicator stuck at 70 mph one time. It would go higher than 70 but not lower. A screw on the face of the indicator had backed out enough...evidently when the needle was higher than 70...and when on approach to landing it would not go below 70 mph. Just something to check.

[bogmonster]

Per my original post, I applied a few lbs of pressure with the pitot blocked off and could hear (inside wing with tip removed) the leak coming from the area around the aluminum pitot tube connection to the pitot line. It appears the line is nylon or polyethylene, not Tygon.

[t j]

Okay I understand. My kit came with an approximately 1/4 inch outside diameter black vinyl pitot line that is connected to the pitot tube with a short section (couple inches) piece of 1/4 inch tygon. When the tygon deteriorated after a few years I replaced that short connecting piece with the blue bing fuel line. Yours may be different.

[bogmonster]

It appears to be the same. I haven't had a chance to cut in yet.

Thanks,

Mike

[Bartman1959]

I would replace the tubing. Connect new tubing to the old at the Pitot tube, pull the old tubing out at the wing root, thus pulling the new tubing with it. Do that all the way to your ASI. Anyway, that's my .02 cents worth.

[bogmonster]

It turned out to be the pitot itself. The top aluminum tubing was never really sealed and must have leaked from day one. I was able to heat it up and extract it and then rebuild it. Works perfectly now.

Mike