Recently I noted some disagreement in airspeed between the ASI and the
uEncoder in the 60kt range. Investigating this led to revisiting the tests
conducted initially on the pitot system.
My technique for testing the pitot is to install a 3 foot length of 1/4 ID
fuel tubing onto the pitot and then insert a length of 1/4 OD NyloSeal (outer
end blocked) into the fuel tubing to increase the pitot pressure in a
controlled way -- the two pieces of tubing work somewhat like a trombone,
where a wipe of grease improves the operation.
The panel plumbing was done with 1/4 OD urethane tubing and NyloSeal fittings
while most of the long runs through the cockpit, etc. used NyloSeal tubing.
Testing of the pitot system using the trombone showed that there was a leak.
Tests prior to first flight had shown leakage such that the ASI reading
dropped 1 kt per 4 minutes. New tests showed a rate of 1 kt per 2 seconds, a
substantial increase in leak rate. Using the trombone at the fitting where
the panel connects to the airframe showed the leak to be within the panel.
Ultimately, it was found that one of the Nylo-Seal fittings within the panel
needed 1/2 turn on one of the nuts where urethane line connected. Further
consideration led to installation of brass tubing inserts of the type
normally used with Weatherhead fittings - the thought being that this would
help prevent creep in the urethane where it is compressed by the fitting.
Are the materials I used appropriate for pitot plumbing? Have others used the
materials I chose and seen leaks develop or is this a case of insufficient
tightening initially? Are the brass inserts appropriate for use in soft
tubing with Nylo-Seal fittings?
John N44EU
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