John,
I don't think that leaks of these sorts of magnitudes can result in
significant, or even measurable, ASI errors. The pitot head is being
supplied by incident air with a source of pressure (and flow if required)
which is effectively constant and which is easily maintained in the presence
of such small leaks. Only when the leak results in a flow upstream of the
gauge of sufficient size that there is a pressure drop through the plumbing
as a result, can the leak result in ASI errors. Remember the ASI is only
measuring pressure, not flow.
More likely positioning error.
Regards,
Dave Simpson
-----Original Message-----
From: JohnJMoran@aol.com <JohnJMoran@aol.com>
Date: 26 November 2000 16:15
Subject: Pitot Testing, etc ??
>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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