Sailplane Construction Project Session #143
When my friend Jerry Burk saw the picture below
of the knurled brass knob soldered to the unsupported stem of the needle
valve, he e-mailed me back the following comment:
The needle valve looks very good, but the
large weight at the
end of the NV might cause a harmonic vibration
at some RPM.
Once again, this feed back pointed out a problem
I hadn't even considered. With the McCoy's high RPM, there is high
probability of this occurring. The approach I used to resolve this issue was
to support the needle valve stem out at the cowl surface. To do this, I
selected a medium size piece of black neoprene fuel tubing. Since the
neoprene tubing was a little large to go through the aluminum tube in the
cowl, I cut one end off on the bias, put a dab of after run oil on the
tubing, and forcefully pulled it through the aluminum tube in cowl with a
pair of hemostats as shown below.
I left about an 1/8" sticking out inside the cowl
as shown below.
This compressed the neoprene tubing down to the
point that the stem of the needle valve would not slide through the hole in
the center of the neoprene tube. So I used increasing drill bit sizes to
ream the hole out until I could push the needle valve stem through the hole
in the tubing as shown below. The stem fits snugly inside the neoprene
tubing, which should dampen out and prevent
harmonic vibration from
occurring in the needle valve.
This is a view of the needle valve
protruding inside the cowl................Tandy
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