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1. 03:38 PM - Dynamic prop balancing (Jim Fillman)
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| Subject: | Dynamic prop balancing |
I have always wanted to dynamically balance my props but never even
static
balanced them. I mentioned on this list that I=92d like to see prop
balancing
discussed at our next Lawrence fly-in and a gracious and trusting reader
offered to loan me his DynaVibe balancing system. The system requires
mounting an accelerometer in vertical orientation near the front of the
engine and an optical sensor pointing at the back of the prop or
backplate,
registering each pass of a reflective tape.
During my fire rebuild in 2009, I changed from an Ivo 3-blade inflight
adjustable prop to an Airmaster 3-blade constant speed prop. The
Airmaster
came static balanced, evidenced by an AN3-4A bolt, area washer &
AN365-1032A
nut (9.5 grams <42.75 gram inches>) in the 240=B0 position. The
Airmaster
backplate came with 3/16=94 holes drilled every 30=B0 at 4.5=94 from
center. I
referenced the hole directly behind blade #1 as 0=B0 and measured
increasing
degrees in the same direction as prop rotation.
The DynaVibe instructions suggested not changing any static balance
weights.
I ran the engine in the initial (static balanced) configuration at
around
1840 rpm prop speed (~4,500 rpm engine speed) and registered the
magnitude
of vibration as 0.26 IPS (inches per second) at 196=B0. According to
what I=92ve
read, anything above 0.25 IPS is considered rough, down to 0.15 IPS is
fair,
0.07 is good and 0.04 is excellent. The identified degrees is the heavy
point. Since I didn=92t want to remove weight (grind away metal) at the
heavy
point, I added weight 180=B0 opposite the heavy point. Only having holes
every
30=B0, I chose to add 10 grams <45 gram inches> (AN3-4A bolt, area
washer, #10
thin washer and nut) at 30=B0.
Running in this configuration produced 0.06 IPS at 82=B0. I added 1 gram
(#10
thick washer) to the original static balance location (240=B0), bringing
the
total mass balance at that location to 47.25 gram inches and ran the
engine
again, registering 0.02 IPS at 54=B0. It was a windy day so I=92ll
verify the
final reading in calmer conditions but I think that=92s where it will
stay.
I believe the DynaVibe balancing unit will be at Lawrence so maybe we
can
demonstrate the process on someone=92s plane and remove a few vibes in
the
process.
Jim
N623JF
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