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The Dynavibe is a good unit in
concept. All Dynamic prop balancing needs is an
instrument to measure amplitude and phase angle, then a scheme to correct any
imbalance found.
But a FYI about the
Dynavibe, when I looked at them couple years ago, their vibration pickup had its
cable integrated in one piece with the MEMS sensor. If
misfortune caused a cable to burn after touching an exhaust pipe, one would have
to send the whole cable and sensor back for repair or replacement, which I was
told was "several hundred dollars".
Another thing I wonder about, Dynavibe provides a
calibration certificate but only after paying additional
cost.
You think they would check each unit after
assembly? Two channels is also additional
cost.
Generic balancing instructions found on the
internet, and is applicable for most piston aircraft and common
turbines.
Rule of thumb, use a 4 gram AN970-3 washer
for each tenth IPS of imbalnce. Move the test weight inboard
for permanent attachment, added another washer along with the bolt &
nut.
Hartzell has instructions for dynamic balance
too. Hartzell does not prohibit adjusting
the shim static weights.
regards
Kent Felkins
Tulsa
----- Original Message -----
Sent: Monday, August 15, 2011 6:40
AM
Subject: [LML] Prop Balancing:
Legacy
Prior to my trip back to Calgary I wanted to get rid of a nasty
vibration. My last A&P actually made the balance worse and now that
I see the results I think it was 180 degrees out of phase. At 2300 rpm I
had a nasty shake.
At Oshkosh I bought the simple Dynavibe unit from
Spruce on sale. Short story is I wasted a day misreading the
procedure. In essence, you measure "in the direction of prop direction"
and I was moving the prop in that direction to get the balance position.
However, once that was understood, I was able to get my prop dialed in
from 0.63 to 0.03 in fast order. It would not be hard to go further with
a miniature weight change but I wanted to fly.
These are photos of the setup. If you ever decide to do this, it is
easy on the Legacy. You can
mount everything on the inside of the cowl which allows the runs
to be made with cowls on. It also means leaving all the cables in place
when changing weights. I used temporary weights on the spinner exterior to
narrow things down form 0.63 to 0.16 which took 4 large washers and about 16g
net added weight on the spinner. At that point, it only took two small
washers (the type that are used to fasten the spinner) to bring it down to
0.03. The final changes are very very sensitive. By moving the last
weights from one spinner screw to another you can easily home in on the exact
spot for final weight.
That final weight was permanently installed on the spinner back and the
final weight was adjusted for the gram-inches between the final radius and the
temporary spinner radius. It worked out darn close. The Dynavibe
is not a feature rich machine but it does work well. I had no problems.
You must guess at the initial test weights but it only takes a few
quick runs to dial things in. It provides IPS at X degrees.
Simple. I made some brackets with spare parts sitting in the hangar.
I taped the cables to the airframe using 3M blue painter tape which held
during runup but pulled off clean. I coiled the wires on top of the cowl
so I could remove the cowl without disconnecting all the cables each time.
Big time saver. When my final weight was installed on the spinner
back plate I did a final run to verify the results and then removed all the
gear.
The resulting 2000 miles back to Calgary was a big improvement.
Whereas I could not really cruise at 2300 before, it was now quite
smooth. I would recommend the procedure to anyone who wants to check out
their balance or verify an existing balance weight or remove a vibration.
I would note that the weights installed by Hartzell on the hub
underneath the spinner are NOT changed or tampered with.
You can play the slideshow here
Paul
Legacy
Calgary
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