----- Original Message -----
Sent: Tuesday, June 21, 2005 9:59
AM
Subject: [FlyRotary] Re: Prop
balancing
What G-force and frequency range
are we talking about?
See attached chart of acceleration limits.
Do you find an RPM of high
amplitude and work there.
The manual suggests a low cruise setting for the
rpm. I'm not sure it matters much, but you need to reproduce the same
rpm from one run to the next for best results.
What sort of weights do you put
on the prop?
None. You put the permanent weights on the spinner back
plate, but the temporary weights generally go under the spinner attachment
screws on the outside of the spinner. On Lycomings, you can put
them on the flywheel, since there are already holes every 30
degrees.
I assume placement is amount is
trial and error.
Yep, no
equipment required :-) Kidding.
Does this approach work on a
three-blade?
Absolutely.
Any info on how this is done
appreciated
Using the
professional equipment, it's pretty easy. Basically, you set
up one vibration sensor, vertical, as close to the prop hub as you
can get (I've got a bracket that clamps to the nose of the redrive). You
also set up an optical tach, with a piece of reflective tape on one
blade. Pick an rpm, and the analyzer tells you what the peak
acceleration reading is, and how many degrees from the tape mark it
occurred at. It then suggests a temporary weight that should be
attached, and tells you what position to install
it.
If you think
about this, you will realize that the distance the sensor is from the
prop, and the distance the weight is from the center of the prop hub
both make a difference on how much weight is required. The
program doesn't know any of these variables up front, and doesn't
care. It initially picks a weight that it knows will make a
measurable change, then you install it, and re-run the test. From
the change that was made, using a known weight, the program can
now calculate what the real weight should be to make the change
required. Each time, the old weights are removed, and the new
weight is installed where the program
suggests
The program
will continue to work well, even if you don't do exactly what it says, as long
as you are honest, and tell the program what you really did. For
example, if it says put 15 grams at 355 degrees, and you have a spinner hole
at 360, then put 15 grams at 360, and enter that for the actual weight in the
program. If you have a hole at 30 and 60 degrees, and it
wants weight at 39 degrees, you can tell it to split the weight, and
give it the two locations you have available. It will then tell you how
much weight to put in each location to make the equivalent weight.
Pretty handy.
When you get to
.06 ips or less, you have to convert the temporary weights to permanent
ones. For that, you measure the radius the temporary
weights are located, and the radius you want to install the permanent weights,
and the program will tell you how much to adjust the weights for the new
location.
Or maybe Rusty has purchased the
professional equipment by now and wants to rent it out J.
We have
vibration analyzers similar to this for measuring site environmental
qualifications for MRI scanners, and it takes just a few trips on the
FedEx plane to convert a new piece of equipment to junk. In other
words, there will be
no renting, or shipping of equipment, but I can certainly offer a reasonably
priced balance job if you bring your airplane to 2R4. Send all your
experimental Lycoming friends too, since those are
simple :-)
Cheers,
Rusty (fighting
Al's font for control)
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