Sender: <marv@lancaironline.net> (Marvin Kaye)
To: lml
Date: Wed, 01 May 2002 11:35:07 -0400
Message-ID: <redirect-1204996@logan.com>
From: "Gary Casey" <glcasey@adelphia.net>
Subject: prop balancing
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<<I am looking for the definition of "ips" (in/sec) as used to describe
the state of a propeller balance/imbalance.  This definition may also
apply to rotating machinery in general.  I used plots of acceleration of
my engine/prop to achieve a better dynamic balance.  The plots were
produced using an accelerometer and scope .  I would like to convert my
results to industry standard units for the sake of comparison with
typically obtained values using commercial balancers.>>

I'm not too much of an expert, but the only measurement of imbalance I know
of is weight times distance and for us Americans inch-ounces are the usual
units.  One inch-ounce is the equivalent of one ounce of weight placed one
inch from the axis of rotation.  since ounces is legitimately a measure of
force, not mass, one can use the true mass measurement, but then
second-squared would be in the result.  I believe that when they use "ips"
they are really talking about the maximum velocity of the resulting movement
of the engine.  An accelerometer is placed on the engine crankcase and if
the signal is integrated to get velocity there is a measurement that can be
used to compare imbalance.  It isn't truly a balance measurement as it
depends on the mass of the engine as well as the mass of the unbalance.  It
does give a good indication of the perceived imbalance to which the airframe
is subjected.  Also, the zero-crossing of the signal can be used to trigger
a strobe, giving a good indication of the position of the imbalance relative
to the location of the accelerometer, which is usually, I think, placed on
the vertical centerline of the engine.

Gary Casey
ES project