From: Epijk@aol.com
Message-ID: <37.2403d991.29bbb717@aol.com>
Date: Sat, 9 Mar 2002 14:05:59 EST
Subject: One more vibration issue
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In a message dated 3/8/2002 5:46:49 AM Pacific Standard Time, 
StarAerospace@aol.com writes:

<<<...pointing out the difference between using the mass of 
a metal prop (actually rotational inertia) for "damping" (actually lowering 
the excitation frequency)...>>>


Eric:
I hate to beat a point to death, but in this case, the concept is important.

Increasing the mass moment of inertia of the crankshaft system 
(with a heavier prop, for example) does NOTHING to change the 
damping of a system nor does it alter the EXCITATION frequency. 
However, it DOES reduce the torsional natural frequency 
(or RESONANT frequency) of the crankshaft system. 

The natural frequency is the number of cycles-per-second 
at which the system will vibrate when excited by a single pulse.  

The excitation FREQUENCY is established (primarily) by the
excitation ORDER and the engine RPM. 

The number of cylinders establishes the primary excitation ORDER.
(example: 4-stroke, 6-cylinder, even-fire engine: 3 power pulses 
per crank rotation, hence 3rd order excitation.)
Secondary factors include piston/conrod geometry. 

The excitation FREQUENCY (in cycles-per-second, or HERTZ) is the
product of ORDER x RPM / 60 
(example: IO-540 at 2700 RPM: 3 x 2700 / 60 = 135 Hz excitation)

The problems get significant when a system having little or no 
DAMPING (like a crankshaft) is being excited at or 
near its resonant frequency (or by an excitation which is some integer-
multiple of the res. freq.) In that neighborhood, because of 
energy storage and feedback, the amplitude of the 
oscillations can grow rapidly until the strength of the 
material is exceeded (aka "breakage"). 

When the excitation exceeds 1.414 times the res.freq.,
the amplitude of the resulting oscillations tends to diminish, and 
depending on how LITTLE damping the system contains, 
the amplitude of the PULSES transmitted to devices 
driven by the engine can diminish to very low numbers.

Arbitrarily raising (lightweight prop) or lowering the Crankshaft
Resonant Frequency (CRF) might by good, or it might be bad. 
Can't tell without knowing the design CRF and the resulting 
margins between CRF and expected excitation IN THE
OPERATING RANGE. (Recall the yellow-band on the tach
for 4-cylinder Lyc's without the pendulous counterweight system?)

For a more detailed treatment of the subject, see:
http://www.epi-eng.com/VibrationBasics.htm

Jack Kane
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