X-Mozilla-Keys:                                                                                 
Jeff Spitzer wrote:

It was recently suggested that having long tubes attached directly to the
engine could lead to harmful resonance in the tubes.  While this is
possible it's important to remember what constitutes "harmful resonance".

When a system is resonant, the input amplitude is multiplied in the
resonant member.  The level of amplification is a function of the ratio of
input frequency to natural frequency and the amount of damping.  All
systems/parts have some level of damping, even a steel bar.  The steel bar
has internal damping and is damped by the air around it.  As a good rule of
thumb, the worst amplification you are going to see is about 10:1.  So, if
you have something that is resonant at frequencies close to the principle
firing frequency (big inputs) a 10:1 amplification could be harmful.  OTOH,
if you have a tube in a brace that is resonant at 200 Hz, a 10:1
amplification of the forcing functions at 200 hz are probably not harmful.

The vibrations coming from your engine are made up of many different
frequencies.  The only ones that will usually lead to harmful resonance are
the principle firing frequency and low integer multipliers of it.  All
airplanes have resonant components at high frequencies buzzing along all
day long and very few lead to any kind of failure.  If the stress produced
is less than the fatigue limit for the material and its not rubbing on
anything then no worries.

Jeff Spitzer

Very interesting post Jeff. Thanks.

Paul Lamar
 
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