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From: "Ed Anderson" <eanderson@carolina.rr.com>
To: "Rotary motors in aircraft" <flyrotary@lancaironline.net>
References: <list-571082@logan.com>
Subject: Spectrunm Analysis was : [FlyRotary] Re: Dynamic balance
Date: Fri, 17 Dec 2004 11:46:17 -0500
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> Perry Mick wrote:
> > I did that too way back when. I was told I need to run it's output
> > through a spectrum analyzer. Audio frequencies so an audio spectrum
> > analyzer would work. You could input to the audio card in a PC and use a
> > PC audio spectrum analyzer.
>
> Ed,
>
> You were calculating the position of your discs by estimating the
> temperature in the exhaust and such.  Have you thought about using
> Perry's method?  Record the sound and analyze that to get the exhaust
> frequencies that are actually present.  It seems that it would also have
> the advantage of telling precisely which ones were stongest and in need
> of attenuation.
>
>
>  Ernest,

Certainly worthwhile to consider if you have the equipment, but here is why
I don't really think it matters too much.

    Its fairly easy to determine analytically what the fundamental exhaust
frequencies will be - the primary frequency (fundamental)   is dependent on
your RPM.  For 6000 rpm its 200 Hz (if both exhaust or combined) or 100HZ if
you treat them separately.  The first few harmonics are also pretty straight
forward.  What is not very amenable to just analysis is the strength (power
level) of each frequency although we know the primary will be the strongest
and they fade off from there.

 Not to say there is no value in spectrum analysis - but, in this case where
your rpm (and therefore your frequency) is varying - not certain exactly
what value going to this trouble would be. While the exhaust temp does
affect the speed at which the wave travels down the tube - it has no affect
on the repetition rate (rpm) of the exhaust pulse, the interval between
pulses would be the same.  Therefore your primary frequency is determined by
rpm.  Now true - there will be all sorts of harmonic frequencies other than
the ones I've mentioned,  but the energy in them compared to say the
fundamental and second, third harmonic is pretty small.  If I can deaden the
fundamental and second harmonic of the fundamental I would be happy.

Now, if you are trying to time the reflected pulse to return in time to help
scavenge the combustion chamber then knowing the speed of the pulse (and
therefore the time it takes to travel distance X ) is very important.  But,
unless I am missing something, I don't see what effect it has the exhaust
repetition rate (frequency).

Ed Anderson