Vibration Monitor (Hyde, Wanttaja?)

Well Jim if you are inventing a vibration monitor, then you need a vibrating
platform to test it on. I have a Safari helicopter that qualifies. I also
have an electronic balancer and accelerometer transducers to provide some
cross checking at engine frequencies of 45hz, rotor frequencies of 8 and
16hz. and possibly with a little fooling, harmonics of all of the above.
Stu Fields

"RST Engineering" wrote in message
...
Necessity, as they say, is a mother.

I am in the process of reinventing a square wheel called a vibration
monitor. The electronics is relatively trivial IF the input and output
parameters are known.

What we know is that the engine is going to have a fundamental frequency
at
cruise RPM. Let's take the math-simple engine RPM of 2400. This gives us
a
fundamental frequency of 2400/60 or 40 Hz.

But wait, he said. There are going to be other (sub) harmonics of that
frequency that will be of some interest. And, those harmonics will change
as a function of the engine being a two or four stroke, four or six
cylinder.

So, oh wise and noble gurus of engine stuffings, what (sub) harmonics are
going to be of most interest to us and what is their mathematical
relationship to the fundamental?

As an extra bonus question, my sensor is going to be an old phonograph
cartridge. Should I use the lightest weight "needle" that I can find?
How
about a tiny little ball of lead at the tip of that needle? Would that
help
the sensor? Or hinder it?

Lastly, once I get this sucker up and running with you all's good ideas,
is
anybody game to bolt it onto their flying machine and report results? I
can
do it for the 182, but I'd really like some other real-world reports.

Jim