O-320 3.5 crank speed vibration

Stuart How many blades on the prop? Variable pitch? What
Stuart balancer are you using? Is it nose wheel or tail dragger?

It's a 2 blade metal prop (Sensenich) and I am using the spectrum mode
of a Smart Avionics PB-1 balancer to look at the vibration spectrum.

As I am actually the manufacturer of the PB-1, I believe it to be
trustworthy!

The spectrum display very clearly shows big peaks at 0.5 and 3.5 crank
speed and not much elsewhere.

A previous post suggested re-indexing the prop and that sounds like it's
worth trying.

What I really would like to find out is if there are any known engine
problems that can give rise to that particular vibration frequency.

Mark

I am unaware of anything within a 4 cyl opposed engine that would
theoretically that would give anything at the 3.5 harmonic. I think
you might be looking at a non-linearity in the mount (is it a dynafocal
or a straight conical mouint?) or in the transducer(s) mounting.

I'd guess the 3.5 order output to be comparatively insignificant
compare to the 0.5 harmonic as far as human discomfort is concerned.

Do try the prop indexing thing. Let us know the results.

Good luck & THX

Thanks for the reply,

Sorry, off the top of my head, I don't know what kind of engine mount
it is. I shall ask the aircraft's owner.

Checking the transducer mounting is a good idea even if it only
removes another unknown from the equation.

What I should have done at the time was vary the RPM more and see how
the 3.5 order vibration magnitude changed. That surely would have
indicated whether it was a resonance at a particular RPM.

What's nagging at me is the magnitude of the 3.5 order vibration. 0.5
IPS equates to quite a lot of acceleration at that frequency and
acceleration means force!

Mark

Checking the transducer mounting is a good idea even if it only
removes another unknown from the equation -

Maybe your xdcr mount is resonant at the 3.5 harmonic? Change the RPM
& see what the measurments show.

What's nagging at me is the magnitude of the 3.5 order vibration. 0.5
IPS equates to quite a lot of acceleration at that frequency

Its still only 1/4 g approx. Not that much for a piece of cheap
machinery.......!

Also - check the prop blade resonant frequency in the flapping mode
without the engine running, hitting it at a tip with the tip of your
index finger.

You might be able to hear it with your ear.

("nrp" wrote)
Also - check the prop blade resonant frequency in the flapping mode
without the engine running, hitting it at a tip with the tip of your index
finger.

You might be able to hear it with your ear.


Oh good. You formed my question for me.

I was wondering if there were any old tricks/shortcuts used back in WWI or
WWII? Or even in the old barnstorming days - out next to someones ...barn?

Not perfect fixes, but 'half-way to the wall' solutions.


Montblack

Its kinda interesting to critically watch the rotational plane of a
brightly lit (daylight) propeller against a dark background, as someone
else varies the engine RPMs. I've seen the blade tips move like 1/2
inch out of plane at certain bad rpms.

nrp wrote:

Its kinda interesting to critically watch the rotational plane of a
brightly lit (daylight) propeller against a dark background, as someone
else varies the engine RPMs. I've seen the blade tips move like 1/2
inch out of plane at certain bad rpms.


Or a variable speed party strobe...

Sheds an interesting light on things like that.



Richard

Possible reasons for out of balance are
propeller static balance bad or
engine mounts bad.

Bingo!

I have just determined that the strip of metal I used to mount the
sensor to the front of the engine has a resonant frequency (with the
sensor attached (of course)) around 95 HZ which is very close to the
frequency of the spike I was seeing.

I think that solves the mystery of the 3.5 crank vibration.

Many thanks to those who replied, I learnt something today!

Mark