O-320 3.5 crank speed vibration

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