Thanks Chris!
Indeed my Smart avionics balancer spectrums were identical to your attached
report. Nice to see I'm not alone! ....and I'm not going crazy ;-)
Question: do you know if your old engine had experienced "case fretting"?
I checked my carb balance throughout the RPM/throttle range, and they are
indeed fine in the 4400-5600rpm and throttle ranges in question. Diaphragm
tensions also felt identical. I did however replace the rubber carb
mounts, which would indeed change the resonant frequency of the carb
assemblies. I have tried moving the spring mounts further away on the tube,
adding tension...no difference. Your suggestion however will prompt me
to try adding some mass to the airbox - to see if the resonance moves down
the rpm range..... sweet. I was running out of things to try LOL.
As my resonance only occurs at cruise speeds and loads and not in the
climbs, I was suspecting possible aero causes (as the three blade prop
pulses are also in that 80ishHz range) and instrumented the tail etc. but
to no avail - nothing different than when I instrument the tunnel mount
points, so nothing is getting excited back there. So I'm back to the
engine, or something on it going into resonance. I have already eliminated
the exhaust from the possibilities by springing and massing it with no
change.
I have obviously also looked for any hard interferences between the engine
and the airframe - none.
The mystery continues..... but I hope messing with the airbox mass may
give me a clue.
Cheers and thx again!
PeteZ
On Tue, Apr 19, 2022 at 11:36 AM n7188u <chmgarb@gmail.com> wrote:
>
> Pete,
>
> I had the same issue with the vibration when I got over 5000 RPM. Did an
> analysis which I include as attachment. You can see how the vibration
> spikes as the engine RPM crosses 5000 RPM and it is indeed a peak
> corresponding to the engine RPM frequency.
>
> Clive Sutton had a theory that maybe the carburetors where resonating and
> causing issues.
>
> According to RPX (DynaVibe) they indicated it was a carb balance issue at
> those RPM. Bud once had a writeup on how he had issues with carbs not
> working in unison because of age and only going to new carbs solved the
> issue. Since the carbs were a bit old that is certainly a possibility.
>
> But I have to report that I bought a new engine and the vibration is gone.
>
> Can I conclude it was a problem with the engine? no I can't because before
> before I flew the airplane with the new engine I removed the cone K&N air
> filters I had and installed the cold air plenum (too many changes). So
> three possible reasons:
>
> 1.- Something off with the old overhauled engine (kind of broad)
> 2.- Old carburetors coming out of balance at high end
> 3.- Vibration on the supported carburetors. I was told my carb support
> spring were a little soft.
>
> For some time I thought the culprit could be something in the airframe
> resonating but with the vibration going away with the engine change that is
> not high on my list.
>
> I hope this helps.
> Chris
>
>
> Read this topic online here:
>
> http://forums.matronics.com/viewtopic.php?p=506582#506582
>
>
> Attachments:
>
>
> http://forums.matronics.com//files/211210_europa_n11eu_vibration_analysis_728.pdf
>
>
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