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#1
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![]() 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 |
#2
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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.......! |
#3
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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. |
#4
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("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 |
#5
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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. |
#6
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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 |
#7
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Possible reasons for out of balance are
propeller static balance bad or engine mounts bad. |
#8
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![]() 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 |
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