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  #11  
Old 11-01-2021, 07:55 PM
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1001001 1001001 is offline
 
Join Date: Dec 2014
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I probably shouldn't do this, but are you sure your airplane isn't a Van Halen fan?

When I first got the album "5150" and put it in the cassette player in my Dad's car in 1986, he got the weirdest look on his face. When the music started, he reached over and yanked the volume knob to zero. I complained, and he said "I have to listen. I think there's something wrong with one of the tires."

If you listen to the song from the beginning, you can figure out what happened.
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  #12  
Old 11-01-2021, 10:06 PM
RFSchaller RFSchaller is offline
 
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Location: Phoenix, AZ
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Iíll chime in with something from my undergrad physics days half a century ago. The sound you describe is typical of two oscillators at nearly the same frequency (beat frequency) which multi engine pilots experience if they donít synchronize the props. It can also arise by driving a system at near resonance when the vibrations build and shrink. I used to see that on the sub when surfaced and rough water would cause us to roll with building angles that would then subside and build again. Probably not helpful in solving your issue, but the geek in me loves it when theory meets practice!😜
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  #13  
Old 11-02-2021, 08:01 PM
Handclutch Handclutch is offline
 
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Location: Bendigo, Australia
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Checked the blade angles and they are within 0.1į, so don't think we can blame them.

I should have thought of the multi-engine props out of sync analogy (lots of time on them) because it describes the sound perfectly. Not quite as pronounced/loud as in a twin but precisely the same sound.

I haven't flown yet to try out some of the other suggestions but will do so in the next day or two.

It's reassuring that others have experienced a similar phenomenon. One thing I haven't thought about is changing the engine mount isolation rubbers. I used to change them fairly regularly with Lycomings and it always made a big difference. They seem to deteriorate with time. Has anybody had any experience in changing them out?

Jack
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  #14  
Old 11-03-2021, 03:30 AM
rgmwa rgmwa is online now
 
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Jack, some kits delivered before mid 2016 had soft engine isolators. Yours would be in that group so perhaps worth checking although it may have nothing to do with the noise. https://www.vansaircraft.com/service.../n-2017-05-19/
Rob
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  #15  
Old 11-03-2021, 10:10 AM
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DaleB DaleB is offline
 
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I'm familiar with the sound you're experiencing, I think. We've had it since Day 1, when I bought the plane with around 160 hours on it from the builder. It always seemed to me like something drifting in and out of resonance as @RFSchaller mentioned. My guess would be an interaction between engine, gearbox, and prop.

Earlier this year we had the prop blades overhauled by Sensenich, and it seemed a little less pronounced after that but still there. Then we had the carbs rebuilt by Lockwood, which reduced vibration a little and made a very slight improvement in the "thrumming". A couple months back we did a dynamic prop balance, reducing the vibration from .6 IPS to about .05, we think (wind came up at the end, so it was impossible to get a really stable reading). That reduced it even further, so it's just perceptible now.

I'm still convinced it's an engine/gearbox/prop resonance thing, and I'll bet it would go away completely if we had the gearbox overhauled. The first couple hundred hours on this plane saw ground idle RPMs well under the currently recommended 2000 or more, so I'm guessing the gearbox has some wear.
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  #16  
Old 11-04-2021, 05:27 PM
Handclutch Handclutch is offline
 
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Thanks, Rob. I had forgotten about that Notice but almost certain I checked at the time and that the isolators are correct, but will check again. Number 2 cylinder is VERY close to the cowling and there is no evidence of excessive movement.

I agree with Dale that itís likely the result of that particular combination of engine, gearbox and prop, and changing one of those elements should modify or eliminate the noise. If I didnít have the new prop coming the first move would be a rebalance but little point in doing that now.

Jack
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  #17  
Old 11-04-2021, 08:21 PM
subpar_bucker subpar_bucker is offline
 
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This seemed like such a coincidence that I couldn't pass it up. We had a friend visiting who is a physics teacher. I asked her if she knew who Dr Julius Sumner Miller was - she did not. So I searched on youtube and showed her one of his videos.

Later tonight youtube suggested I watch his 1973 presentation to the Air Force Academy:

https://www.youtube.com/watch?v=4wa8IKMYwK8

If you watch through the first 3 minutes you'll see a great demonstration of what I believe is being discussed in this thread.
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