Prop slightly cycling in flight?

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Rich
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Prop slightly cycling in flight?

Post by Rich » Sun Aug 25, 2019 5:10 pm

I was on a flight up to Victoria a couple of days ago and the air was extremely smooth pretty much all the way. Prop set at 2440 and I'm cruising steadily at 141 KTAS at 10,000 ft. (11.5 DA). I notice that there seems to be two different vibration frequencies. The normal obvious engine-driven vibration at something like 40 Hz, but a second one at maybe 1/3-1/4 Hz. It's really subtle and it resembles what you get when flying a twin and the props are not quite in synch. The observed result is the amplitude of overall vibration consistently ebbs and flows at this secondary frequency. I theorize that maybe the prop governor is hunting a bit. The RPM does show some (2440-2450) variation, but since it only reads to the nearest 10 RPM I can't really make anything of this.

Has anyone else has noticed this and come to resolve anything about it? It requires really calm air and constant airspeed over an extended period to notice it. And it's pretty easy to overlook or ignore. My prop governor hasn't had any attention since 2009, FWIW.
2002 DA40: MT, PF, 530W/430W, KAP140, ext. baggage, 1090 ES out, 2646 MTOW, 40gal.
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pietromarx
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Re: Prop slightly cycling in flight?

Post by pietromarx » Sun Aug 25, 2019 6:22 pm

Does your setup log the engine, etc.? On the G---- (don't want to mention it here for fear of thread drift!) the log will show your RPM to the nearest 0.1. You can graph it and see if there is a waveform.
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Rich
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Re: Prop slightly cycling in flight?

Post by Rich » Sun Aug 25, 2019 6:42 pm

pietromarx wrote:
Sun Aug 25, 2019 6:22 pm
Does your setup log the engine, etc.? On the G---- (don't want to mention it here for fear of thread drift!) the log will show your RPM to the nearest 0.1. You can graph it and see if there is a waveform.
Nope. VM1000. Very limited tracking. I might be able to fiddle with this in flight and gain some insight, though. Thanks for the suggestion.
2002 DA40: MT, PF, 530W/430W, KAP140, ext. baggage, 1090 ES out, 2646 MTOW, 40gal.
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Steve
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Re: Prop slightly cycling in flight?

Post by Steve » Sun Aug 25, 2019 9:23 pm

Rich:

Not a direct answer to your issue, but perhaps a tool to assist you in diagnosis...

I have this App on my iPad:

https://apps.apple.com/us/app/vibration/id301097580

Called "Vibration", it records vibration in 3 axes using the built-in accelerometers in the iPad, does analysis, and graphs the results.

Steve
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Rich
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Re: Prop slightly cycling in flight?

Post by Rich » Sun Aug 25, 2019 10:01 pm

Thanks, Steve. I'll try it out.
2002 DA40: MT, PF, 530W/430W, KAP140, ext. baggage, 1090 ES out, 2646 MTOW, 40gal.
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perossichi
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Re: Prop slightly cycling in flight?

Post by perossichi » Wed Sep 11, 2019 8:01 am

Have you had the prop dynamically balanced? I just had this done and it dramatically reduced the vibration and in the process various harmonics are measured and the prop is balanced to reduce specific harmonics.

My prop was dynamically balanced when it was installed by previous owner six years ago but I installed Powerflo recently and they recommend it.
2002. Powerflo, Hartzell composite two blade, 530W/430, 345 transponder.
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Re: Prop slightly cycling in flight?

Post by Rich » Wed Sep 11, 2019 3:37 pm

perossichi wrote:
Wed Sep 11, 2019 8:01 am
Have you had the prop dynamically balanced? I just had this done and it dramatically reduced the vibration and in the process various harmonics are measured and the prop is balanced to reduce specific harmonics.

My prop was dynamically balanced when it was installed by previous owner six years ago but I installed Powerflo recently and they recommend it.
Not a bad suggestion. The base vibration isn't bad, but this harmonic is something I just recently noticed - it takes an absolutely smooth extended flight to notice - something in short supply where I live :D
2002 DA40: MT, PF, 530W/430W, KAP140, ext. baggage, 1090 ES out, 2646 MTOW, 40gal.
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perossichi
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Re: Prop slightly cycling in flight?

Post by perossichi » Fri Sep 13, 2019 5:29 am

Rich. I feel your pain. I get many very smooth flight here in socal. Haven’t noticed the harmonic you reference.
2002. Powerflo, Hartzell composite two blade, 530W/430, 345 transponder.
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