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  #1  
Old 11-01-2021, 06:14 AM
Handclutch Handclutch is offline
 
Join Date: Feb 2015
Location: Bendigo, Australia
Posts: 135
Default Rhythmic 'noise'

I have been noticing this occasionally but it's becoming more pronounced. Above 5150 rpm I'm getting, for want of a better description, a rhythmic, thrumming noise. And 'noise' is a bit of an overstatement - just audible with a good noise cancelling headset, wouldn't be heard with a passive headset.

Absolutely nothing at less than 5150, and I can't feel feel it through the air frame. The prop has been dynamically balanced, although some time ago, and is quite smooth.

I suspect it is some sort of resonance. Wondering if anybody has had a similar experience?

By way of trying to provide a better description it is somewhat similar to the noise from a worn rear axle bearing on a car. In my (limited) experience a worn bearing produces that rhythmic sound at pretty much any speed, but this is limited to a narrow rpm range.

I appreciate that many people cruise at less than 5150, and probably wouldn't notice it, but I'm usually a little above that.

Jack
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  #2  
Old 11-01-2021, 07:00 AM
Cth6 Cth6 is offline
 
Join Date: Apr 2017
Location: Central Florida
Posts: 180
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Could it be oil canning of the skin at a certain speed?
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  #3  
Old 11-01-2021, 08:08 AM
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Piper J3 Piper J3 is offline
 
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Location: Hinckley, Ohio
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Could be resonant or harmonic frequency. Lots of things in motion and playing together - sometimes well / sometimes not.
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  #4  
Old 11-01-2021, 09:44 AM
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rvbuilder2002 rvbuilder2002 is offline
 
Join Date: Jul 2005
Location: Hubbard Oregon
Posts: 9,667
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It could be a very early sign that the gear box will be needing some attention in the future.
You didn't mention how many hrs. are on the airplane.
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  #5  
Old 11-01-2021, 09:55 AM
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rcarsey rcarsey is offline
 
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Location: North Brunswick, NJ
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Hmmm I have to join the club here too. I've noticed this at higher RPM settings. It doesn't seem to happen EVERY time though (maybe I've learned to be ignorant).

Its a thump about 1-2 times per second. The first time it happened was during the 5-hour flyoff.. and I quickly dialed back on the throttle -- I was hypersensitive to every noise and vibration. Since then, now at about 125hrs, I ignore it.

My initial assumption was that its a wheel that was rotating during flight. But I do not think it is that because I have since balanced all the wheels (and I applied brakes in flight). I have yet to place external cameras on the plane in an effort to see the issue.
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  #6  
Old 11-01-2021, 10:27 AM
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rvbuilder2002 rvbuilder2002 is offline
 
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Quote:
Originally Posted by rcarsey View Post
Hmmm I have to join the club here too. I've noticed this at higher RPM settings. It doesn't seem to happen EVERY time though (maybe I've learned to be ignorant).

Its a thump about 1-2 times per second. The first time it happened was during the 5-hour flyoff.. and I quickly dialed back on the throttle -- I was hypersensitive to every noise and vibration. Since then, now at about 125hrs, I ignore it.

My initial assumption was that its a wheel that was rotating during flight. But I do not think it is that because I have since balanced all the wheels (and I applied brakes in flight). I have yet to place external cameras on the plane in an effort to see the issue.
1-2 X's per second is a pretty low freq. vibration, which I don't think is anything similar to the op's description.

There are a lot of different things that can induce a vibration. Start with the simple stuff....confirm prop. blades are the same angle, etc. Have you had a dynamic prop balance done since detecting the vibration during phase 1?
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  #7  
Old 11-01-2021, 11:53 AM
myv65 myv65 is offline
 
Join Date: Aug 2020
Location: Beloit, WI
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If something "thumps" 1-2 times per second, you don't hear it, you feel it. If you do hear something, it's resulting vibrations of the "thumped" system at higher frequencies. Bang anything structural and it will ring at one or more resonant frequencies, with the frequencies and duration a function of what you hit. You *can* hear beat frequencies well below 20Hz, but that's just two or more similar frequencies going in and out of phase, and doesn't seem like what you're describing.

To the OP, it may be worth loading up FFT software on a smartphone. There are a few free alternatives for Android and Apple. Problem will be figuring out which frequency corresponds to the noise you're sensing. I've had success using this with electric motors looking at bearing frequencies, but that's a much more constrained case than all the frequencies you'll experience in a flying airplane.
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  #8  
Old 11-01-2021, 04:41 PM
Tacco Tacco is offline
 
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Location: White Salmon, WA
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I chased this through 200 hours and two annuals. I am convinced, given a meticulously completed dynamic prop balance, that my problem was a slight, very slight, pitch mismatch (71.45 versus 71.30) between the blades coupling with the natural harmonic resonances of the Rotax power train (crankshaft, gear box). I still have it occasionally but barely noticeable and at variable RPMs, mostly because I'm looking for it. .

Here are some ideas I used to eliminate other vibration sources:

Airframe: Yaw and pitch the aircraft slightly to see if the vibration changes.

Noise cancelling headset interaction: Just turn it off.

Main Landing gear: Not an issue here but just tap the brakes to stop any rotation. Properly installed wheels bearings will have a brake-out torque that stops the wheel pretty quickly after take-off (see https://youtu.be/P5N8C4_f8eE)

Nose gear: An out of round or balance condition might cause it to rotate but you will likely be able to discern this from other sources (see video above)

For what it's worth.
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Last edited by Tacco : 11-01-2021 at 04:48 PM. Reason: spelling
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  #9  
Old 11-01-2021, 06:10 PM
NinerBikes NinerBikes is offline
 
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Location: Granada Hills
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I experienced this noise when under full throttle climbing out at 5150-5250 rpms and 65-85 kts airspeed, and noticed that it vanished if I let up just slightly on the right rudder pedal, creating a left yaw.

So far, on 3 different RV-12 Legacy models at the airport I am at, it seems to be normal. All 3 of us pilots run David Clark headsets, nothing fancy.

See if it vanishes when you slightly reduce foot pressure on the right or left rudder pedal on climb out. If it does, I suspect it's gearbox related, and it appears to be normal.
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  #10  
Old 11-01-2021, 06:51 PM
Handclutch Handclutch is offline
 
Join Date: Feb 2015
Location: Bendigo, Australia
Posts: 135
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Thanks for all the suggestions and comments.

The aircraft has done 550 hours with very little circuit (pattern) work. I wouldn't expect the gearbox to be a problem yet but it's always possible.

The possibility of out of sync blades is a good thought. They were adjusted a few months ago when I felt that the prop was slightly under pitched. It was done carefully and, as I recall, the the difference was no more than 0.1 - 0.2, but definitely worth rechecking. May have got it wrong. And that time since the re-pitch coincides with when I have been more aware of the issue.

I haven't tried yawing the aircraft to see what happens but have noticed that if I raise the nose slightly at 5300 the sound seems to reduce/disappear.

Doubt that it has anything to do with the wheels because it definitely disappears below 5150.

I have an Airmaster 3 blade CS on order (no regulatory restrictions in Australia) and expect it to be fitted in a couple of months. If the noise persists beyond that the gearbox would have to be a strong candidate. In the meantime I'll double check the blade sync and report back.

Jack
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