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Rhythmic 'noise'

Handclutch

Well Known Member
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
 
Could be resonant or harmonic frequency. Lots of things in motion and playing together - sometimes well / sometimes not.
 
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.
 
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.
 
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?
 
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.
 
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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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.
 
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
 
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. :D
 
I’ll 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 don’t 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!😜
 
Checked the blade angles and they are within 0.1°, 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
 
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.
 
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 it’s 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 didn’t have the new prop coming the first move would be a rebalance but little point in doing that now.

Jack
 
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.
 
One for the brains trust

Summer has come to an end here in southern Australia and I've started using the heater again. I have a Bender baffle that is a fairly close fit, necessarily so I have found in order to bring the CHTs up to around 100° C and produce decent heat.

To my great surprise, the resonant 'thrumming' disappears when the baffle is closed, or very nearly closed. I've tried it a few times now to make sure it wasn't my imagination!

So, what to make of it? The pulsating airflow through the bottom opening divides, of course.. It seems that shutting off the column of air to the radiator changes whatever interaction is causing the noise. It could also be an interaction between the top and bottom airflows. The shroud came off my engine about two years ago which would have changed airflow through the cowling as well. Lots of moving parts.

Has anybody else with a Bender baffle had a similar experience?

My case at least, the gearbox appears innocent. The Julius Sumner Miller video in the previous post dealing with resonance in wind pipes now seems remarkably relevant.

Jack
 
Huh, sounds sort of like what happens when cruising down the highway and both back windows are partially open in the car. Open only one, or open them fully and the primary sound is wind noise. Get them both open just the right (or wrong) amount, and the echo effect is quite dramatic. Amazing what resonance can do.
 
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