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Engine knocking sound

ronrapp

ronrapp

Well Known Member
Patron
I recently noticed what I will call a knocking sound from the engine and can't seem to identify what's going on. I thought perhaps the collective knowledgebase here might be able to help.

Here's a link to a sound file which I recorded with my phone. (Apologies for the fact that you can hear me breath at certain points -- I wanted to keep the iPhone near my head so the microphone would pick up what my ears did.):


The recording starts at cruise speed and power setting, and I slowly pull the throttle out. At about 0:50, RPM is down to about 1800 and the knocking sound begins. The knocking start to abate around 1300 RPM. The sound is repeatable and does not seem to change with airspeed, attitude, or altitude.

De-cowled the aircraft and everything looks normal. My mechanic looked through it as well and couldn't see anything abnormal. I submitted G3X logs for analysis to Savvy and they compared it with other flights over the past year and do not see anything out of the ordinary. The knocking sound is not accompanied by any roughness or vibration, no power loss or surging, no unusual indications or anything else. Pressures, temperatures, etc all completely normal.

A little bit about the plane: IO-360-C1A powered RV-6 with a fixed pitch Sensenich GA composite prop. 9.5:1 compression. Dual electronic ignition: a Lightspeed Plasma III and a P-Mag.

Thanks for any help you can provide. This one has a bunch of people stumped.

--Ron
 
ronrapp said:
I recently noticed what I will call a knocking sound from the engine and can't seem to identify what's going on. I thought perhaps the collective knowledgebase here might be able to help.

Here's a link to a sound file which I recorded with my phone. (Apologies for the fact that you can hear me breath at certain points -- I wanted to keep the iPhone near my head so the microphone would pick up what my ears did.):


The recording starts at cruise speed and power setting, and I slowly pull the throttle out. At about 0:50, RPM is down to about 1800 and the knocking sound begins. The knocking start to abate around 1300 RPM. The sound is repeatable and does not seem to change with airspeed, attitude, or altitude.

De-cowled the aircraft and everything looks normal. My mechanic looked through it as well and couldn't see anything abnormal. I submitted G3X logs for analysis to Savvy and they compared it with other flights over the past year and do not see anything out of the ordinary. The knocking sound is not accompanied by any roughness or vibration, no power loss or surging, no unusual indications or anything else. Pressures, temperatures, etc all completely normal.

A little bit about the plane: IO-360-C1A powered RV-6 with a fixed pitch Sensenich GA composite prop. 9.5:1 compression. Dual electronic ignition: a Lightspeed Plasma III and a P-Mag.

Thanks for any help you can provide. This one has a bunch of people stumped.

--Ron
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Interesting. Sounds like blades of a helicopter beating the air. :unsure:

So: no loss of power? No abnormal reading from your engine monitors?

Air from the prop hitting something like it isn't supposed to. I'm sure you have given the prop a tug and made sure the spinner is tight. I'd almost want to take the spinner off. Something that starts to bump at a certain RPM range that is consistent with the RPM of the engine......... I would doubt it was the engine itself unless something outside the engine is loose. Air dams loose?

No signs of an exhaust leak.

Thinking........... :unsure:🤷‍♂️
 
Roadjunkie1 said:
Interesting. Sounds like blades of a helicopter beating the air. :unsure:

So: no loss of power? No abnormal reading from your engine monitors?

Air from the prop hitting something like it isn't supposed to. I'm sure you have given the prop a tug and made sure the spinner is tight. I'd almost want to take the spinner off. Something that starts to bump at a certain RPM range that is consistent with the RPM of the engine......... I would doubt it was the engine itself unless something outside the engine is loose. Air dams loose?

No signs of an exhaust leak.

Thinking........... :unsure:🤷‍♂️
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Good thoughts. I have given the prop a good inspection and checked the blades, all looks normal. Haven't removed the spinner, but that can't hurt so I'll add it to my list.

My first thought was that it was a piece of baffling or perhaps a fairing component vibrating, but everything seems secure.

My next idea is to take a mechanic up with me to hear it themselves. Savvy had suggested perhaps putting a camera inside the cowl to record what's happening, but I don't have a suitable camera and don't like the idea of putting anything in there.

--Ron
 
ronrapp said:
I recently noticed what I will call a knocking sound from the engine and can't seem to identify what's going on. I thought perhaps the collective knowledgebase here might be able to help.

Here's a link to a sound file which I recorded with my phone. (Apologies for the fact that you can hear me breath at certain points -- I wanted to keep the iPhone near my head so the microphone would pick up what my ears did.):


The recording starts at cruise speed and power setting, and I slowly pull the throttle out. At about 0:50, RPM is down to about 1800 and the knocking sound begins. The knocking start to abate around 1300 RPM. The sound is repeatable and does not seem to change with airspeed, attitude, or altitude.

De-cowled the aircraft and everything looks normal. My mechanic looked through it as well and couldn't see anything abnormal. I submitted G3X logs for analysis to Savvy and they compared it with other flights over the past year and do not see anything out of the ordinary. The knocking sound is not accompanied by any roughness or vibration, no power loss or surging, no unusual indications or anything else. Pressures, temperatures, etc all completely normal.

A little bit about the plane: IO-360-C1A powered RV-6 with a fixed pitch Sensenich GA composite prop. 9.5:1 compression. Dual electronic ignition: a Lightspeed Plasma III and a P-Mag.

Thanks for any help you can provide. This one has a bunch of people stumped.

--Ron
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Soooo ....

1300-1800 RPM. By any chance, would you know the manifold pressure? (I understand that you have a fixed pitch prop and thus might not have a MP Gauge.
 
Are you positive it is coming from engine area? The sound can resonate through the hollow metal fuselage and make it hard to determine exact location of noise.
Frequency sounds like 3-5 times per sec, way less than engine RPM. I have found flapping noises from loose fairing maybe on gear leg. Also on a loose pieces of weather stripping under/around canopy or on the edge of the wing to body fairing. Also possibly a loose corner of anti-chaff tape under flap leading edges. Flapping only occurs at certain rpm when there is a local turbulent flow exciting a certain frequency of the loose part which may smooth out at higher rpm’s.
 
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Check the spinner and its backplate and confirm there's nothing going on around the prop. I'd also check the engine mounts for looseness, as well as where the engine mounts attach to the firewall. You state that it happens at specific RPM ranges regardless of airspeed, attitude or altitude. This would seem to point that it's something engine or prop related, and less likely a piece of trim being buffeted by airspeed (my first inclination was loose trim as well). Can you replicate the noise on the ground?
 
My guesses are (in no particular order) :

1. Oil cooler flanges banging on the motor mount
2. Alternator belt slapping the front baffle
3. Oil hoses, Fuel hoses touching motor mount
4. Exhaust pipes touching motor mount or fuselage.
 
plehrke said:
Are you positive it is coming from engine area? The sound can resonate through the hollow metal fuselage and make it hard to determine exact location of noise.
Frequency sounds like 3-5 times per sec, way less than engine RPM. I have found flapping noises from loose fairing maybe on gear leg. Also on a loose pieces of weather stripping under/around canopy or on the edge of the wing to body fairing. Also possibly a loose corner of anti-chaff tape under flap leading edges. Flapping only occurs at certain rpm when there is a local turbulent flow exciting a certain frequency of the loose part which may smooth out at higher rpm’s.
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This ^^
 
That definately sounds like a rythmic vibration and doesn't sound like anything thast would come from the engine internals. Though I would suspect it is related to engine or prop rotation and not something related to airspeed, like loose external parts. Might be worth looking for witness marks on the cowl where the baffling could be hitting. the engine movement could be larger in that RPM range and therefore only touch cowl then. Many folks leave a small area just a bit too long and it only contacts when vibrations rates are high. Look for anything in the engine area that could be making contact with something else as the engine moves around on its mounts.
 
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Record and quantify the knocking frequency and RPM, at the same time. Report back.
 
Great suggestions all! Thank you. I’m in London right now but will check into these suggestions when I get back.

In answer to the question about manifold pressure, at 1800 RPM it was 10" MP. At 1300 RPM it was 5.4". That's from the G3X data log.

Re: Dan's suggestion to "record and quantify the knocking frequency and RPM, at the same time", the only way I can think of to do that would be to repeat the recording and match it up with a video of the RPM display. Or were you thinking of something else?

--Ron
 
dpansier said:
View attachment 79760
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:ROFLMAO: 😂🤣😂🤣 This gave me a laugh-out-loud this morning!!! Thanks!!

But I digress. I drive a 2005 Toyota RAV4 (with 217,000+ miles; just starting to get broken in) that has an exhaust pipe heat shield that, at a certain predicable RPM range, gives off a buzzing sound. It is rhythmic in its own way. It annoyed my former roommate which means now I kind of LIKE it!!😁 I have often thought a good fix would be to attach a length of thicker welding rod or aluminum to disrupt the frequency of the vibration......but now I might just leave it!!😆 Something else to consider......

Enjoy London............
 
ronrapp said:
Re: Dan's suggestion to "record and quantify the knocking frequency and RPM, at the same time", the only way I can think of to do that would be to repeat the recording and match it up with a video of the RPM display. Or were you thinking of something else?
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Hadn't thought much about how to do it. I was suggesting the first step is to determine if the noise is proportional to an engine rotational frequency, or not. For example, at 1800 RPM is the noise at 30 hz? That would be first order, something happening once per crankshaft revolution. Or perhaps at 60 hz, the firing frequency, twice per rev. Or perhaps the noise is a thumping away at some random number like 17 hz, in which case it's time to look at the airframe. At 1800, there is nothing engine related happening at 17 hz.

Can you feel it? I have a vibration app on my phone which would ID the frequency.
 
Check for a lose bolt on the isolator/mounts, check the actual engine mount for a cracked tube (usually at a cluster, (lower bottom right or upper right, at firewall is frequently a place to break), check the inside of the cowl for a rub/ witness mark that would indicate something is too close to the cowl. Have someone grab the prop ( don't turn it and make sure the mags are off) and aggressively pull up, down left and right with the cowl on and again with the cowl off, while you observe and listen.
 
DanH said:
Hadn't thought much about how to do it. I was suggesting the first step is to determine if the noise is proportional to an engine rotational frequency, or not. For example, at 1800 RPM is the noise at 30 hz? That would be first order, something happening once per crankshaft revolution. Or perhaps at 60 hz, the firing frequency, twice per rev. Or perhaps the noise is a thumping away at some random number like 17 hz, in which case it's time to look at the airframe. At 1800, there is nothing engine related happening at 17 hz.

Can you feel it? I have a vibration app on my phone which would ID the frequency.
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No, I can't really feel any difference in vibration. But I did make another recording of it, this time with video that shows the full G3X engine page. This was taken today with the autopilot engaged so I could record. As I reduced power, the knocking sound did not occur until the power was at idle and the plane slowed to nearly 80 knots and the yellow "MIN SPEED" annunciation appeared. I started adding power in, and then (at around 1:00 into the video), you can clearly hear the sound.

Dan, does this help at all? I still can't tell if the noise is proportional to the RPM or not. For what it's worth, I'm able to duplicate the sound in the run up area on the ground, so it's not airspeed-dependent.


--Ron
 
Does appear to be proportional to RPM. Beyond that observation, I dunno. I would ID the frequency and compare to RPM.

BTW, the phone app I use is called "Resonance". Plots vibratory amplitude in three axis. Just hold you phone against some structure. If one of the axis gains amplitude as you hear the noise, that's it. If there is no change in any axis as the noise gets louder, that also tells you something...it's probably not an airframe item.
 
ronrapp said:
No, I can't really feel any difference in vibration. But I did make another recording of it, this time with video that shows the full G3X engine page. This was taken today with the autopilot engaged so I could record. As I reduced power, the knocking sound did not occur until the power was at idle and the plane slowed to nearly 80 knots and the yellow "MIN SPEED" annunciation appeared. I started adding power in, and then (at around 1:00 into the video), you can clearly hear the sound.

Dan, does this help at all? I still can't tell if the noise is proportional to the RPM or not. For what it's worth, I'm able to duplicate the sound in the run up area on the ground, so it's not airspeed-dependent.


--Ron
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Kinda of sounds like drumming. I would be looking for something exhaust related. Definately not knocking. The fact that it appears immediately with application of power points away from vibrations causing parts contact and towards something related to the combustion cycle. Have you checked ign timing? That will change the exh note.

I am hearing changes in the exh note and not parts banging together and definately not knocking in the classic definition (i.e. detonation).
 
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It's interesting that several people have mentioned the tail pipe area; I just reviewed the logbook from the last condition inspection and the tailpipe hangar was re-fabricated and clamp was replaced. I think we might have a good place to start...

--Ron
 
Life gets in the way, so I haven't had the chance to get out to the airport until recently. Turns out one of the exhaust pipe hangars was broken, and the pipe had rotated enough to be directing the exhaust pulses right onto the lower skin of the airframe. That's what was causing the sound heard in the video and audio clips. I always check the exhaust pipes for play during the preflight, but I didn't pay enough attention to the orientation of the pipe itself. Lesson learned. (I seem to say that a lot...).

I flew the plane for an hour or two today, at various attitudes, altitudes, power settings, and speeds, and I didn't hear anything abnormal, so I guess we can call this one fixed.

Interestingly, the OTHER exhaust pipe's hangar was broken at the last annual and was fixed then. Now both of them have beefier metal holding it, so with any luck this problem won't happen again.

My sincere thanks to all who took the time to chime in with observations, suggestions, and ideas. It's always appreciated.

--Ron
 
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