Propeller noise

[Pablo Canales]

Hi Guys! I own a Cessna 205 with a 3 blade Continental I-O 550

The thing is, the propeller has started to make an almost unnoticeable Pulsating Sound during cruise flight.(you can only hear it with the headsets on)

No real vibration has been felt, just the sound, like when you are flying a twin and the props are not well synchronized.

If a set the RPM’s lower than 2,200 the pulsating sound slows down until it goes away.

what could it be? Does it sound dangerous?

 

[Unit74]

Off the top of my head, I’d look at the prop balance, then the engine mounts. After that, I got nuthin….

 

[Doug Reid]

Only hear it with headsets on...what type headset ? If ANR, do you still hear it with ANR off ?

 

[Hang 4]

One blade bent? Can check with reference to a fixed object for one out of track. Not sure how to check if all same in pitch.

 

[Dan Thomas]

Loose alternator or a failing belt might do that.

Only in the headsets?. Failed alternator diode comes to mind.

 

[Bell206]

what could it be?

Really not enough info to make a call. Only "hearing" it under ANR is interesting. Perhaps try and define it more. For example, with it dialed in at 2200 and ANRs on gradually climb and dive and see if "noise" changes, etc. Work that from various directions until you can specifically duplicate it at will if possible. Need to try to see if mechanical. airspeed influenced, RF issues, etc. Then report back. However, before you you move forward give the aircraft good tactile going over just to be sure all the pieces are in place and secure.

 

[PaulS]

First I would confirm it is not a headset issue before I started chasing the propellor.

 

[Let'sgoflying!]

I have heard of propeller clocking causing this in smaller engines, 2-blade props.
Does it happen in large bores, with 3-blades?

 

[Dan Thomas]

I have heard of propeller clocking causing this in smaller engines, 2-blade props.
Does it happen in large bores, with 3-blades?

It can. The bigger Cessnas with the 470 or 520 engines used to used a shock-mounted alternator/generator that had the habit of throwing its belt. Reclocking the prop, whether 2-blade or 3-blade, often cured it. Going to the newer solid mount eliminated it. That shockmounting let the alternator start resonating or something. Too much vibration.

 

[Pablo Canales]

Well, its not really the headsets, you can hear the same pulsating noise by just covering your ears with your hands.

as you climb and the propeller pitch changes the sound goes away, but when you level in cruise, the pulsating comes back

 

[Pablo Canales]

It can. The bigger Cessnas with the 470 or 520 engines used to used a shock-mounted alternator/generator that had the habit of throwing its belt. Reclocking the prop, whether 2-blade or 3-blade, often cured it. Going to the newer solid mount eliminated it. That shockmounting let the alternator start resonating or something. Too much vibration.

This sounds quite interesting, because we’ve had 3 broken alternators in the past 6 months

 

[Bell206]

we’ve had 3 broken alternators in the past 6 months

In detective work thats called a clue. Definitely need some additional work on that point. While a prop balance/engine spectrum check might give some more clues 3 busted mounts in 6 mos usually indicates something seen or felt manually.

 

[Racerx]

This sounds quite interesting, because we’ve had 3 broken alternators in the past 6 months

I would have dug deep when the second one went. What's the failure?

 

[Sinistar]

Total long shot. We started getting a vibration somewhat like this in our 182. Pulled the prop governor and looked at a few other things. Still there.. Would only do it once leveled out in cruise. Turned out to be the nose wheel pant wasnt snugged tight enough.

 

[Sinistar]

When you say broken alternator what exactly broke? Was it the same problem all 3 times?

 

[Pablo Canales]

When you say broken alternator what exactly broke? Was it the same problem all 3 times?

I mean like “literally” broken apart in half
The external screws of the alternator broke and the alternator opened itself up in half

 

[Pablo Canales]

Good news guys!

We found the problem, turn out to be the metal plate that sits in front of the 6th Cylinder to redirect the airflow

It was loose, and it was dancing with the wind, making that pulsating noise


But…. The alternators keep breaking in 2 pieces and we cant figure out why


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[Dan Thomas]

But…. The alternators keep breaking in 2 pieces and we cant figure out why

So what's breaking? The aluminum housings? The three case screws? Can't help if we don't know what it looks like. Is it on a shockmounted setup or is it rigid? How tight is the belt being set? Were all the busted alternators from the same shop or technician? Installed by the same mechanic?

 

[Pablo Canales]

So what's breaking? The aluminum housings? The three case screws? Can't help if we don't know what it looks like. Is it on a shockmounted setup or is it rigid? How tight is the belt being set? Were all the busted alternators from the same shop or technician? Installed by the same mechanic?

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[Pablo Canales]

So what's breaking? The aluminum housings? The three case screws? Can't help if we don't know what it looks like. Is it on a shockmounted setup or is it rigid? How tight is the belt being set? Were all the busted alternators from the same shop or technician? Installed by the same mechanic?

Literally the 3 screws breaking in half

We replaced the first alternator with a new one after the third time it broke

(Today its the 4th time the new alternador starts to loosen up on the screws)


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[PaulS]

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Yeah, that should never happen. Are the bolts just fatiguing a breaking? Are they loosening up? Are the bearings failing? At number 4 I would be thinking about a different manufacturer.

 

[Dan Thomas]

Literally the 3 screws breaking in half

We replaced the first alternator with a new one after the third time it broke

(Today its the 4th time the new alternador starts to loosen up on the screws)

Sounds like a serious vibration problem. Might need a dynamic prop balance. Or you're buying Kelly/Hartzell alternators, or maybe they're by someone else who is doing a bad rebuilding job of them. Might be overtorqued screws cracking or stretching or pulling the threads out of the aluminum housings. Or cheap screws. Are these failures being SDR'd like they should be?

 

[Bell206]

Literally the 3 screws breaking in half

If you have inspected/verified all other items are within spec on your engine (eng mts, prop, alt drive pulley, etc) I would look to get a complete prop balance and engine vibration spectrum. Repeatedly breaking screws like that usually indicates a serious problem which I would not fly with until you can isolate it. I've seen many different failures due to vibrations but don't recall having 4 alternators fail in the same fashion on the same engine/airframe.

 

[Pablo Canales]

Sounds like a serious vibration problem. Might need a dynamic prop balance. Or you're buying Kelly/Hartzell alternators, or maybe they're by someone else who is doing a bad rebuilding job of them. Might be overtorqued screws cracking or stretching or pulling the threads out of the aluminum housings. Or cheap screws. Are these failures being SDR'd like they should be?

Actually yes, the first alternator was Hartzell and the one we have right now is Kelly Aerospace.
Are they bad?

Also, we will have the prop balance, we don’t actually feel the vibration in the cockpit. But just in case


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[PaulS]

Actually yes, the first alternator was Hartzell and the one we have right now is Kelly Aerospace.
Are they bad?

Also, we will have the prop balance, we don’t actually feel the vibration in the cockpit. But just in case


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That's the first place I would look, at the alternators. I assume you have other rotating accessories on that engine that are not failing like this. It could be a case of a couple companies labling one company's product. Did you take pics of the other failures? Should be pretty easy to see if they are made by the same people.

 

[Jdm]

Holy crap, you’re description of having a “broken” alternator was spot on!!!
That thing looks horrible. Is that the direct drive type that’s geared at the front right sided of the engine? Not belt drive right?

 

[Dan Thomas]

Holy crap, you’re description of having a “broken” alternator was spot on!!!
That thing looks horrible. Is that the direct drive type that’s geared at the front right sided of the engine? Not belt drive right?

He has an IO-550. It's on the left aft corner of the engine, belt-driven.

Overtightened belts can destroy bearings and cause that sort of damage. Look at the aluminum smearing on the slip rings. That might have happened as the thing came apart and the slip rings contacted the aluminum shield cast into the rear housing, or it might be due to a failed bearing.

Too often the belts are overtightened. The '69 - '76 172 manual has this to say about installation on a Lycoming:



Try it sometime and see that it might be rather loose compared to what you usually do or see. The Continental installation has a longer arc of belt contact in the alternator pulley and surely wouldn't need much more tension than the Lyc. The Cessna 185 has the IO-520, very similar to the 550, and it has this to say:



The 206 manual says the same thing, for both 12- and 24-volt systems.

 

[Pablo Canales]

Take a look at the alternator pulley guys

Is that amount of movement normal?

Could that be causing the alternators to vibrate and break?


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[Dan Thomas]

Take a look at the alternator pulley guys

Is that amount of movement normal?

Could that be causing the alternators to vibrate and break?


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Could be a factor but I'd suspect other problems. In any case, you have a problem there. The drive is coming apart. Too much belt tension might have done that. The steel shaft gets compressed under the ball bearing's inner race and so works loose. If the bearing fails it drops hardened steel balls into the accessory drive gears. That is not good at all. Get it looked at.

 

[JustViper]

If you are hearing a pulsating noise, might want to check the leading and trailing edge of the propeller, if there if filed down areas, check with the manufactures manual and ensure that it is within standards. It could be airflow over the propeller area being different from the surrounding area producing a small vibration and abnormal pulsating noises.

 

[EdFred]

If you are hearing a pulsating noise, might want to check the leading and trailing edge of the propeller, if there if filed down areas, check with the manufactures manual and ensure that it is within standards. It could be airflow over the propeller area being different from the surrounding area producing a small vibration and abnormal pulsating noises.

Or it could be the problem he already found and fixed.

 

[JustViper]

Or it could be the problem he already found and fixed.

Noticed that after he posted, no need to be so presumptuous.